Atom
What is an Atom?
An atom is the
tiniest unit of matter that forms a chemical element. Each and every type of
solid, liquid, gas, and form of plasma is composed of neutral or ionized atoms.
In general, atoms are 100 picometers across. Due to quantum effects, they are
so small that it is impossible to predict their behavior with sufficient
accuracy using classical physics, as would be the case, for example, if they
were tennis balls.
A nucleus and
one or more electrons connected to it make up each atom. The nucleus is
composed of one or more protons and a sizable number of neutrons. Only the most
common kind of hydrogen has no neutrons. More than 99.94% of the mass of an
atom is found in its nucleus. The electric charges of the protons are positive,
those of the electrons are negative, and those of the neutrons are zero. The
atom is electrically neutral if the number of protons and electrons is equal.
An atom has an overall charge of positive or negative if it contains more
electrons than protons; these atoms are known as ions.
The
electromagnetic force pulls an atom's electrons toward the protons in its
atomic nucleus. The nuclear force draws the protons and neutrons in the nucleus
together. The electromagnetic force that keeps positively charged protons apart
is typically weaker than this force. The electromagnetic force that repels
objects can occasionally outweigh the nuclear force. The nucleus breaks in this
instance, leaving behind several parts. This type of nuclear decay exists.
The atomic
number, which designates which chemical element the atom belongs to, is
determined by the quantity of protons in the nucleus. For instance, a copper atom
is any atom with 29 protons. The element's isotope is determined by the number
of neutrons in it. For instance, a copper atom with 34 neutrons is copper-63
(29+34), and one with 36 neutrons is copper-65; naturally occurring copper is
composed of roughly 70% Cu-63 and the remaining 30% Cu-65.
Atoms can create chemical compounds like molecules or crystals by joining up with one or more other atoms through chemical bonds. For instance, the Statue of Liberty in New York City was initially built of pure copper, but over time, oxygen, carbon, and Sulphur atoms reacted with the surface to give the copper a green patina. The majority of the physical changes seen in nature are caused by atoms' capacity to bind and detach. The science that investigates these changes is chemistry.
In philosophy
Many ancient
societies, including those of Greece and India, adopted the fundamental notion
that matter is composed of tiny, inseparable components. The ancient Greek term Atomos, which meaning "uncuttable," is the source of the word atom.
Modern atomic theory is not based on these antiquated ideas; rather, it is
founded on philosophical reasoning as opposed to scientific reasoning. John
Dalton, a scientist who lived in the early 19th century, observed that chemical
elements appeared to join with one another in basic weight units. On the idea
that these weight units were the basic building blocks of matter, he chose to
call these units "atoms." Although it was later found that Dalton's
atoms are not truly indivisible, the phrase persisted.
Atom Structure
Although the
term "atom" originally referred to a particle that could not be
divided into smaller particles, it is now used to refer to a group of subatomic
particles that make up an atom. The electron, proton, and neutron are the
building blocks of an atom.
Using a mass of
9.11 10-³¹ kg, the electron is by far the least massive of these particles. It
also has a negative electrical charge and is too tiny to be measured with
current methods. Prior to the discovery of neutrino mass, it was the lightest
particle with a positive measured rest mass. Under normal circumstances, the
attraction between the positively charged nucleus's positively and negatively
charged electrons binds them to it. An atom becomes positively or negatively
charged overall if it has more or less electrons than its atomic number; such
an atom is referred to as an ion. J.J. Thomson had a major role in the
discovery of electrons in the late 19th century; for further information, see
the history of subatomic physics.
With a mass of
1.6726 10-²⁷ kg, protons are 1,836 times heavier than electrons and have a
positive charge. The atomic number of an atom is the sum of all of its protons.
Ernest Rutherford noted in 1909 that nitrogen emits what looked to be hydrogen
nuclei when subjected to an alpha particle bombardment. He recognized the
hydrogen nucleus as a separate particle within the atom in 1920 and gave it the
name proton.
The free mass of a neutron is 1.6749 10⁻²⁷ kg, or 1,839 times that of an electron. Neutrons have no electrical charge. Although neutrons are the heaviest of the three component particles, the nuclear binding energy can diminish their mass. Nucleons, which are made up of protons and neutrons, are similar in size—on the order of 2.5 10⁻¹⁵ m—but their'surfaces' are not clearly defined. James Chadwick, an English physicist, discovered the neutron in Year 1932..
In the Standard
Model of physics, protons and neutrons are composite particles made up of
genuinely elementary particles called quarks, whereas electrons are truly
elementary particles with no intrinsic structure. Atoms contain two different
types of quarks, each of which has a little electric charge. Two up quarks
(each having a charge of +2/3) and one down quark (with a charge of 1/3) make
up protons.
One up quark and
two down quarks make up neutrons. The two particles' different masses and
charges are explained by this disparity.
The strong
interaction (also known as the strong force), which is mediated by gluons,
holds the quarks together. The nuclear force, a remnant of the strong force
with somewhat modified range-properties, holds the protons and neutrons to one
another in the nucleus (see the article on the nuclear force for more). The
gluon is an elementary particle that mediates physical forces. It belongs to
the family of gauge bosons.
Nucleus
Nucleus of an Atom
Nucleons are the
collective name for the bonded protons and neutrons that make up the small
atomic nucleus of an atom. A nucleus's radius is about equivalent to 1.07 3√ A femtometers,
where 1.07 3√ A is the sum of all its nucleons. This is substantially less than
the atom's radius, which is approximately 105 fm. The residual strong force, a
short-range attractive potential, holds the nucleons together. This force is
substantially stronger than the electrostatic force that prevents positively
charged protons from repelling one another at distances lower than 2.5 fm.
The atomic
number, or number of protons in an atom, is the same for all atoms of the same
element. The isotope of an element can differ depending on the amount of
neutrons in that element. The nuclide is determined by the total amount of
protons and neutrons. The stability of the nucleus is determined by the ratio
of neutrons to protons, with some isotopes undergoing radioactive decay.
Fermions are
defined as the proton, electron, and neutron. The Pauli exclusion principle,
which applies to fermions, forbids identical fermions—such as multiple
protons—from existing in the same quantum state simultaneously. As a result,
each proton in the nucleus must exist in a distinct quantum state from every
other proton, and the same holds true for each neutron in the nucleus and each
electron in the electron cloud.
Through a
radioactive decay that brings the number of protons and neutrons closer
together, a nucleus with a different number of protons than neutrons may be
able to transition to a lower energy state. As a result, atoms with equal
numbers of protons and neutrons are more resistant to decay. However, as the
atomic number increases, an increasing proportion of neutrons are needed to
maintain the stability of the nucleus due to the mutual repulsion of the
protons.
It is possible
to change the proportion of protons and neutrons in the atomic nucleus, albeit
the strong force may make this need extremely high energy. When several atomic
particles combine to form a heavier nucleus, such as when two nuclei collide
energetically, this process is known as nuclear fusion. Protons, for instance,
need energy between 3 and 10 keV to break through the coulomb barrier and
combine into a single nucleus at the center of the Sun. The opposing process,
known as nuclear fission, causes a nucleus to split into two smaller nuclei,
typically by radioactive decay. High energy subatomic or photon bombardment of
the nucleus is another way to alter it. The atom transforms into a different element
if this changes the number of protons in a nucleus.
According to
Albert Einstein's mass-energy equivalence formula, E=mc2, where m is the mass
loss and c is the speed of light, if the mass of the nucleus after a fusion
reaction is less than the sum of the masses of the separate particles, then the
difference between these two values can be emitted as a type of usable energy (example
gamma ray or the kinetic energy of a beta particle). The non-recoverable loss
of the energy is what keeps the fused particles together in a state that calls
for this energy to separate them. This deficit is a component of the binding
energy of the new nucleus.
In most cases,
the fusing of two nuclei that results in bigger nuclei with atomic numbers less
than those of iron and nickel—a total nucleon number of roughly 60—occurs as an
exothermic process that releases more energy than is necessary to bring the
nuclei together. Because of this process of energy release, nuclear fusion in
stars is a self-sustaining reaction. The binding energy per nucleon in the
nucleus starts to decline for heavier nuclei. Therefore, fusion events that
result in nuclei with atomic weights more than about 60 and atomic numbers
greater than about 26 are endothermic reactions. These more powerful nuclei are
unable to undertake an energetic fusion reaction that would maintain a star's
hydrostatic stability.
Through a
radioactive decay that brings the number of protons and neutrons closer
together, a nucleus with a different number of protons than neutrons may be able
to transition to a lower energy state. As a result, atoms with equal numbers of
protons and neutrons are more resistant to decay. However, as the atomic number
increases, an increasing proportion of neutrons are needed to maintain the
stability of the nucleus due to the mutual repulsion of the protons.
Electron cloud
The
electromagnetic force pulls the electrons of an atom toward the protons in the
nucleus. The electrons are constrained by this force inside an electric
potential well surrounding the smaller nucleus, therefore an external energy
source is required for the electron to escape. The attractive force between an
electron and the nucleus increases with proximity. As a result, electrons
trapped close to the potential well's core need more energy to free themselves
than electrons trapped farther apart.
Like other
particles, electrons exhibit both wavelike and particle like characteristics.
Inside the potential well, there is an area known as the electron cloud, where
each electron creates a particular kind of standing wave in three dimensions
that is stationary with respect to the nucleus. An atomic orbital, a
mathematical function that describes the likelihood that an electron would
appear to be at a specific location when its position is measured, defines this
behaviour. Around the nucleus, there is only a definite (or quantized)
collection of these orbitals since all other potential wave patterns quickly
decay into a more stable form. Orbitals vary from one another in size, shape,
and orientation, and they can have one or many ring or node structures.
Each atomic
orbital corresponds to a specific electron energy level. A photon with enough
energy to propel it into the new quantum state can be absorbed by an electron
to change its state to one at a higher energy level. Similar to how a photon
can emit extra energy as a photon, an electron in a higher energy state can
drop to a lower energy level spontaneously. Atomic spectral lines are caused by
these distinctive energy values, which are determined by the variations in the
energies of the quantum states.
The electron
binding energy is much smaller than the binding energy of nucleons since it is
required to remove or add an electron. For instance, splitting a deuterium
nucleus only requires 2.23 million eV, whereas stripping a ground-state
electron from a hydrogen atom only needs 13.6 eV. If protons and electrons are
distributed equally across the atom, the atom is electrically neutral. Ions are
defined as atoms with either a surplus or a shortage of electrons. The electrons
that are most distant from the nucleus may be shared or transferred to other neighboring atoms. Atoms can form molecules and other kinds of chemical
compounds, such as ionic and covalent network crystals, by this method.
Properties of Atom
Nuclear properties
Any two atoms that
have the same number of protons in their nuclei are said to be members of the
same chemical element. Differing isotopes of the same element are atoms that
have different numbers of neutrons but equal quantities of protons. For
instance, all hydrogen atoms can accept exactly one proton, but there are other
isotopes of hydrogen that can have one, two, or more neutrons. Hydrogen-1 is
the isotope that is most prevalent and is also known as protium. From the
single proton element hydrogen through the 118 proton element oganesson, the
known elements are grouped by their atomic numbers. The radioactivity of
element 83 (bismuth) is so minute as to be essentially nonexistent, while all
isotopes of known elements with atomic numbers greater than 82 are radioactive.
The term
"stable isotopes" refers to the 339 nuclides that naturally occur on
Earth, of which 252 (or around 74%), have not been seen to decay. Only 90
nuclides have been observed to decay, despite the fact that another 162 (for a
total of 252) are theoretically capable of doing so. Additionally, they are
formally categorized as "stable" Another 34 radioactive nuclides are
long-lived enough to have existed since the Solar System's formation, with
half-lives more than 100 million years. Primordial nuclides are the 286
nuclides in this group. Last but not least, there are 53 more short-lived
nuclides that are known to be produced naturally, either as daughter products
of primordial nuclide disintegration (such as radium from uranium) or as
byproducts of the Earth's natural energetic processes, such as cosmic ray
bombardment (for example, carbon-14).
There is at
least one stable isotope for each of the 80 chemical elements. For each of
these elements, there are often just a few stable isotopes, with an average of
3.2 stable isotopes per element. While ten stable isotopes have been found for
the element tin, which has the most stable isotopes of any element, 26 other
elements have only one. There are no stable isotopes for elements 43, 61, or 83
or above.
The ratio of protons to neutrons and the presence of specific "magic numbers" of protons or neutrons, which stand for closed and filled quantum shells, both influence the stability of isotopes. The filled shells, like the filled shell of 50 protons for tin, confer remarkable stability on the nuclide. These quantum shells correspond to a set of energy levels within the shell model of the nucleus of an atom. Only four of the 252 known stable nuclides—hydrogen-2 (deuterium), lithium-6, boron-10, and nitrogen-14—have odd numbers of both protons and neutrons. Additionally, only four naturally occurring radioactive odd-odd nuclides—potassium-40, vanadium-50, lanthanum-138, and tantalum-180m—have a half-life of over a billion years. Due to nuclear pairing effects, most odd-odd nuclei are extremely unstable to beta decay because the decay products are even-even and thus more tightly bonded.
Atomic Mass
The protons and
neutrons that make up an atom account for a significant portion of its mass.
The mass number refers to the total number of these particles (also known as
"nucleons") in an atom. Because it expresses a count, it is a
positive integer and dimensionless (rather than having a dimension of mass).
The mass number "carbon-12," which possesses 12 nucleons, is an
illustration of its utilization (six protons and six neutrons).
Daltons (Da),
commonly known as the unified atomic mass unit, are frequently used to express
the actual mass of an atom at rest (u). This quantity is equal to one-twelfth
of a carbon-12 free neutral atom's mass, or around 1.66×10−27
kg The atomic weight of hydrogen-1, the lightest isotope of hydrogen and
the nuclide with the lowest mass, is 1.007825 Da. The atomic mass is the value
of this number. A given atom has an atomic mass that is generally equal (within
1%) to its mass number times the atomic mass unit (for instance, the mass of a
nitrogen-14 is around 14 Da), although this value won't be an exact integer
unless it's carbon-12 (by definition). Lead-208 has the heaviest stable atom,
with a mass of 207.9766521 Da.
Chemists employ
the mole instead of working directly with atoms since even the most massive
ones are far too light. Any element's mole of atoms always has the same amount
of atoms (about 6.022×10²³). This value was set so that a mole of
atoms of an element with an atomic mass of 1 u would have a mass close to that
of 1 gramme. Because each carbon-12 atom has an exact atomic mass of 12 Da
according to the definition of the unified atomic mass unit, a mole of
carbon-12 atoms weighs precisely 0.012 kg.
Shape and size of Atom
Since an atom's outer boundary isn't clearly defined, its dimensions are typically expressed in terms of an atomic radius. This is a measurement of how far the electron cloud may be seen to stretch from the nucleus. This presupposes that the atom has a spherical shape, which only holds true for atoms in free space or a vacuum. The distances between two nuclei when two atoms are linked together in a chemical bond can be used to calculate atomic radii. The radius is dependent on an atom's position on the atomic structure diagram, the kind of chemical bond, the number of nearby atoms (coordination number), and a quantum mechanical characteristic called spin. Atom size tends to increase while travelling down columns but decrease when moving across rows on the periodic table of the elements (left to right). As a result, helium has the smallest atom, with a radius of 32 pm, whereas caesium has one of the largest, with a radius of 225 pm.
An atom's form
can depart from spherical symmetry when subjected to outside pressures, such
electrical fields. Group-theoretical analysis demonstrates that the deformation
is dependent on the field strength and the orbital type of the outer shell
electrons. For instance, in crystals, where strong crystal-electrical fields
may develop at low-symmetry lattice locations, aspherical deviations may be
induced. It has been demonstrated that Sulphur ions and chalcogen ions in
pyrite-type compounds experience significant ellipsoidal deformations.
Although
individual atoms can be seen using a scanning tunneling microscope, individual
atoms cannot be seen with an optical microscope because their dimensions are
hundreds of times smaller than the wavelengths of light (400-700 nm). Consider
that an average human hair is around 1 million carbon atoms wide to get an idea
of how tiny an atom is. About 2 sextillion (2×10²¹) hydrogen atoms
and 2 sextillion (2×10²¹) oxygen atoms make up each individual drop
of water. The average number of carbon atoms in a single carat diamond, which
has a mass of 2×10⁻⁴ kg, is 10 sextillion (10²²).
The atoms in an apple would be around the same size as the original apple if it
were enlarged to the size of the Earth.
Radioactive decay
Every element
has one or more isotopes with radioactively unstable nuclei that can decay,
releasing electromagnetic radiation or nuclear particles. When a nucleus'
radius exceeds that of the strong force, which only functions across distances
of the order of one fm, radioactivity can result.
The most common forms of radioactive decay are as below:
When a helium
nucleus with two protons and two neutrons emits an alpha particle, the process
of alpha decay is initiated. With a lower atomic number, a new element is
produced as a result of the emission.
The weak force
controls the processes of beta decay (and electron capture), which are caused
by the conversion of a neutron into a proton or a proton into a proton. Proton
to neutron transitions (with the exception of electron capture) result in the
emission of a positron and a neutrino, while neutron to proton transitions are
accompanied by the emission of an electron and an antineutrino. The discharges
of electrons or positrons are known as beta particles. The atomic number of the
nucleus is altered by one during beta decay. Because electron capture uses less
energy than positron emission, it is more frequently used. Instead of a
positron being released from the nucleus, an electron is instead taken up by
the nucleus in this form of decay. In the course of this process, a proton
turns into a neutron while still emitting neutrinos.
Gamma decay:
this process, which causes the emission of electromagnetic radiation, happens
when the energy level of the nucleus changes to a lower state. The excited
nucleus state that causes gamma emission often happens after the emission of an
alpha or beta particle. Gamma decay consequently frequently follows alpha or
beta decay.
Ejection of
neutrons, protons, nucleon clusters, or more than one beta particle from a
nucleus are some of the other, more uncommon types of radioactive decay.
Internal conversion is a process that generates high-speed electrons that are
not beta rays, followed by the generation of high-energy photons that are not
gamma rays, and is an analogue of gamma emission that permits excited nuclei to
lose energy in a different way. In a process known as spontaneous nuclear
fission, a few big nuclei explode into two or more charged pieces of various masses
and a number of neutrons.
The half-life,
or characteristic decay time period for any radioactive isotope, is defined by
how long it takes for half of a sample to decay. Every half-life, this
exponential decay mechanism gradually reduces the amount of the remaining
isotope by 50%. Thus, only 25% of the isotope is present after two half-lives,
and so on.
Magnetic moment of Atom
Spin is a
fundamental quantum mechanical feature of elementary particles. Although these
particles are thought to be point-like and cannot be considered to be rotating
strictly speaking, this is equivalent to the angular momentum of an object that
is spinning around its centre of mass. The reduced Planck constant (ħ) is used
to measure spin, and electrons, protons, and neutrons all have spin 1⁄2 ħ, or
"spin-1⁄2"," respectively. In an atom, the nucleus itself has
angular momentum because of its nuclear spin, whereas electrons moving around
the nucleus have orbital angular momentum in addition to their spin.
Just as a
revolving charged item produces a magnetic field conventionally, the magnetic
field created by an atom—its magnetic moment—is governed by these several types
of angular momentum, although electron spin makes the most significant
contribution. Bound electrons couple up with each other as a result of the
Pauli exclusion principle, which states that no two electrons may exist in the
same quantum state. In each pair, one electron is in a spin-up state while the
other is in a spin-down state. As a result, in some atoms with an even number
of electrons, these spins cancel one another out, bringing the total magnetic
dipole moment to zero.
An unpaired
electron and a net overall magnetic moment result from an odd number of
electrons in ferromagnetic materials like iron, cobalt, and nickel. When the
spins of unpaired electrons are aligned with one another, a spontaneous process
known as an exchange contact, it results in the orbitals of nearby atoms
overlapping and a lower energy state. A quantifiable macroscopic field can be
produced by the substance when the magnetic moments of ferromagnetic atoms are
lined up. When there is no magnetic field, the magnetic moments of the
individual atoms in paramagnetic materials line up in random directions, but
when there is a field, they line up.
When both
neutrons and protons are present in even amounts, the nucleus of an atom will
not have a spin; however, the nucleus may have a spin in other situations.
Because of thermal equilibrium, spin-aligned nuclei typically face random
orientations, but for some elements, such as xenon-129, it is feasible to
polarise a sizable fraction of the nuclear spin states, causing them to face
the same direction. This phenomenon is known as hyperpolarization. Magnetic
resonance imaging will benefit greatly from this.
Energy levels of Atom
When the
distance from the nucleus reaches infinity, the potential energy of an electron
in an atom is negative; the dependency on the electron's position reaches its
minimum inside the nucleus, roughly in inverse proportion to the distance. A
bound electron can only inhabit a limited range of states in the
quantum-mechanical model, each of which corresponds to a distinct energy level;
for a theoretical justification, see the time-independent Schrödinger equation.
The amount of energy required to release an electron from an atom can be used
to determine an energy level, which is often expressed in electronvolts (eV). The
ground state, or stationary state, of a bound electron is its lowest energy
level, while an electron transitioning to a higher level produces an excited
state. Due to a rise in the (average) distance from the nucleus, the energy of
the electron also increases along with n. The electrostatic potential of the nucleus
does not contribute to the energy's dependence on â„“, rather, electron
interaction does.
According to the
Niels Bohr model, which can be precisely calculated by the Schrödinger
equation, for an electron to transition between two states, such as from the
ground state to the first excited state, it must absorb or emit a photon at an
energy matching the difference in the potential energy of those levels.
Electrons move like particles when they hop between orbitals. For instance,
just one electron will change states in response to a single photon striking
the electrons; for further information, see Electron characteristics.
These particular
energy levels manifest as separate bands in the electromagnetic spectrum
because the energy of a photon is proportional to its frequency when it is
emitted. Each element has a distinctive spectrum, which can vary depending on
the nuclear charge, the electron subshells it occupies, how the electrons
interact electromagnetically, and other things.
Some photons are
absorbed by atoms when a continuous spectrum of energy is conveyed through a
gas or plasma, changing the energy state of the electrons. Excited electrons
that are still attached to their atom spontaneously release their energy as a
photon, which travels in an arbitrary direction. As a result, they lose energy
and fall to lower energy levels. In order to create a sequence of dark
absorption bands in the energy output, the atoms act as a filter. (An observer
looking at the atoms from a perspective that excludes the background continuous
spectrum sees a series of emission lines from the photons the atoms emit.) The
strength and width of atomic spectral lines can be measured using spectroscopy
in order to determine a substance's composition and physical characteristics.
Some of the
spectral lines have a fine structural splitting, as may be seen by closely
examining them. The interaction between the spin and velocity of the outermost
electron, known as spin-orbit coupling, is what causes this. The Zeeman effect
is a phenomenon whereby spectral lines break into three or more components when
an atom is exposed to an external magnetic field. This results from the
magnetic field's interaction with the atom's and its electrons' magnetic
moment. A single spectral line can be seen when an atom has numerous electron
configurations at the same energy level. Multiple spectral lines are produced
as a result of the atom's shifting of these electron configurations to slightly
different energy levels as a result of the magnetic field's interaction with
it. The Stark effect, which modifies the electron energy levels, can result in
a similar splitting and shifting of spectral lines when an external electric
field is present.
An interacting
photon with the right energy can promote the emission of a photon with the
right energy level if a bound electron is in an excited state. To make this
happen, the electron must transition to a state with a lower energy and an
energy difference that matches the energy of the photon that is interacting
with it. The photons that were released and those that had interacted move away
in parallel and with synchronized phases. In other words, the two photons' wave
patterns are coordinated. With the help of this physical characteristic, lasers
may produce coherent light beams with a specific frequency range.
Valence and bonding behavior of Atom
An element's
valency is its capacity for combination. The amount of bonds it can establish
with other atoms or groups is what determines its size. The valence shell and
the electrons that reside in it are referred to as an atom's outermost electron
shell and its valence electrons, respectively. The bonding behaviour with other
atoms is governed by the quantity of valence electrons. Chemical interactions
between atoms frequently result in the filling (or emptying) of their outer
valence shells. For bonds that form between atoms with one more electron than a
filled shell and others that are one electron short of a full shell, such as
occurs in the compound sodium chloride and other chemical ionic salts, a
transfer of one electron between atoms is a suitable approximation. Multiple
valences, or propensities to share different numbers of electrons in various
compounds, are characteristics of several elements. Thus, there are numerous
types of electron-sharing that go beyond straightforward electron transfers in
the chemical bonds formed between these components. Examples include organic
compounds and the atom carbon.
A periodic
table, which is designed to show recurrent chemical features, is frequently
used to illustrate the chemical elements. Elements with the same amount of
valence electrons form a group and are arranged in the same column of the
table. The elements at the extreme right of the chart have their outer shell
entirely filled with electrons, resulting in chemically inert substances known
as the noble gases. (The horizontal rows correspond to the filling of a quantum
shell with electrons.)
States of Atom
Amounts of atoms
are present in various states of matter, which depend on external physical
factors like pressure and temperature. Materials can change between solids,
liquids, gases, and plasmas by adjusting the circumstances. A material can also
exist in many allotropes withing a state. Solid carbon, which can be found as
graphite or diamond, is one illustration of this. Additionally, there are
gaseous allotropes like ozone and dioxygen.
Atoms can create
a Bose-Einstein condensate at temperatures very close to absolute zero, at
which point quantum mechanical effects that are typically only seen at the
atomic scale become visible on a macroscopic scale. The behaviour of this group
of supercooled atoms therefore resembles that of a single super atom, which
would allow for important verifications of quantum mechanical behaviour.
Identification of Atom
Atoms are so
small that they cannot be seen, but tools like the scanning tunnelling
microscope (STM) allow us to see them at the surfaces of materials. The quantum
tunnelling phenomenon, which is used in the microscope, enables particles to
pass through a barrier that would be impossible from a classical perspective.
Between two biassed electrodes, electrons can tunnel through the vacuum and
produce a tunnelling current that depends exponentially on the distance between
the electrodes. One electrode is ideally an atom-containing sharp tip. The
tip's height is changed at each point of the surface scan in order to maintain
a constant tunnelling current. The height profile is determined by how close to
the surface the tip moves and how far it moves away. For low bias, the
microscope captures the local density of the electronic states close to the
Fermi level by averaging electron orbitals across densely clustered energy
levels. Due to the distances involved, both electrodes must be exceedingly
stable; only then can periodicities that are specific to individual atoms be recognized. The technique is not chemically specialised and is unable to
distinguish between the atomic species that are present at the surface.
The mass of an
atom can be used to quickly identify it. A magnetic field will cause an atom's
route to bend if one of its electrons is removed, creating an ion. The mass of
the atom determines the radius by which the magnetic field rotates an ion's
trajectory. This idea is the basis for the mass spectrometer's measurement of
the mass-to-charge ratio of ions. By monitoring the strength of the several ion
streams, the mass spectrometer can calculate the percentage of each isotope
present in a sample if it contains multiple isotopes. Both inductively coupled
plasma mass spectrometry and atomic emission spectroscopy, which use a plasma
to evaporate materials for analysis, are methods for vaporizing atoms.
Using time-of-flight mass spectrometry, the atom-probe tomograph can chemically detect individual atoms with sub-nanometer resolution in three dimensions.
Non-destructively
identifying the atomic species present in a sample involves the use of electron
emission techniques like X-ray photoelectron spectroscopy (XPS) and Auger
electron spectroscopy (AES), which measure the binding energies of the core
electrons. Both of these can be made area-specific with the right emphasis.
Electron energy loss spectroscopy (EELS), which analyses the energy loss of an
electron beam inside a transmission electron microscope when it comes into
contact with a part of a material, is another such technique.
The atomic
makeup of far-off stars can be examined using excited state spectra. It is
possible to isolate and tie particular light wavelengths present in the observed
light from stars to the quantum transitions in free gas atoms. A gas-discharge
lamp made of the same substance can reproduce these hues. This method allowed
for the discovery of helium 23 years before it was identified on Earth.
Origin and current state of Atom
With an average
density of roughly 0.25 particles/m3, baryonic matter makes up about 4% of the
observable universe's total energy density (mostly protons and electrons).
Particles are far more concentrated within galaxies like the Milky Way, where
the interstellar medium (ISM) has a matter density of 105 to 109 atoms/m3. The
Local Bubble is thought to contain the Sun, making the density in the area
around the Sun merely 103 atoms/m3. The ISM undergoes constant enrichment with
elements more massive than hydrogen and helium as a result of the formation of
stars, which arise from thick clouds in the ISM.
Up to 95% of the
baryonic substance in the Milky Way is confined inside stars, which are hostile
environments for atomic matter. About 10% of the galaxy's mass is made up of
dark matter, the balance being entirely made up of total baryonic material.
Most "atoms" get totally ionised, or have all of their electrons
detached from their nuclei, as a result of the high temperatures found inside
stars. Electron shells are not conceivable in stellar remnants, with the
exception of their surface layers, due to extreme pressure.
Formation of Atom
It is believed that electrons have existed in the universe since before the Big Bang. Atomic nuclei forms in nucleosynthesis reactions. In about three minutes Big Bang nucleosynthesis produced most of the helium, lithium, and deuterium in the Universe, and perhaps some of the beryllium and boron.
Atoms' ubiquity
and stability depend on their binding energy, which is lower than the energy of
a nucleus and electrons in an unbound system. Plasma is a gas of positively
charged ions (perhaps even bare nuclei) and electrons that exists where the
temperature is substantially higher than the ionization potential. Atoms become
statistically advantageous when the temperature falls below the ionization potential. 380,000 years after the Big Bang, during a period known as
recombination, the expanding Universe cooled enough for electrons to connect to
nuclei, atoms (complete with bound electrons) overtook charged particles.
Since the Big Bang, which did not result in the production of carbon or heavier elements, atomic nuclei have been fused in stars to form additional helium and the series of elements from carbon to iron (via the triple alpha process).
Through cosmic
ray spallation, isotopes like lithium-6, as well as some beryllium and boron,
are produced in space. This happens when a high-energy proton collides with an
atomic nucleus, ejecting a significant number of nucleons.
The r-process,
which involves the capture of neutrons by atomic nuclei in supernovae and
colliding neutron stars, and the s-process, which occurs in AGB stars, both
entail the production of elements heavier than iron. Lead is an example of an
element that was primarily created through the radioactive decay of heavier
elements.
Earth
The majority of the atoms that make up the Earth and its inhabitants were present in the nebula that formed when the Solar System broke out from a molecular cloud. Radiometric dating can be used to calculate the age of the Earth by comparing the relative proportions of the remaining elements, which are the product of radioactive decay. Because of its lower abundance of helium-3, the majority of the helium in the Earth's crust (about 99% of the helium from gas wells) is a byproduct of alpha decay..
There are a few
trace atoms on Earth that are neither products of radioactive decay nor were
there in the beginning (i.e., not "primordial"). Cosmic rays cause
ongoing atmospheric production of carbon-14. On Earth, some atoms have been
produced artificially, either on purpose or as a byproduct of nuclear reactions
or explosions. The only naturally occurring transuranic elements—those with
atomic numbers more than 92—on Earth are plutonium and neptunium. With the
exception of residues of plutonium-244, which may have been left behind by
cosmic dust, all detectable amounts of transuranic elements have long since
decayed because their radioactive lives are less than the Earth's age today.
Neutron capture in uranium ore results in the production of natural reserves of
plutonium and neptunium.
There are
roughly 1.33 x 1050 atoms on Earth. 99% of the atmosphere is made up
of molecules, including carbon dioxide as well as diatomic oxygen and nitrogen,
despite the fact that a few isolated atoms of noble gases like argon, neon, and
helium do exist in minute quantities. The vast majority of atoms mix near the
Earth's surface to create a variety of compounds, including water, salt,
silicates, and oxides. Crystals, liquid, and solid metals are examples of
materials made up entirely of atoms rather than distinct molecules. This atomic
matter develops networked arrangements without the specific kind of molecular
matter's small-scale disrupted order.
Rare and theoretical forms
It is known that
all nuclides with atomic numbers greater than 82 (lead) are radioactive. On
Earth, there is no primordial nuclide with an atomic number greater than 92
(uranium), and heavier elements typically have shorter half-lives. Even yet,
moderately stable isotopes of super heavy elements with atomic numbers 110 to
114 may exist as a "island of stability." The half-life of the
island's most stable nuclide has been estimated to be anywhere from a few
minutes to millions of years. In any event, in the absence of any stabilizing effects, super heavy atoms (with Z > 104) would not exist due to growing
Coulomb repulsion (which causes spontaneous fission with progressively shorter
half-lives).
Exotic matter
There is an
antimatter particle with the opposing electrical charge for every matter
particle. As a result, the antiproton is a negatively charged counterpart to a
proton, while the positron is a positively charged antielectron. A matter
particle and its matching antimatter particle destroy one another when they
collide. Antimatter particles are therefore uncommon in the cosmos, along with
an imbalance between the numbers of matter and antimatter particles. Although
hypotheses of baryogenesis may provide an explanation, the underlying origins
of this imbalance are still not entirely understood. Therefore, no antimatter
atoms have been found in the natural world. At the CERN laboratory in Geneva,
antihydrogen—the hydrogen atom's antimatter counterpart—was created in 1996.
By substituting
another particle with the same charge for one of the protons, neutrons, or
electrons, other unusual atoms have been made. For instance, a more powerful
muon can take the place of an electron to create a muonic atom. These atoms can
be used to test the basic physics hypotheses.
Conclusion to
Atoms
Numerous
meticulous physicists, chemists, and mathematicians conducted a number of
experiments that led to the development of the quantum model of the atom. The
nucleus of the atom, which is a concentrated region of positive charge, is
surrounded by an electron-filled, largely empty void. The electrons are in
orbitals, which represent likely locations depending on energy, and the protons
and neutrons make up the dense nucleus. The electron clouds are another name
for the orbitals. All hues and emission spectra are the result of electrons
changing energy levels.
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