Atom
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| A semi-accurate depiction of the helium atom. The darkness of the electron cloud corresponds to the line-of-sight integral over the probability function of the 1s electron orbital. The magnified nucleus is schematic, showing protons in pink and neutrons in purple. In reality, the nucleus (and the wavefunction of each of the nucleons) is also spherically symmetric. (For more complex nuclei this is not the case.) |
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In
chemistry and
physics, an
atom (
Greek άτομος or
átomos meaning "indivisible") is the smallest possible particle of a
chemical element that retains its chemical properties. The word
atom originally meant the smallest possible indivisible particle, but after the term came to have a specific meaning in science, atoms were found to be divisible and composed of smaller
subatomic particles.
Most atoms are composed of three types of
subatomic particles which govern their external properties:
*
electrons, which have a negative
charge and are the least massive of the three;
*
protons, which have a positive charge and are about 1836 times more massive than electrons; and
*
neutrons, which have no charge and are about 1839 times more massive than electrons.
Protons and neutrons make up a dense, massive
atomic nucleus, and are collectively called
nucleons. The electrons form the much larger
electron cloud surrounding the nucleus.
Atoms differ in the number of each of the subatomic particles they contain. The number of protons in an atom (called the
atomic number) determines the
element of the atom. Within a single element, the number of neutrons may also vary, determining the
isotope of that element. The number of protons and neutrons in the atomic nucleus may also change, via
nuclear fusion,
nuclear fission or
radioactive decay. The number of electrons associated with an atom is most easily changed, due to the lower energy of binding of electrons.
Atoms are
electrically neutral if they have an equal number of protons and electrons. Atoms which have either a deficit or a surplus of electrons are called
ions. Electrons that are furthest from the nucleus may be transferred to other nearby atoms or shared between atoms. By this mechanism atoms are able to
bond into
molecules and other types of
chemical compounds like ionic and covalent network crystals. For gases and certain molecular liquids and solids (such as
water and
sugar), molecules are the smallest division of matter which retains chemical properties; however, there are also many solids and liquids which are made of atoms, but do not contain discrete molecules (such as
salts,
rocks, and liquid and solid
metals). Most molecules are made up of multiple atoms; for example, a molecule of water is a combination of two
hydrogen atoms and one
oxygen atom. A few types of molecules (for example gas molecules of elements that do not form compounds, such as
helium), are composed of only a single atom.
Atoms are the fundamental building blocks of
chemistry, and are
conserved in
chemical reactions.
Philosophical atomic theories date as far back to the ancient Greeks and Indians in the fifth and sixth centuries BC. It was the Greeks who coined the term
atomos, which meant "the smallest possible division of matter".
The first scientific theory of atoms was developed in the early years of the 19
th century by
John Dalton, who found it elegantly explained why substances always broke down into proportional constituents. For Dalton, each chemical element was represented by a type of atom, and vice versa. Near the turn of the 20
th century,
J.J. Thomson, through his work on
cathode rays discovered that atoms were in fact divisible, being partially composed of very light negatively-charged particles (which proved identical, no matter the element they originated from), and which were later named
electrons. In 1911,
Ernest Rutherford discovered that that the electrons orbited a compact
nucleus. Rutherford found the lightest nucleus belonged to hydrogen, a particle he named the
proton. In order to explain why the orbiting electrons never spiralled into the nucleus,
Niels Bohr incorporated
quantum mechanics into
his own model of the atom, in which the electrons could only orbit the nucleus in fixed circles. With the discovery of the
Werner Heisenberg's
uncertainty principle, the concept of circular orbits was replaced with that of a "cloud", wherein the distribution of electrons was described through probabilistic equations. Finally, with the discovery of the
neutron in 1932, the atomic nucleus for atoms heavier than common hydrogen was shown to be composed of
protons and
neutrons, and the modern conception of basic atomic structure was complete.
Subatomic particles
Although the name "atom" was applied at a time when atoms were thought to be indivisible, it is now known that the atom can be broken down into a number of smaller components. The first of these to be discovered was the negatively charged
electron, which is easily ejected from atoms during
ionization. The electrons orbit a small, dense body containing all of the positive charge in the atom, called the
atomic nucleus. This nucleus is itself made up of
nucleons: positively charged
protons and chargeless
neutrons.
|
This model of a helium atom shows the electrons(yellow), the protons(grey), and the neutrons(pink). Also shown are the up quarks(red), and the down quarks(blue) that make up the nucleons as well as the gluons(black) which hold the quarks together. |
Before 1961, the subatomic particles were thought to consist of only protons, neutrons and electrons. However, protons and neutrons themselves are now known to consist of still smaller particles called
quarks. In addition, the electron is known to have a nearly massless neutral partner called a
neutrino. Together, the electron and neutrino are both
leptons.Ordinary atoms are composed only of quarks and leptons of the first
generation. The proton is composed of two
up quarks and one
down quark, whereas the neutron is composed of one up quark and two down quarks. Although they do not occur in ordinary matter, two other heavier generations of quarks and leptons may be generated in
high-energy collisions.
The subatomic
force carrying particles (called
gauge bosons) are also important to atoms. Electrons are bound to the nucleus by
photons carrying the
electromagnetic force. Protons and neutrons are bound together in the nucleus by
gluons carrying the
strong nuclear force.
Electron configuration
The
chemical behavior of atoms is due to interactions between electrons. Electrons of an atom remain within certain, predictable
electron configurations. These configurations are determined by the
quantum mechanics of electrons in the
electric potential of the atom; the
principal quantum number determines particular
electron shells with distinct
energy levels. Generally, the higher the energy level of a shell, the further away it is from the nucleus. The electrons in the outermost shell, called the
valence electrons, have the greatest influence on chemical behavior. Core electrons (those not in the outer shell) play a role, but it is usually in terms of a secondary effect due to screening of the positive charge in the atomic nucleus.
An electron shell can hold up to 2
n2 electrons, where
n is the principal quantum number of the shell. The occupied shell of greatest
n is the valence shell, even if it only has one electron. In the most stable
ground state, an atom's electrons will fill up its shells in order of increasing energy. Under some circumstances an electron may be
excited to a higher energy level (that is, it absorbs energy from an external source and leaps to a higher shell), leaving a space in a lower shell. An excited atom's electrons will
spontaneously fall into lower levels, emitting excess energy as a
photons, until it returns to the ground state.
In addition to its principal quantum number
n, an electron is distinguished by three other quantum numbers: the
azimuthal quantum number l (describing the
orbital angular momentum of the electron), the
magnetic quantum number m (describing the direction of the angular momentum vector), and the
spin quantum number s (describing the direction of the electron's
intrinsic angular momentum). Electrons with varying
l and
m have distinctive shapes denoted by
spectroscopic notation. In the illustration, the letters
s,
p,
d and
f (corresponding to
l = 0, 1, 2, 3) describe the shape of the
atomic orbital. In most atoms, orbitals of differing
l are not exactly
degenerate but separated into a
fine structure. Orbitals of differing
m are degenerate but may be separated by applying a
magnetic field, creating the
Zeeman effect. Electrons with differing
s have very slight energy differences called
hyperfine splitting.
Nucleon properties
The constituent
protons and
neutrons of the
atomic nucleus are collectively called
nucleons. The nucleons are held together in the nucleus by the
strong nuclear force which is caried by
gluons.
Nuclei can undergo transformations that affect the number of protons and neutrons they contain, a process called
radioactive decay. When nuclei transformations take place spontaneously, this process is called
radioactivity. Radioactive transformations proceed by a wide variety of modes, but the most common are
alpha decay (emission of a
helium nucleus) and
beta decay (emission of an electron). Decays involving electrons or
positrons are due to the
weak nuclear interaction.
In addition, like the electrons of the atom, the nucleons of nuclei may be pushed into
excited states of higher energy. However, these transitions typically require thousands of times more energy than electron excitations. When an excited nucleus emits a photon to return to the
ground state, the photon has very high energy and is called a
gamma ray.
Nuclear transformations also take place in
nuclear reactions. In
nuclear fusion, two light nuclei come together and merge into a single heavier nucleus. In
nuclear fission, a single large nucleus is divided into two or more smaller nuclei.
Atom size and speed
Atoms are much smaller than the
wavelengths of
light that human vision can detect, so atoms cannot be seen in any kind of optical
microscope. However, there are ways of detecting the positions of atoms on the surface of a solid or a
thin film so as to obtain images. These include:
electron microscopes (such as in
scanning tunneling microscopy (STM)),
atomic force microscopy (AFM),
nuclear magnetic resonance (NMR) and
x-ray microscopy.
Since the
electron cloud does not have a sharp cutoff, the size of an atom is not easily defined. For atoms that can form solid
crystal lattices, the distance between the centers of adjacent atoms can be easily determined by
x-ray diffraction, giving an estimate of the atoms' size. For any atom, one might use the radius at which the electrons of the
valence shell are most likely to be found. As an example, the size of a
hydrogen atom is estimated to be approximately 1.06×10 m (twice the
Bohr radius). Compare this to the size of the
proton (the only particle in the nucleus of the hydrogen atom), which is approximately 10 m. So the ratio of the size of the hydrogen atom to its nucleus is about 100,000:1. If an atom were the size of a
stadium, the nucleus would be the size of a
marble. Nearly all the mass of an atom is in its nucleus, yet almost all the space in an atom is occupied by its electrons.
Atoms of different
elements do vary in size, but the sizes do not scale linearly with the mass of the atom. Their sizes (diameters) are roughly the same to within a factor of about 3. The reason for this is that heavy elements have large positive charge on their nuclei, which strongly attract the electrons to the center of the atom. This contracts the size of the
electron shells, so that more electrons fit into only a slightly greater volume.
The
temperature of a collection of atoms is a measure of the average energy of motion of those atoms above the minimum zero-point energy demanded by quantum mechanics; at 0
kelvins (
absolute zero) atoms would have no extra energy above the minimum. As the temperature of the system is increased, the kinetic energy of the particles in the system is increased, and their speed of motion increases. At
room temperature, atoms making up gases in the air move at a speed of 500 m/s (about 1100 mph or 1800 km/h).
Elements, isotopes and ions
Atoms are generally classified by their
atomic number Z, which corresponds to the number of protons in the atom. The atomic number determines which
chemical element the atom is. For example,
carbon atoms are atoms containing six protons (
Z = 6). All atoms with the same atomic number Z share a wide variety of physical properties and exhibit almost identical
chemical properties (for the closest instance to an exception to this principle, see
deuterium and
heavy water). The elements may be sorted according to the
periodic table in order of increasing atomic number.
The
mass number A, or nucleon number of an element is the total number of protons and neutrons in an atom of that element, so-called because each proton and neutron has a mass of about 1
amu. The number of neutrons
A-
Z in an atom has no effect on which element it is. Each element can have numerous kinds of atoms with the same number of protons and electrons but varying numbers of neutrons. Each has the same atomic number but a different mass number. These are called the
isotopes of an element. When writing the name of an isotope, the element name is followed by the mass number. For example,
carbon-14 contains 6 protons and 8 neutrons in each atom, for a total mass number of 14. For a complete table of known isotopes, including radioactive and stable isotopes, see
isotope table (divided).
The atomic mass listed for each element in the periodic table is an average of the isotope masses found in nature,
weighted by their
abundance.
The simplest atom is the
hydrogen isotope
protium, which has atomic number 1 and atomic mass number 1; it consists of one proton and one electron. The hydrogen isotope which also contains one neutron so is called
deuterium or hydrogen-2; the hydrogen isotope with two neutrons is called
tritium or hydrogen-3. Tritium is an unstable isotope which decays through a process called
radioactivity. Many isotopes of each element are radioactive; the number which are
stable varies greatly with the element (
tin has 10 stable isotopes; see
list of stable isotopes). Lead (
Z = 82) is the last element which has stable isotopes. The elements with atomic number 83 (
bismuth) and greater have no stable isotopes and are all radioactive.
Virtually all elements heavier than hydrogen and
helium were created through
stellar nucleosynthesis and
supernova nucleosynthesis. The solar system is thought to be formed of clouds of elements from many such supernovae, which date from more than 4.6 billion years ago. Most of the elements lighter than
uranium (
Z = 92) have either stable isotopes, or else radioisotopes long-lived enough to occur naturally on
Earth. Two notable exceptions of light but short-lived radioactive elements are
technetium Z = 43 (although some technetium has been found on Earth, this occurred only after the element was first synthesized artificially), and
promethium Z = 61, which is found naturally only in stars where it was recently made. Several other short-lived heavy elements that do not occur on Earth have been found to be present in
stars. Elements not normally found in nature have been artificially created by
nuclear bombardment;
as of 2006, elements have been created through atomic number 116 (given the temporary name
ununhexium). These ultra-heavy elements are generally highly unstable and decay quickly.
Atoms that have lost or gained electrons to become electrically non-neutral, are called atomic
ions. Ions are divided into
cations with positive (+) electric charge; or
anions with negative (-) charge.
Valence and bonding
The number of electrons in an atom's outermost shell (the
valence shell) governs its bonding behavior. Therefore, elements with the same number of valence electrons are
grouped together in the columns of the
periodic table of the elements.
Alkali metals contain one electron on their outer shell;
alkaline earth metals, two electrons;
halogens, seven electrons; and various others.
Every atom is most stable with a full valence shell. This means that atoms with full valence shells (the
noble gases) are very unreactive. Conversely, atoms with few electrons in their valence shell are more
reactive. Alkali metals are therefore very reactive, with
caesium,
rubidium, and
francium being the most reactive of all metals. Also, atoms that need only few electrons (such as the halogens) to fill their valence shells are reactive.
Fluorine is the most reactive of all elements.
Atoms may fill their valence shells by
chemical bonding. This can be achieved one of two ways: an atom can either share electrons with other atoms (a
covalent bond), or it can remove electrons from (or donate electrons to) other atoms (an
ionic bond). The formation of a bond causes a strong attraction between two atoms, creating
molecules or
ionic compounds. Many other types of bonds exist, including:
*
polar covalent bonds;
*
coordinate covalent bonds;
*
metallic bonds;
*
hydrogen bonds; and
*
van der Waals bonds.
Atomic spectrum
Since each element in the
periodic table consists of an atom in a unique configuration with different numbers of
protons and
electrons, each element can also be uniquely described by the
energies of its
atomic orbitals and the number of electrons within them. Normally, an atom is found in its lowest-energy
ground state; states with higher energy are called
excited states. An electron may move from a lower-energy orbital to a higher-energy orbital by absorbing a
photon with energy equal to the difference between the energies of the two levels. An electron in a higher-energy orbital may drop to a lower-energy orbital by emitting a photon. Since each element has a unique set of energy levels, each creates its own
light pattern unique to itself: its own spectral signature.
If a set of atoms is heated (such as in an
arc lamp), their electrons will move into excited states. When these atoms fall back toward the ground state, they will produce an
emission spectrum. If a set of atoms is illuminated by a
continuous spectrum, it will only absorb specific
wavelengths (energies) of photon that correspond to the differences in its energy levels. The resulting pattern of gaps is called the
absorption spectrum.
In spectroscopic analysis, scientists can use a
spectrometer to study the atoms in
stars and other distant objects. Due to the distinctive spectral lines that each element produces, they are able to tell the chemical composition of distant
planets, stars and
nebulae.
Not all parts of the atomic spectrum are in visible light part of the
electromagnetic spectrum. For example, the
hyperfine transitions (including the important
21 cm line) produce low-energy
radio waves. When electrons deep inside atoms of high atomic number are knocked out (for example by
beta radiation), replacement electrons fall deep into the
electric potential of the high-
Z nucleus, producing high-energy
x-rays.
An
exotic atom is usually made from a normal matter atom with a substitution from abnormal or rarely encountered matter, such as
antimatter,
muons,
mesons, or other objects. A few exotic atoms (such as atoms of antimatter) are not made of any normal atomic constituents at all. All exotic atoms (save antiatoms made from antinucleons and
positrons), are highly unstable, decaying with lifetimes of a few microseconds or less. The antimatter counterparts of stable particles are also stable, but difficult to store for more than short periods, since they
annihilate if allowed to contact ordinary matter.
The most familiar examples of exotic atoms are the antiatom
antihydrogen (composed of an
antiproton and positron) which has been produced in tiny quantities, and
positronium, an analogue to the hydrogen atom in which a positron is substituted for the usual proton nucleus.
Positronium is unstable; it is a common phase in the attraction between an electron and positron before the annihilation reaction in which the matter particles are destroyed and two
gamma rays are emitted.
In models of the
Big Bang,
Big Bang nucleosynthesis predicts that within one to three minutes of the Big Bang almost all atomic material in the universe was created. During this process,
nuclei of
hydrogen and
helium formed abundantly, but almost no elements heavier than
lithium. Hydrogen makes up approximately 75% of the atoms in the universe; helium makes up 24%; and all other elements make up just 1%. However, although nuclei (fully-
ionized atoms) were created, neutral atoms themselves could not form in the intense heat.
Big Bang chronology of the atom continues to approximately 379,000 years after the Big Bang when the cosmic temperature had dropped to just 3,000
K. It was then cool enough to allow the nuclei to capture
electrons. This process is called
recombination, during which the first neutral atoms took form. Once atoms become neutral, they only absorb
photons of a discrete
absorption spectrum. This allows most of the photons in the universe to travel unimpeded for billions of years. These photons are still detectable today in the
cosmic microwave background.
After Big Bang nucleosynthesis, no heavier elements could be created until the
formation of the first stars. These stars
fused heavier elements through
stellar nucleosynthesis during their lives and through
supernova nucleosynthesis as they died. The seeding of the
interstellar medium by heavy elements eventually allowed the formation of
terrestrial planets like the
Earth.
The atom is quite small. Here are some notes on how petite they are:
*In the width of a hair, there is approximately 1x10 atoms.
*In a speck of dust, there's about 3x10 atoms.
*
Atomism*
Basic quantum mechanics*
Chemical bond*
Exotic atom*
Infinite divisibility*
Ionization*
List of particles*
Periodic table*
Radioactive isotope*
Superatom*
Super-heavy atom*
Transuranium element* Kenneth S. Krane,
Introductory Nuclear Physics (1987)
*
How Atoms Work*
Wikibooks FHSST Physics Atom:The Atom*
Wikibooks Atomic structure*
Science aid - atomic structure A guide to the atom for teens.
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