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Atoms:Subcomponents and quantum theory
The physicistJ. J. Thomson, through his work oncathode raysin
1897, discovered the electron, and
concluded that they were a component of every atom. Thus he
overturned the belief that atoms are the
indivisible, ultimate particles of matter.[25]
Thomson postulated that the low mass, negatively charged
electrons were distributed throughout the atom, possibly
rotating in rings, with their charge balanced by
the presence of a uniform sea of positive charge. This later
became known as the plum pudding model.
In 1909,Hans GeigerandErnest Marsden, under the direction of
physicistErnest Rutherford, bombarded
a sheet of gold foil withalpha raysby then known to be
positively charged helium atomsand
discovered that a small percentage of these particles were
deflected through much larger angles than
was predicted using Thomson's proposal. Rutherford interpreted
the gold foil experimentas suggesting
that the positive charge of a heavy gold atom and most of its
mass was concentrated in a nucleus at the
center of the atomtheRutherford model.[26]
While experimenting with the products ofradioactive decay, in
1913radiochemistFrederick
Soddydiscovered that there appeared to be more than one type of
atom at each position on the periodic
table.[27]
The termisotopewas coined byMargaret Toddas a suitable name for
different atoms that
belong to the same element. J.J. Thomson created a technique for
separating atom types through his
work on ionized gases, which subsequently led to the discovery
ofstable isotopes.[28]
ABohr modelof the hydrogen atom, showing an electron jumping
between fixed orbits and emitting aphotonof energy with
a specific frequency.
Meanwhile, in 1913, physicistNiels Bohrsuggested that the
electrons were confined into clearly defined,
quantized orbits, and could jump between these, but could not
freely spiral inward or outward in
intermediate states.[29]
An electron must absorb or emit specific amounts of energy to
transition between
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these fixed orbits. When thelightfrom a heated material was
passed through aprism, it produced a multi-
coloredspectrum. The appearance of fixedlines in this
spectrumwas successfully explained by these
orbital transitions.[30]
Chemical bondsbetween atoms were now explained, byGilbert Newton
Lewisin 1916, as the
interactions between their constituent electrons.[31]
As thechemical propertiesof the elements were
known to largely repeat themselves according to theperiodic
law,[32]
in 1919 the American chemistIrving
Langmuirsuggested that this could be explained if the electrons
in an atom were connected or clustered
in some manner. Groups of electrons were thought to occupy a set
ofelectron shellsabout the
nucleus.[33]
TheSternGerlach experimentof 1922 provided further evidence of
the quantum nature of the atom.
When a beam of silver atoms was passed through a specially
shaped magnetic field, the beam was split
based on the direction of an atom's angular momentum, or spin.
As this direction is random, the beam
could be expected to spread into a line. Instead, the beam was
split into two parts, depending on whether
the atomic spin was oriented up or down.[34]
In 1924,Louis de Broglieproposed that all particles behave to an
extent like waves. In 1926,Erwin
Schrdingerused this idea to develop a mathematical model of the
atom that described the electrons as
three-dimensionalwaveformsrather than point particles. A
consequence of using waveforms to describe
particles is that it is mathematically impossible to obtain
precise values for both
thepositionandmomentumof a particle at the same time; this
became known as the uncertainty
principle, formulated byWerner Heisenbergin 1926. In this
concept, for a given accuracy in measuring a
position one could only obtain a range of probable values for
momentum, and vice versa. This model was
able to explain observations of atomic behavior that previous
models could not, such as certain structural
andspectralpatterns of atoms larger than hydrogen. Thus, the
planetary model of the atom was
discarded in favor of one that describedatomic orbitalzones
around the nucleus where a given electron
is most likely to be observed.[35][36]
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a.org/wiki/Irving_Langmuirhttp://en.wikipedia.org/wiki/Irving_Langmuirhttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-FOOTNOTEScerri2007205.E2.80.93226-32http://en.wikipedia.org/wiki/Periodic_lawhttp://en.wikipedia.org/wiki/Chemical_propertyhttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-jacs38_4_762-31http://en.wikipedia.org/wiki/Gilbert_Newton_Lewishttp://en.wikipedia.org/wiki/Chemical_bondhttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-bohr19221211-30http://en.wikipedia.org/wiki/Spectral_linehttp://en.wikipedia.org/wiki/Spectrumhttp://en.wikipedia.org/wiki/Prism_(optics)http://en.wikipedia.org/wiki/Light
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Schematic diagram of a simple mass spectrometer.
The development of themass spectrometerallowed the exact mass of
atoms to be measured. The device
uses a magnet to bend the trajectory of a beam of ions, and the
amount of deflection is determined by the
ratio of an atom's mass to its charge. The chemistFrancis
William Astonused this instrument to show that
isotopes had different masses. Theatomic massof these isotopes
varied by integer amounts, called
thewhole number rule.[37]
The explanation for these different isotopes awaited the
discovery of
theneutron, a neutral-charged particle with a mass similar to
theproton, by the physicistJames
Chadwickin 1932. Isotopes were then explained as elements with
the same number of protons, but
different numbers of neutrons within the nucleus.[38]
Fission, high energy physics and condensed matter
In 1938, the German chemistOtto Hahn, a student of Rutherford,
directed neutrons onto uranium atoms
expecting to gettransuranium elements. Instead, his chemical
experiments showedbariumas a
product.[39]
A year later,Lise Meitnerand her nephewOtto Frischverified that
Hahn's result were the first
experimental nuclear fission.[40][41]
In 1944, Hahn received theNobel prizein chemistry. Despite
Hahn's
efforts, the contributions of Meitner and Frisch were not
recognized.[42]
In the 1950s, the development of improvedparticle
acceleratorsandparticle detectorsallowed scientists
to study the impacts of atoms moving at high energies.[43]
Neutrons and protons were found to
behadrons, or composites of smaller particles calledquarks.
Standard models of nuclear physics were
developed that successfully explained the properties of the
nucleus in terms of these sub-atomic particles
and the forces that govern their interactions.[44]
Components
Subatomic particles
Main article:Subatomic particle
Though the word atom originally denoted a particle that cannot
be cut into smaller particles, in modern
scientific usage the atom is composed of varioussubatomic
particles. The constituent particles of an atom
are theelectron, theprotonand theneutron. However,
thehydrogen-1atom has no neutrons and a
positivehydrogen ionhas no electrons.
The electron is by far the least massive of these particles at
9.111031
kg, with a negativeelectrical chargeand a size that is too small
to be measured using available
techniques.[45]
Protons have a positive charge and a mass 1,836 times that of
the electron,
at 1.67261027
kg, although this can be reduced by changes to theenergy
bindingthe proton into an atom. Neutrons
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dia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Neutronhttp://en.wikipedia.org/wiki/Protonhttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Subatomic_particlehttp://en.wikipedia.org/wiki/Subatomic_particlehttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-44http://en.wikipedia.org/wiki/Quarkhttp://en.wikipedia.org/wiki/Hadronhttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-43http://en.wikipedia.org/wiki/Particle_detectorhttp://en.wikipedia.org/wiki/Particle_acceleratorhttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-42http://en.wikipedia.org/wiki/Nobel_prizehttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-40http://en.wikipedia.org/wiki/Atomic_structure#cite_note-40http://en.wikipedia.org/wiki/Otto_Frischhttp://en.wikipedia.org/wiki/Lise_Meitnerhttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-39http://en.wikipedia.org/wiki/Bariumhttp://en.wikipedia.org/wiki/Transuranium_elementhttp://en.wikipedia.org/wiki/Otto_Hahnhttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-chadwick1935-38http://en.wikipedia.org/wiki/James_Chadwickhttp://en.wikipedia.org/wiki/James_Chadwickhttp://en.wikipedia.org/wiki/Protonhttp://en.wikipedia.org/wiki/Neutronhttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-pm39_6_449-37http://en.wikipedia.org/wiki/Whole_number_rulehttp://en.wikipedia.org/wiki/Atomic_masshttp://en.wikipedia.org/wiki/Francis_William_Astonhttp://en.wikipedia.org/wiki/Mass_spectrometry
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have no electrical charge and have a free mass of 1,839 times
the mass of electrons ,[46]
or 1.69291027
kg. Neutrons and protons have comparable dimensionson the order
of 2.51015
malthough the 'surface' of these particles is not sharply
defined.[47]
In theStandard Modelof physics, both protons and neutrons are
composed ofelementary
particlescalledquarks. The quark belongs to thefermiongroup of
particles, and is one of the two basic
constituents of matterthe other being thelepton, of which the
electron is an example. There are six
types of quarks, each having a fractional electric charge of
either +23or
13. Protons are composed of
twoup quarksand onedown quark, while a neutron consists of one
up quark and two down quarks. This
distinction accounts for the difference in mass and charge
between the two particles. The quarks are held
together by thestrong nuclear force, which is mediated bygluons.
The gluon is a member of the family
ofgaugebosons, which are elementary particles that mediate
physicalforces.[48][49]
Nucleus
Main article:Atomic nucleus
Thebinding energyneeded for a nucleon to escape the nucleus, for
various isotopes.
All the bound protons and neutrons in an atom make up a
tinyatomic nucleus, and are collectively
callednucleons. The radius of a nucleus is approximately equal
to , whereA is the total
number of nucleons.[50]
This is much smaller than the radius of the atom, which is on
the order of 105
fm.
The nucleons are bound together by a short-ranged attractive
potential called the residual strong force. At
distances smaller than 2.5 fm this force is much more powerful
than theelectrostatic forcethat causes
positively charged protons to repel each other.[51]
Atoms of the sameelementhave the same number of protons, called
theatomic number. Within a single
element, the number of neutrons may vary, determining
theisotopeof that element. The total number of
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/Gluonhttp://en.wikipedia.org/wiki/Strong_nuclear_forcehttp://en.wikipedia.org/wiki/Down_quarkhttp://en.wikipedia.org/wiki/Up_quarkhttp://en.wikipedia.org/wiki/Leptonhttp://en.wikipedia.org/wiki/Fermionhttp://en.wikipedia.org/wiki/Quarkhttp://en.wikipedia.org/wiki/Elementary_particlehttp://en.wikipedia.org/wiki/Elementary_particlehttp://en.wikipedia.org/wiki/Standard_Modelhttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-FOOTNOTEMacGregor199233.E2.80.9337-47http://en.wikipedia.org/wiki/Atomic_structure#cite_note-FOOTNOTEWoan20008-46
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protons and neutrons determine thenuclide. The number of
neutrons relative to the protons determines
the stability of the nucleus, with certain isotopes
undergoingradioactive decay.[52]
The neutron and the proton are different types offermions.
ThePauli exclusion principleis aquantum
mechanicaleffect that prohibits identicalfermions, such as
multiple protons, from occupying the same
quantum physical state at the same time. Thus every proton in
the nucleus must occupy a different state,
with its own energy level, and the same rule applies to all of
the neutrons. This prohibition does not apply
to a proton and neutron occupying the same quantum
state.[53]
For atoms with low atomic numbers, a nucleus that has a
different number of protons than neutrons can
potentially drop to a lower energy state through a radioactive
decay that causes the number of protons
and neutrons to more closely match. As a result, atoms with
roughly matching numbers of protons and
neutrons are more stable against decay. However, with increasing
atomic number, the mutual repulsion
of the protons requires an increasing proportion of neutrons to
maintain the stability of the nucleus, which
modifies this trend. Thus, there are no stable nuclei with equal
proton and neutron numbers above atomic
number Z = 20 (calcium); and as Z increases toward the heaviest
nuclei, the ratio of neutrons per proton
required for stability increases to about 1.5.[53]
Illustration of a nuclear fusion process that forms a deuterium
nucleus, consisting of a proton and a neutron, from two
protons. Apositron(e+)anantimatterelectronis emitted along with
an electronneutrino.
The number of protons and neutrons in the atomic nucleus can be
modified, although this can require
very high energies because of the strong force.Nuclear
fusionoccurs when multiple atomic particles join
to form a heavier nucleus, such as through the energetic
collision of two nuclei. For example, at the core
of the Sun protons require energies of 310 keV to overcome their
mutual repulsionthecoulomb
barrierand fuse together into a single nucleus.[54]
Nuclear fissionis the opposite process, causing a
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nucleus to split into two smaller nucleiusually through
radioactive decay. The nucleus can also be
modified through bombardment by high energy subatomic particles
or photons. If this modifies the
number of protons in a nucleus, the atom changes to a different
chemical element.[55][56]
If the mass of the nucleus following 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 (such
as agamma ray, or the kinetic energy of abeta particle), as
described byAlbert Einstein'smassenergy
equivalenceformula, E= mc2, where m is the mass loss and cis
thespeed of light. This deficit is part of
thebinding energyof the new nucleus, and it is the
non-recoverable loss of the energy that causes the
fused particles to remain together in a state that requires this
energy to separate.[57]
The fusion of two nuclei that create larger nuclei with lower
atomic numbers thanironandnickela total
nucleon number of about 60is usually anexothermic processthat
releases more energy than is
required to bring them together.[58]
It is this energy-releasing process that makes nuclear fusion
instarsa
self-sustaining reaction. For heavier nuclei, the binding energy
pernucleonin the nucleus begins to
decrease. That means fusion processes producing nuclei that have
atomic numbers higher than about
26, andatomic masseshigher than about 60, is anendothermic
process. These more massive nuclei can
not undergo an energy-producing fusion reaction that can sustain
thehydrostatic equilibriumof a star.[53]
Electron cloud
Main articles:Atomic orbitalandElectron configuration
A potential well, showing, according toclassical mechanics, the
minimum energyV(x) needed to reach each positionx.
Classically, a particle with energy Eis constrained to a range
of positions betweenx1 andx2.
The electrons in an atom are attracted to the protons in the
nucleus by theelectromagnetic force. This
force binds the electrons inside anelectrostaticpotential
wellsurrounding the smaller nucleus, which
means that an external source of energy is needed for the
electron to escape. The closer an electron is to
the nucleus, the greater the attractive force. Hence electrons
bound near the center of the potential well
require more energy to escape than those at greater
separations.
Electrons, like other particles, have properties of both
aparticle and a wave. The electron cloud is a
region inside the potential well where each electron forms a
type of three-dimensional standing wavea
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gy_equivalencehttp://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalencehttp://en.wikipedia.org/wiki/Albert_Einsteinhttp://en.wikipedia.org/wiki/Beta_particlehttp://en.wikipedia.org/wiki/Gamma_rayhttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-55http://en.wikipedia.org/wiki/Atomic_structure#cite_note-55
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wave form that does not move relative to the nucleus. This
behavior is defined by an atomic orbital, a
mathematical function that characterises the probability that an
electron appears to be at a particular
location when its position is measured.[59]
Only a discrete (orquantized) set of these orbitals exist
around
the nucleus, as other possible wave patterns rapidly decay into
a more stable form .[60]
Orbitals can have
one or more ring or node structures, and they differ from each
other in size, shape and orientation.[61]
Wave functions of the first five atomic orbitals. The three 2p
orbitals each display a single angularnodethat has an
orientation and a minimum at the center.
Each atomic orbital corresponds to a particularenergy levelof
the electron. The electron can change its
state to a higher energy level by absorbing aphotonwith
sufficient energy to boost it into the new
quantum state. Likewise, throughspontaneous emission, an
electron in a higher energy state can drop to
a lower energy state while radiating the excess energy as a
photon. These characteristic energy values,
defined by the differences in the energies of the quantum
states, are responsible foratomic spectral
lines.[60]
The amount of energy needed to remove or add an
electrontheelectron binding energyis far less
than thebinding energy of nucleons. For example, it requires
only 13.6 eV to strip aground-stateelectron
from a hydrogen atom,[62]
compared to 2.23 million eV for splitting
adeuteriumnucleus.[63]
Atoms
areelectricallyneutral if they have an equal number of protons
and electrons. Atoms that have either a
deficit or a surplus of electrons are calledions. Electrons that
are farthest from the nucleus may be
transferred to other nearby atoms or shared between atoms. By
this mechanism, atoms are able
tobondintomoleculesand other types ofchemical
compoundslikeionicandcovalentnetworkcrystals.[64]
Properties
Nuclear properties
Main articles:Isotope,Stable isotope,List of nuclides, andList
of elements by stability of isotopes
By definition, any two atoms with an identical number ofprotons
in their nuclei belong to the
samechemical element. Atoms with equal numbers of protons but a
different number ofneutrons are
different isotopes of the same element. For example, all
hydrogen atoms admit exactly one proton, but
isotopes exist with no neutronshydrogen-1, one neutron
(deuterium), two neutrons (tritium) andmore
than two neutrons. The hydrogen-1 is by far the most common
form, and is sometimes called
http://en.wikipedia.org/wiki/Atomic_orbitalhttp://en.wikipedia.org/wiki/Atomic_orbitalhttp://en.wikipedia.org/wiki/Atomic_orbitalhttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-59http://en.wikipedia.org/wiki/Atomic_structure#cite_note-59http://en.wikipedia.org/wiki/Atomic_structure#cite_note-59http://en.wiktionary.org/wiki/quantizehttp://en.wiktionary.org/wiki/quantizehttp://en.wiktionary.org/wiki/quantizehttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-Brucat-60http://en.wikipedia.org/wiki/Atomic_structure#cite_note-Brucat-60http://en.wikipedia.org/wiki/Atomic_structure#cite_note-Brucat-60http://en.wikipedia.org/wiki/Atomic_structure#cite_note-61http://en.wikipedia.org/wiki/Atomic_structure#cite_note-61http://en.wikipedia.org/wiki/Atomic_structure#cite_note-61http://en.wikipedia.org/wiki/Node_(physics)http://en.wikipedia.org/wiki/Node_(physics)http://en.wikipedia.org/wiki/Node_(physics)http://en.wikipedia.org/wiki/Energy_levelhttp://en.wikipedia.org/wiki/Energy_levelhttp://en.wikipedia.org/wiki/Energy_levelhttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Spontaneous_emissionhttp://en.wikipedia.org/wiki/Spontaneous_emissionhttp://en.wikipedia.org/wiki/Spontaneous_emissionhttp://en.wikipedia.org/wiki/Atomic_spectral_linehttp://en.wikipedia.org/wiki/Atomic_spectral_linehttp://en.wikipedia.org/wiki/Atomic_spectral_linehttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-Brucat-60http://en.wikipedia.org/wiki/Atomic_structure#cite_note-Brucat-60http://en.wikipedia.org/wiki/Atomic_structure#cite_note-Brucat-60http://en.wikipedia.org/wiki/Electron_binding_energyhttp://en.wikipedia.org/wiki/Electron_binding_energyhttp://en.wikipedia.org/wiki/Binding_energyhttp://en.wikipedia.org/wiki/Binding_energyhttp://en.wikipedia.org/wiki/Binding_energyhttp://en.wikipedia.org/wiki/Stationary_statehttp://en.wikipedia.org/wiki/Stationary_statehttp://en.wikipedia.org/wiki/Stationary_statehttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-62http://en.wikipedia.org/wiki/Atomic_structure#cite_note-62http://en.wikipedia.org/wiki/Atomic_structure#cite_note-62http://en.wikipedia.org/wiki/Deuteriumhttp://en.wikipedia.org/wiki/Deuteriumhttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-63http://en.wikipedia.org/wiki/Atomic_structure#cite_note-63http://en.wikipedia.org/wiki/Atomic_structure#cite_note-63http://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Chemical_bondhttp://en.wikipedia.org/wiki/Chemical_bondhttp://en.wikipedia.org/wiki/Chemical_bondhttp://en.wikipedia.org/wiki/Moleculehttp://en.wikipedia.org/wiki/Moleculehttp://en.wikipedia.org/wiki/Moleculehttp://en.wikipedia.org/wiki/Chemical_compoundhttp://en.wikipedia.org/wiki/Chemical_compoundhttp://en.wikipedia.org/wiki/Chemical_compoundhttp://en.wikipedia.org/wiki/Chemical_compoundhttp://en.wikipedia.org/wiki/Ionic_crystalhttp://en.wikipedia.org/wiki/Ionic_crystalhttp://en.wikipedia.org/wiki/Ionic_crystalhttp://en.wikipedia.org/wiki/Covalent_bondhttp://en.wikipedia.org/wiki/Covalent_bondhttp://en.wikipedia.org/wiki/Covalent_bondhttp://en.wikipedia.org/wiki/Crystallizationhttp://en.wikipedia.org/wiki/Crystallizationhttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-FOOTNOTESmirnov2003249.E2.80.93272-64http://en.wikipedia.org/wiki/Atomic_structure#cite_note-FOOTNOTESmirnov2003249.E2.80.93272-64http://en.wikipedia.org/wiki/Atomic_structure#cite_note-FOOTNOTESmirnov2003249.E2.80.93272-64http://en.wikipedia.org/wiki/Isotopehttp://en.wikipedia.org/wiki/Isotopehttp://en.wikipedia.org/wiki/Isotopehttp://en.wikipedia.org/wiki/Stable_isotopehttp://en.wikipedia.org/wiki/Stable_isotopehttp://en.wikipedia.org/wiki/Stable_isotopehttp://en.wikipedia.org/wiki/List_of_nuclideshttp://en.wikipedia.org/wiki/List_of_nuclideshttp://en.wikipedia.org/wiki/List_of_nuclideshttp://en.wikipedia.org/wiki/List_of_elements_by_stability_of_isotopeshttp://en.wikipedia.org/wiki/List_of_elements_by_stability_of_isotopeshttp://en.wikipedia.org/wiki/List_of_elements_by_stability_of_isotopeshttp://en.wikipedia.org/wiki/Chemical_elementhttp://en.wikipedia.org/wiki/Chemical_elementhttp://en.wikipedia.org/wiki/Chemical_elementhttp://en.wikipedia.org/wiki/Hydrogen_atomhttp://en.wikipedia.org/wiki/Hydrogen_atomhttp://en.wikipedia.org/wiki/Hydrogen_atomhttp://en.wikipedia.org/wiki/Deuteriumhttp://en.wikipedia.org/wiki/Deuteriumhttp://en.wikipedia.org/wiki/Deuteriumhttp://en.wikipedia.org/wiki/Tritiumhttp://en.wikipedia.org/wiki/Tritiumhttp://en.wikipedia.org/wiki/Isotopes_of_hydrogenhttp://en.wikipedia.org/wiki/Isotopes_of_hydrogenhttp://en.wikipedia.org/wiki/Isotopes_of_hydrogenhttp://en.wikipedia.org/wiki/Isotopes_of_hydrogenhttp://en.wikipedia.org/wiki/File:S-p-Orbitals.svghttp://en.wikipedia.org/wiki/File:S-p-Orbitals.svghttp://en.wikipedia.org/wiki/File:S-p-Orbitals.svghttp://en.wikipedia.org/wiki/File:S-p-Orbitals.svghttp://en.wikipedia.org/wiki/Isotopes_of_hydrogenhttp://en.wikipedia.org/wiki/Isotopes_of_hydrogenhttp://en.wikipedia.org/wiki/Tritiumhttp://en.wikipedia.org/wiki/Deuteriumhttp://en.wikipedia.org/wiki/Hydrogen_atomhttp://en.wikipedia.org/wiki/Chemical_elementhttp://en.wikipedia.org/wiki/List_of_elements_by_stability_of_isotopeshttp://en.wikipedia.org/wiki/List_of_nuclideshttp://en.wikipedia.org/wiki/Stable_isotopehttp://en.wikipedia.org/wiki/Isotopehttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-FOOTNOTESmirnov2003249.E2.80.93272-64http://en.wikipedia.org/wiki/Crystallizationhttp://en.wikipedia.org/wiki/Covalent_bondhttp://en.wikipedia.org/wiki/Ionic_crystalhttp://en.wikipedia.org/wiki/Chemical_compoundhttp://en.wikipedia.org/wiki/Chemical_compoundhttp://en.wikipedia.org/wiki/Moleculehttp://en.wikipedia.org/wiki/Chemical_bondhttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-63http://en.wikipedia.org/wiki/Deuteriumhttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-62http://en.wikipedia.org/wiki/Stationary_statehttp://en.wikipedia.org/wiki/Binding_energyhttp://en.wikipedia.org/wiki/Electron_binding_energyhttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-Brucat-60http://en.wikipedia.org/wiki/Atomic_spectral_linehttp://en.wikipedia.org/wiki/Atomic_spectral_linehttp://en.wikipedia.org/wiki/Spontaneous_emissionhttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Energy_levelhttp://en.wikipedia.org/wiki/Node_(physics)http://en.wikipedia.org/wiki/Atomic_structure#cite_note-61http://en.wikipedia.org/wiki/Atomic_structure#cite_note-Brucat-60http://en.wiktionary.org/wiki/quantizehttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-59http://en.wikipedia.org/wiki/Atomic_orbital
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protium.[65]
The known elements form a set of atomic numbers from hydrogen
with a single proton up to
the 118-proton elementununoctium.[66]
All known isotopes of elements with atomic numbers greater
than
82 are radioactive.[67][68]
About 339 nuclides occur naturally onEarth,[69]
of which 255 (about 75%) have not been observed to
decay, and are referred to as "stable isotopes". However, only
90 of these nuclides are stable to all
decay, evenin theory. Another 165 (bringing the total to 255)
have not been observed to decay, even
though in theory it is energetically possible. These are also
formally classified as "stable". An additional
33 radioactive nuclides have half lives longer than 80 million
years, and are long-lived enough to be
present from the birth of thesolar system. This collection of
288 nuclides are known asprimordial
nuclides. Finally, an additional 51 short-lived nuclides are
known to occur naturally, as daughter products
of primordial nuclide decay (such asradiumfromuranium), or else
as products of natural energetic
processes on Earth, such as cosmic ray bombardment (for example,
carbon-14).[70][71]
For 80 of the chemical elements, at least one stable
isotopeexists. Elements43,61, and all elements
numbered83or higher have no stable isotopes. As a rule, there
is, for each element, only a handful of
stable isotopes, the average being 3.2 stable isotopes per
element among those that have stable
isotopes. Twenty-six elements have only a single stable isotope,
while the largest number of stable
isotopes observed for any element is ten, for the
elementtin.[72]
Stability of isotopes is affected by the ratio of protons to
neutrons, and also by the presence of certain
"magic numbers" of neutrons or protons that represent closed and
filled quantum shells. These quantum
shells correspond to a set of energy levels within theshell
modelof the nucleus; filled shells, such as the
filled shell of 50 protons for tin, confers unusual stability on
the nuclide. Of the 255 known stable nuclides,
only four have both an odd number of protons andodd number of
neutrons:hydrogen-
2(deuterium),lithium-6,boron-10andnitrogen-14. Also, only four
naturally occurring, radioactive odd-
odd nuclides have a half-life over a billion
years:potassium-40,vanadium-50,lanthanum-
138andtantalum-180m. Most odd-odd nuclei are highly unstable
with respect tobeta decay, because the
decay products are even-even, and are therefore more strongly
bound, due tonuclear pairing effects.[72]
Mass
Main article:Atomic mass
The large majority of an atom's mass comes from the protons and
neutrons, the total number of these
particles in an atom is called themass number. Themass of an
atom at restis often expressed using
theunified atomic mass unit(u), which is also called a Dalton
(Da). This unit is defined as a twelfth of the
mass of a free neutral atom ofcarbon-12, which is approximately
1.661027
kg.[73]
Hydrogen-1, the lightest isotope of hydrogen and the atom with
the lowest mass, has an atomic
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weight of 1.007825 u.[74]
An atom has a mass approximately equal to the mass number times
the atomic
mass unit.[75]
The heavieststable atomis lead-208,[67]
with a mass of 207.9766521 u.[76]
As even the most massive atoms are far too light to work with
directly, chemists instead use the unit
ofmoles. The mole is defined such that one mole of any element
always has the same number of atoms
(about6.0221023
). This number was chosen so that if an element has an atomic
mass of 1 u, a mole of atoms of that
element has a mass close to one gram. Because of the definition
of theunified atomic mass
unit,carbonhas an atomic mass ofexactly12 u, and so a mole of
carbon atoms weighs exactly
0.012 kg.[73]
Shape and size
Main article:Atomic radius
Atoms lack a well-defined outer boundary, so their dimensions
are usually described in terms of
anatomic radius. This is a measure of the distance out to which
the electron cloud extends from the
nucleus. However, this assumes the atom to exhibit a spherical
shape, which is only obeyed for atoms in
vacuum or free space. Atomic radii may be derived from the
distances between two nuclei when the two
atoms are joined in achemical bond. The radius varies with the
location of an atom on the atomic chart,
the type of chemical bond, the number of neighboring atoms
(coordination number) and aquantum
mechanicalproperty known asspin.[77]
On theperiodic tableof the elements, atom size tends to
increase
when moving down columns, but decrease when moving across rows
(left to right).[78]
Consequently, the
smallest atom is helium with a radius of 32pm, while one of the
largest iscaesiumat 225 pm.[79]
When subjected to external fields, like anelectrical field, the
shape of an atom may deviate from that of a
sphere. The deformation depends on the field magnitude and the
orbital type of outer shell electrons, as
shown bygroup-theoreticalconsiderations. Aspherical deviations
might be elicited for instance incrystals,
where large crystal-electrical fields may occur
atlow-symmetrylattice
sites.[80]
Significantellipsoidaldeformations have recently been shown to
occur for sulfur ions inpyrite-
type compounds[81]
Atomic dimensions are thousands of times smaller than the
wavelengths oflight(400700nm) so they
can not be viewed using anoptical microscope. However,
individual atoms can be observed using
ascanning tunneling microscope. To visualize the minuteness of
the atom, consider that a typical humanhair is about 1 million
carbon atoms in width.
[82]A single drop of water contains about 2sextillion(21021
) atoms of oxygen, and twice the number of hydrogen
atoms.[83]
A singlecaratdiamondwith a mass
of 2104
kg contains about 10 sextillion (1022
) atoms ofcarbon.[note 2]
If an apple were magnified to the size of the
Earth, then the atoms in the apple would be approximately the
size of the original apple.[84]
Radioactive decay
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Main article:Radioactive decay
This diagram shows thehalf-life(T) of various isotopes with Z
protons and N neutrons.
Every element has one or more isotopes that have unstable nuclei
that are subject to radioactive decay,
causing the nucleus to emit particles or electromagnetic
radiation. Radioactivity can occur when the
radius of a nucleus is large compared with the radius of the
strong force, which only acts over distanceson the order of 1
fm.
[85]
The most common forms of radioactive decay are:[86][87]
Alpha decayis caused when the nucleus emits an alpha particle,
which is a helium nucleus consisting
of two protons and two neutrons. The result of the emission is a
new element with a loweratomic
number.
Beta decayis regulated by theweak force, and results from a
transformation of a neutron into a
proton, or a proton into a neutron. The first is accompanied by
the emission of an electron and
anantineutrino, while the second causes the emission of
apositronand aneutrino. The electron or
positron emissions are called beta particles. Beta decay either
increases or decreases the atomic
number of the nucleus by one.
Gamma decayresults from a change in the energy level of the
nucleus to a lower state, resulting in
the emission of electromagnetic radiation. This can occur
following the emission of an alpha or a beta
particle from radioactive decay.
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Other more rare types ofradioactive decayinclude ejection of
neutrons or protons or clusters
ofnucleonsfrom a nucleus, or more than onebeta particle, or
result (throughinternal conversion) in
production of high-speed electrons that are not beta rays, and
high-energy photons that are not gamma
rays.
Each radioactive isotope has a characteristic decay time
periodthehalf-lifethat is determined by the
amount of time needed for half of a sample to decay. This is
anexponential decayprocess that steadily
decreases the proportion of the remaining isotope by 50% every
half life. Hence after two half-lives have
passed only 25% of the isotope is present, and so forth
.[85]
Magnetic moment
Main articles:Electron magnetic dipole momentandNuclear magnetic
moment
Elementary particles possess an intrinsic quantum mechanical
property known as spin. This is analogous
to theangular momentumof an object that is spinning around
itscenter of mass, although strictly
speaking these particles are believed to be point-like and
cannot be said to be rotating. Spin is measured
in units of the reducedPlanck constant(), with electrons,
protons and neutrons all having spin , or
"spin-". In an atom, electrons in motion around
thenucleuspossess orbitalangular momentumin
addition to their spin, while the nucleus itself possesses
angular momentum due to its nuclear spin .[88]
Themagnetic fieldproduced by an atomitsmagnetic momentis
determined by these various forms of
angular momentum, just as a rotating charged object classically
produces a magnetic field. However, the
most dominant contribution comes from spin. Due to the nature of
electrons to obey thePauli exclusion
principle, in which no two electrons may be found in the
samequantum state, bound electrons pair up
with each other, with one member of each pair in a spin up state
and the other in the opposite, spin down
state. Thus these spins cancel each other out, reducing the
total magnetic dipole moment to zero in some
atoms with even number of electrons.[89]
Inferromagneticelements such as iron, an odd number of electrons
leads to an unpaired electron and a
net overall magnetic moment. The orbitals of neighboring atoms
overlap and a lower energy state is
achieved when the spins of unpaired electrons are aligned with
each other, a process known as
anexchange interaction. When the magnetic moments of
ferromagnetic atoms are lined up, the material
can produce a measurable macroscopic field.Paramagnetic
materialshave atoms with magnetic
moments that line up in random directions when no magnetic field
is present, but the magnetic moments
of the individual atoms line up in the presence of a field
.[89][90]
The nucleus of an atom can also have a net spin. Normally these
nuclei are aligned in random directions
because ofthermal equilibrium. However, for certain elements
(such asxenon-129) it is possible
topolarizea significant proportion of the nuclear spin states so
that they are aligned in the same
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)http://en.wikipedia.org/wiki/Nuclear_magnetic_momenthttp://en.wikipedia.org/wiki/Electron_magnetic_dipole_momenthttp://en.wikipedia.org/wiki/Atomic_structure#cite_note-splung-86http://en.wikipedia.org/wiki/Exponential_decayhttp://en.wikipedia.org/wiki/Half-lifehttp://en.wikipedia.org/wiki/Internal_conversionhttp://en.wikipedia.org/wiki/Beta_particlehttp://en.wikipedia.org/wiki/Nucleonhttp://en.wikipedia.org/wiki/Radioactive_decay
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directiona condition calledhyperpolarization. This has important
applications inmagnetic resonance
imaging.[91][92]
Energy levels
Main articles:Energy levelandAtomic spectral line
When an electron is bound to an atom, it has apotential
energythat is inversely proportional to its
distance from the nucleus. This is measured by the amount of
energy needed to unbind the electron from
the atom, and is usually given in units ofelectronvolts(eV). In
the quantum mechanical model, a bound
electron can only occupy a set of states centere
