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Atom
Atomic Nucleus
Atom
An atom is the smallest, irreducible
constituent of a chemical system. The word is derived from the
Greek atomos, indivisible, from a-, not, and tomos,
a cut. It usually means chemical atoms, the basic constituents of
molecules and ordinary
matter. These atoms are not divisible by
chemical reactions but are now known to be composed of even smaller
subatomic particles. The sizes of these atoms are generally in the range
from
10 pm to
100 pm. This article discusses these chemical atom(s).
The variety of matter that is dealt with in everyday
experience consists of discrete atoms. The existence of such particles was first
proposed by Greek philosophers such as
Democritus,
Leucippus, and the
Epicureans, but without any real way to be sure, the concept disappeared
until it was revived by
Rudjer Boscovich in the 18th century, and after that applied to chemistry by
John Dalton.
Rudjer Boscovich based his theory on
Newtonian mechanics and published it in
1758
within his Theoria philosophiae naturalis redacta ad unicam legem virium in
natura existentium. According to Boscovich, atoms are stuctureless points,
which exhibit repelling and attracting forces on each other, depending on
distance.
John Dalton used the atomic theory to explain why
gases
always combine in simple ratios. It was with
Amedeo Avogadro's work, in the 19th century, that scientists began to
distinguish atoms and
molecules. In modern times atoms have been observed experimentally.
As it turns out, atoms are themselves made out of smaller
particles. In fact, almost all of an atom is empty space. At the
center is a tiny positive
nucleus composed of
nucleons (protons
and
neutrons), and the rest of the atom contains only the fairly flexible
electron shells. Usually atoms are
electrically neutral with as many
electrons as
protons. Atoms are generally classified by the atomic number, which
corresponds to the number of protons in the atom. For example,
carbon atoms are those atoms containing 6 protons. All atoms with the same
atomic number share a wide variety of physical properties and exhibit the same
chemical behavior. The various kinds of atoms are listed in the
Periodic table. Atoms having the same atomic number, but different atomic
masses (due to their different numbers of neutrons), are called
isotopes.
The simplest atom is the
hydrogen atom, having atomic number 1 and consisting of one proton and one
electron. It has been the subject of much interest in science, particularly in
the early development of
quantum theory.
The chemical behavior of atoms is largely due to interactions
between the electrons. In particular 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.
There is a strong tendency for atoms to completely fill (or
empty) the outer electron shell, which in hydrogen and helium has space for two
electrons, and in all other atoms has space for eight. This is achieved either
by sharing electrons with neighboring atoms or by completely removing electrons
from other atoms. When electrons are shared a
covalent bond is formed between the two atoms. Covalent bonds are the
strongest type of atomic bond.
When one or more electrons are completely removed from one
atom by another,
ions
are formed. Ions are atoms that possess a net
charge due to an imbalance in the number of protons and electrons. The ion
that stole the electron(s) is called an anion and is negatively
charged. The atom that lost the electron(s) is called a cation and is
positively charged. Cations and anions are attracted to each other due to
coulombic forces between the positive and negative charges. This attraction is
called
ionic bonding and is weaker than covalent bonding.
As mentioned above covalent bonding implies a state in which
electrons are shared equally between atoms, while ionic bonding implies that the
electrons are completely confined to the anion. Except for a limited number of
extreme cases, neither of these pictures is completely accurate. In most cases
of covalent bonding, the electron is unequally shared, spending more time around
the more
electronegative atom, resulting in the covalent bond having some ionic
character. Similarly, in ionic bonding the electrons often spend a small
fraction of time around the more electropositive atom, resulting in some
covalent character for the ionic bond.
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Particle Radiation | Phonon | Roton
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