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 atoms.

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 try 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.