Definition of atom

An atom is the smallest particle of matter impervious to attack with energies short of nuclear reactions. There are only 90 chemically distinct kinds of atoms found in all of nature. Thus atoms are the building blocks on which we baseour understanding of the composition of and the formation of minerals under geological conditions; atoms are rearranged - but not changed - during geological processes.

At the center of an atom is the nucleus, a combination of positively-charged protons and uncharged neutrons. The outside the nucleus are negatively-charged electrons. The number of protons in the nucleus determines the chemical behavior of the atom.

An atom has the same number of electrons as protons (thus; electrically neutral). Most atoms, when mixed with atoms of a different element, lose or gain electrons to form ions. Negatively charged ions (atoms with extra electrons) are anions; positively charged ions (atoms with a deficit of electrons) are cations.