AtomAn 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.