Atom

From EvoWiki

An atom (from Greek atomos, "uncut") is the smallest possible physical quantity of a chemical element (i.e. hydrogen, silicon, etc). While matter does come in smaller pieces, none of those pieces are themselves chemical elements.

The concept of atoms dates back at least 2,400 years, to the Greek philosopher Democritus. His idea differs from the modern concept in that Democritus held that an atom was an indivisible unit.

The notion was placed on a rigorous basis by chemist John Dalton, who showed in the early 1800's that the atom hypothesis readily explained the recently-discovered Law of Definite Proportions of chemical reactions. Many compounds, especially inorganic ones, could be explained as having only a few of various elements' atoms.

Dalton even invented some symbols for atoms, consisting of circles with various marks inside. But Jöns Jakob Berzelius's alphabetic abbreviations proved more convenient, and are still used today.

Further support emerged in the mid 19th century when the kinetic theory of gases was worked out - gases consistent of free-flying molecules that seldom interact with each other, making it easy to work out their "equation of state" (interrelationship of density, pressure, temperature, etc.).

But in 1897, evidence emerged that atoms were not really "uncuttable". Electrons were discovered by J.J. Thomson; they are particles about 1/2000 the mass of the least massive atom, that of hydrogen. This discovery was followed in 1909 by Ernest Rutherford's discovery of atomic nuclei. A few decades later, nuclei were shown to contain protons and neutrons, and in the 1960's and 1970's, those particles were shown to contain quarks and gluons.

(should add something on chemical bonds and molecules)