ATOM

Atom the smallest part of an element that retains its chemical identity and can take part in a chemical reaction.
Atoms are very, very tiny being typically about one ten-millionth of a millimetre (10 -7 mm) in radius. Each of the 100 or so chemical elements contains a different kind of atom. Atoms differ in the number of particles they contain.
Atom means that which cannot be divided for it was long thought that atoms were indivisible. The Greek democritos first suggested that substances were made up of atoms in about 400BC. But the idea was forgotten until John Dalton suggested it again in 1801. Experiments by J. J. Thomson and others near the turn of the century indicated that the atom contains smaller particles. In 1911 Ernest Rutherford build up a new theory of atomic structure which saw the atom as a positively charged nucleus (sun) around which circle negatively charged electrons (planets) and like the solar system, most of the atom consists of space. By the 1930s the modern concept of the atom was established, though it has since been much modified and extended.

In essence the atom consists of a nucleus containing two main particles - Positively charged protons and neutral neutrons - with negatively charged electrons orbiting around it.
There are as many electrons as protons, making the atom electrically neutral. Many other atomic particles are known to exist, baryons, hadrons, leptons, mesons, neutrinos, antiparticles.
And all atomic particles could be made up of different kinds of particles called quarks.
Nevertheless the structure of matter and the chemistry of the elements can still be best explained in terms of the three main particles - protons, neutrons and electrons. Different atoms have different numbers of these particles
Most atoms are stable. Others are unstable - they break down and give off particles and radiation. This phenomenon is called Radioactivity