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Atom

(Science: chemistry, physics, radiobiology) a particle of matter indivisible by chemical means. It is the fundamental building block of the chemical elements.

The elements, such as iron, lead and sulphur, differ from each other because they contain different kinds of atoms. There are about six sextillion (6 followed by 21 zeros or 6 x 1021) atoms in an ordinary drop of water.

According to present day theory, an atom contains a dense inner core (the nucleus) and a much less dense outer domain consisting of electrons in motion around the nucleus. Atoms are electrically neutral.