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.
(16 Dec 1997)
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