atom Smallest part of an element which exists chemically. The atom consists of a positively charged core, called the nucleus, about which the negatively charged electrons revolve in orbits. The nucleus contains the positively charged protons and neutral neutrons. Almost whole mass of the atom is contained in the nucleus, since proton and neutron are 1840 times heavier than electrons. In an atom the number of electrons is equal to number protons. Therefore the atom is electrically neutral.

The number of protons in an atom is called atomic number, Z. The number obtained by adding number of neutrons, n and number of protons, p gives the atomic mass number, A.

An atom may convert itself into a positive ion by losing electrons from outermost orbit, or it may become a negative ion by gaining electrons.