The smallest particle of an element that is not further divisible by chemical means. An atom is composed of an electron shell and a nucleus. The atomic nucleus is composed of protons with positive electric charge and electrically neutral neutrons. In the atomic shell negatively charged electrons equal in number to the protons orbit around the nucleus. To the outside atoms are therefore electrically neutral. Every element is characterised by the number of protons in the nucleus, the atomic number Z.

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