ATOMIC NUCLEUS
Atomic nucleus is the central part of an atom, where the major part of its mass is concentrated. Atomic nucleus consists of nucleons, which are protons and neutrons. The number of protons Z determines the positive electric charge of a nucleus in units of the elementary electric charge. Z is equal to the index number of the given element in the periodic system and is called atomic number of the element. The sum of Z and the number of neutrons N is called mass number A (the nearest whole number to the value of the atomic mass).