atomic number. The number of protons in an atomic nucleus; it indicates the position of an element in the periodic table. The number of protons in the nucleus of an atom. In electrically neutral atoms, this number is also equal to the number of electrons orbiting about the atom's nucleus. read more
The number of neutrons in an atom is referred to as the N-number, or the neutron number. Both the neutron number and the atomic number are expressed as whole numbers. When an atom has the same atomic number but a different neutron number, it is called an isotope. read more