The chemical behaviour of an element depends on the number of electrons, which is Z . This is the number of neutrons in the nucleus. All atoms of the same element have the same Z , but they do not all have the same value of N . Atoms having the same number of protons, but different numbers of neutrons are called isotopes. They belong to the same element and behave the same in chemistry, but they do differ slightly in properties such as melting point.
Most elements have more than one isotope.
You may have wondered why chlorine has an atomic mass of 35.5. A nuclide is a type of nucleus. The upper number is A and the lower number is Z . There is a big uranium separation plant at Capenhurst, near Chester.