If Z, the atomic number = 20, there MUST be 20 nuclear protons, and the element MUST be calcium, with 20 electrons associated with it.
By definition, the atomic number is the number of protons, massive, positively charged nuclear particles. If there are 20 positively charged nuclear particles, there MUST be 20 fundamental, negatively charged particles also associated with the ATOM. And there are; they are called ELECTRONS, which are conceived to orbit the nuclear core. How do I know that there are such electrons? Well, because calcium, like all matter, is electrically neutral. For every positive charge, there is a negative charge. The mass of a single calcium atom is 40 atomic mass units
. Since, electrons have negligible mass, the difference in atomic mass
and atomic number can be accounted for the presence of another 20 or so neutrons, massive, neutrally charged nuclear particles. Because an element can have varying numbers of neutrons, the identity of the element always depends on Z
, the atomic number, a slight excess or deficiency of neutrons can give rise to the existence of isotopes
. The weighted average of the isotopes gives rise to the quoted atomic mass on the Periodic Table