# The atomic number for the element calcium (Ca) is 20. How many protons and electrons does each atom have?

Mar 21, 2016

If $Z$, the atomic number $=$ $20$, there MUST be 20 nuclear protons, and the element MUST be calcium, with 20 electrons associated with it.
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 $\text{40 atomic mass units}$. 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.