# What makes up most of the mass of a Calcium atom?

$\text{Nuclear protons, and neutrons............}$
The standard calcium isotope is ""^(40)Ca. Because it is a calcium nucleus, WE KNOW that the nucleus contains 20 protons, 20 massive, positively-charged nuclear particles. The calcium nucleus also contains approx. 20 neutrons, 20 massive, neutrally-charged nuclear particles.