The element calcium has the atomic number of 20 and the mass number of 40, how many neutrons does it have?

An atom of the calcium-40 isotope has ##20## neutrons in its nucleus.

are named for their mass numbers, for example, carbon-14. The of an isotope is the sum of the number of protons and neutrons in the atomic nucleus. The is the number of protons.

The calcium-40 isotope has a mass number of ##40##, and an atomic number of ##20##, therefore it has ##20## protons.

##"Number of neutrons"="(mass number)"-"(atomic number)"##

##"Number of neutrons"=40-20=20##