# Question #0139f

Calcium in the periodic table is shown as $_ {\left(40.1\right)}^{20} C a$. '20' is the atomic number of calcium which is the number of protons an atom of calcium has. '40.1' shows the mass number or weighted average relative mass of calcium. To calculate the average number of neutrons, take $40.1 - 20 = 20.1$. The most common isotope of calcium has 20 neutrons.
Other isotopes will have more neutrons than the most common form of Calcium $C {a}_{20}^{40}$ but all isotopes of Calcium have 20 protons.