Calcium has 20 protons and 20 neutrons. This element is number 20 on the Periodic Table of the Elements and has an atomic weight of 40.078.
The easiest way to determine the number of protons in an element is to look at its atomic number. The atomic number is printed above the element's symbol on the periodic table. Calcium has an atomic number of 20, so it has 20 protons.
Finding the number of neutrons in an element is slightly more difficult. The number of neutrons present corresponds to the element's mass number less the number of protons in the element. Rounding an element's atomic weight, which is listed below the element on the periodic table, is the best way to find the element's mass number.
Calcium's atomic weight of 40.078 rounds to a mass number of 40. Subtracting 20 from 40 yields a result of 20. This means an atom with 20 protons and 20 neutrons must be an atom of calcium.
In some cases, calcium does not have 20 neutrons. The element has several different isotopes, which are atoms of the same element that have different numbers of neutrons. Some calcium isotopes are stable, but others decay over time. Ca-49 has a half-life of just 8.7 minutes, while Ca-41 has a half-life of 103,000 years.