The atomic mass of an atom is determined by the sum of its protons and neutrons. In the case of calcium, the atomic number is 20, which means it has 20 protons. However, the atomic mass of calcium is not exactly 40. It is actually closer to 40.08.
The atomic mass of an element listed on the periodic table is an average value that takes into account the different isotopes of that element and their relative abundances. Calcium has several isotopes, including calcium-40, calcium-42, calcium-43, calcium-44, calcium-46, and calcium-48. These isotopes have different numbers of neutrons.
The most abundant isotope of calcium is calcium-40, which has 20 protons and 20 neutrons. However, there are also other isotopes present in smaller amounts. The average atomic mass of calcium is calculated by considering the mass of each isotope and its abundance in nature.
When these values are taken into account, the weighted average of the isotopes' masses results in an atomic mass of approximately 40.08 for calcium. Therefore, the atomic mass of calcium is commonly rounded to 40 for simplicity when referring to it in general terms.