Every single discovered atom have protons, electrons, and neutrons. The number of each depends on its assigned atomic number. Protons have a positive charge, electrons have a negative charge and, as the name implies, neutrons have no charge.
When looking at an element in the periodic table, there are two numbers: the atomic number, which is found at the top, and the atomic weight, which is found at the bottom of the box.
The number of protons found in the nucleus of calcium is the same as its atomic number: 20.
The number of electrons and the number of protons are always the same, unless the element is given another charge.
The atomic mass, the number at the bottom, is the sum of the number of protons and the number of neutrons. Just subtract the number of protons, 20 in this case, from the rounded number of the mass.
Changing the number of neutrons will change the element to another version of itself, called an isotope.
