Atoms of the same element with different weights is because the neutrons make the difference on of weight in the atom and they can have more neutrons without changing the element like the protons would. The protons do have weight but they never have more than their supposed to. Protium is the original form of Hydrogen and hydrogen originally has one proton, one proton, and no neutrons. Deuterium adds a neutron to the element and makes it heavier than it was originally and then tritium adds another neutron and makes it heavier than it was.
