Each element on the periodic table has a unique atomic number, which represents the number of protons in the nucleus of its atoms. For example, hydrogen has an atomic number of 1, meaning it has one proton. Helium has an atomic number of 2, indicating it has two protons. The atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes, taking into account the abundance of each isotope.
So, while all atoms of a specific element have the same number of protons (atomic number) and electrons (in a neutral atom), the number of neutrons and hence the atomic mass can vary among isotopes of the same element. This is why elements can have different isotopes with different numbers of neutrons, resulting in different atomic masses and different numbers of atoms per given mass of the element.