Can atoms of the same element have different atomic numbers?
NO. An element always has its own unique atomic number, because the atoms of that element all have identical amounts of protons. Counting the number of protons in a nucleus of an atom reveals the identity of the atom. So, all atoms of the same element have the same atomic number.
However atoms of one element CAN have a different mass
caused by different number of neutrons in nucleus: these are called
isotopes of that element.
No. The only thing that can change within an given element, is the atomic mass usually referred to as isotopes, which means there is a different abundance of each isotope.
For example chlorine has two isotopes one which is 75.78% and a atomic mass of 34.969 and the second is 24.22% and a atomic mass of 36.966.
Now calculate the average atomic mass and you have a answer of 35.45.