Atom/Proton Number and Isotopes
Each element of a specific element contains the same number of protons, for example all the atoms of carbon have 6 protons in their nucleus.
Every different element has a different atom number, this idea is used to order the periodic table.
All the atoms of a particular element have the same amount of protons. however, the number of protons can vary within the atoms of an element.
An isotope is the same element with the same number of protons but a different number of neutrons.
For example the proton number of carbon is 6, so all atoms of carbon have 6 protons but their number of neutrons can differ.
Most carbon atoms contain 6 protons and 6 neutrons making the mass number (protons + neutrons) 12. However, some carbon atom isotopes also exist with 6 protons and 8 neutrons giving it a mass number of 14.