The atomic number of an element is based on what?
The atomic number of an element is based on the number of protons in the atomic nuclei of its atoms.
Each element has an atomic number that is unique to it. The atomic number is the number of protons in all of the nuclei of the same element. For example, the atomic number of hydrogen is 1, so all hydrogen atoms have one proton in their nuclei. Another example is manganese (Mn) with atomic number 25, therefore its atoms have 25 protons in their nuclei. It is the atomic number that gives an element its identity.
In chemical reactions, the number of protons in the atoms of an element never changes. The only way the number of protons can change is by radioactive decay.