The answers should be Protons. The number of Protons in the nucleus is the atomic number of an element.
The number of protons in an element is like an ID card number. Each element has their own number of protons and it is never changed no matter how the number of neutrons or electrons are changed.
For example, fluorine has 9 protons, (protons only locates in the nucleus of an atom by the way), so, the atomic number of fluorine is 9.
