Answer:
The number of protons in an atom define each individual element because the number of protons is the atomic number which is the defining property of an atom and never changes. The number of protons determines an atom's identity. For example, Oxygen has an atomic number of 8, so it has 8 protons, so no matter what ionic compound or molecule it's part of, every single Oxygen atom will always have 8 protons each.
