Atoms of an element differ from those of all other elements in ___________.

Option A

The atomic number of an element, as in the periodic table, is equal to the number of protons in its nucleus. The number of neutrons (also in the nucleus) adds to the protons to give the mass number. Thus there may be different numbers of neutrons in different atoms of the same element (called isotopes). Even the number of electrons may vary, when an atom becomes an ion, but the number of protons is always the same for the same element.