Answer: The number of protons in the nucleus of an atom of that element.
Explanation: the atomic number of an element is simply the proton number of that element. This is the most reason while the ion of a particular element still retain its atomic number whether it gains or loss electrons. For example, sodium atom (Na) has 11 electrons, 11 protons and 12 neutrons. Sodium ion (Na+) on the other hand has 10 electrons because it loss one electron, 11 proton and 12 neutrons. If the atomic number was to be determined by the number of electrons, then the atomic number of sodium atom and sodium ion would have been different. So the best way to determine the atomic number is by the number of protons. Neutron number will only bring about isotope and not atomic number
