Protons, electrons and isotopes

Atomic number is proton count and identifies an element. A neutral atom has equal proton and electron counts. Neutrons have no electrical charge. Mass number counts protons plus neutrons in one nucleus, so neutrons = mass number − atomic number. Do not confuse this whole-number count with the averaged relative atomic mass on a periodic table.

Isotopes have the same proton count but different neutron counts. Carbon−12 and carbon−14 both have six protons, with six and eight neutrons respectively. Changing protons changes the element. Forming an ion instead involves electron gain or loss: losing one electron gives charge +1. Check whether an atom is neutral before calculating its electron count.

Watch out: Their proton counts are identical. Different neutron counts give different mass numbers.