1.2.2—The notation x

Read isotope notation in the form ^A_ZX by locating the mass (nucleon) number A at the upper left and the atomic (proton) number Z at the lower left. X is the chemical symbol, so Z identifies the element.

Use a fixed sequence: (1) copy A and Z, (2) calculate neutron number as A − Z, and (3) check the symbol against Z. The mass number counts protons plus neutrons; it is not the number of electrons.

For isotopes of one element, Z and X stay the same while A changes because the neutron number changes. Thus a different upper-left number does not by itself mean a different element.

Only change the electron count when a charge is shown: a positive ion has lost electrons and a negative ion has gained electrons. Keep this charge adjustment separate from the nuclear counts A, Z and A − Z.