Superscript left: total number of nucleons (i.e., number of protons plus number of neutrons); also written in normal font to the right of the chemical symbol (C-14, U-238)
Subscript left: total number of protons (should match the chemical symbol, i.e. ^14_7 C is bogus, as 'C' has 6 protons, not 7)
Superscript right: charge of the ion in electrons, designating a surplus (-) or deficiency (+); OR: number of unpaired electrons in case you're dealing with a radical---but in the latter case you write a dot, not a number
Subscript right: amount of atoms making up the molecule.
Funny things happen in case of nucleons and leptons:
neutron = ^1_0 n
proton = ^1_1 p = ^1_1 H+
electron = ^0_-1 e-
anti-electron = positron = ^0_+1 e+
which seem like notational quirks but are still very useful.
Contrary to popular belief and high school teachings, neutrons *do* have an effect on chemistry, but it's subtle and for all intents and purposes can be ignored. For example, it has been noted that in some reactions involving the transfer of hydrogen atoms, substituting them by their twice as heavy brother atom deuterium slows the reaction rate by over 30%. It has been hypothesised that drinking sufficient amounts of D2O will kill you because of this, but I think the consensus was that you will die from drinking too much water first. (Which, by the way, has actually been reported in medical literature.)
