valence shell

The outermost shell of an atom.

valence electron

A valence electron is an electron in the valence shell of an atom.

valence

The 'combining power' of an atom, or the number of hydrogen atoms it will bond with. It can be seen that valence is equal to the absolute value of the most common charge. For example, Oxygen has the most common charge $-2$, so its valence is $|-2|=2$.

core electron

Any electron which is not a valence electron.

nuclear charge

The number of protons in an atom. Equal to the atomic number.

mass number

The number of protons plus the number of neutrons in an atom.

core charge

Also known as effective nuclear charge. This is equal to the number of protons minus the number of non-valence electrons. This represents the force of attraction experienced by the valence electrons of an atom to its nucleus.