What does valence mean?

Answer 1

'Valence' is the combining capacity of an element. It refers to the number of electrons in the outer shell. The number of valence electrons, versus the maximum number of possible electrons in that shell, affects the ability of atoms to combine.

For instance, water is H2O, or two atoms of hydrogen and one atom of oxygen per molecule. Each atom of hydrogen has one electron in the first shell, with a maximum of two. Each atom of oxygen has 6 electrons in the second shell (2 in the first), with a maximum of eight. The electron in hydrogen "fills in the gap" in the outer shell of oxygen. Since there are two "gaps", then two atoms of hydrogen are required. This is why water is H2O, and not H3O, or something else.

Another example is plain oxygen. Since it has a "gap" of two, it "prefers" to make molecules of two atoms, i.e. O2. Each atom will share two valence electrons with the other atom, effectively filling the valence shell.

If the outer shell is filled, then the atom is not reactive and does not combine with other elements. This is the case for the noble gases Helium, Neon, Argon, Krypton, Xenon, and Radon.
Valence as in Valence Electrons are the electrons in the outer "belt" of electrons.

Answer 2

Valency is the number of electrons in the outer shell of an atom. Each shell needs a certain number of electron in it. The valence electrons are the ones that can be shared between atoms to complete the shell and to form chemical bonds between atoms.