How do you count the valence electrons of an atom?

Counting Valence Electrons

The number of valence electrons is just how many electrons an atom has in its outer shell. It's easy to figure out if you've got a periodic table. (See the link below this answer for a good periodic table).

All the elements in each column have the same number of electrons in their outer shells. All the elements in the first column all have a single valence electron (H, Li, Na, K, etc.).

The second column elements all have 2 valence electrons (Be, Mg, Ca, Sr, etc.).
Skipping over the gap, go to the Group 3 elements, which all have 3 valence electrons (B, Al, Ga, etc.).

The elements in the next column (C, Si, Ge, etc.) all have 4 valence electrons.
The elements in the next column (N, P, As, etc.) all have, yes, you guessed it, 5 valence electrons.
O, S, Se, and the others in this column have 6 valence electrons.
The halogens in the next-to-last column (F, Cl, Br, etc.) have 7 valence electrons.

The noble gases in the right-most column (Ne, Ar, Kr, etc.) all have 8 electrons in their out except for He, which only has 2 electrons.

If an atom is an ion, you must include the charge also:
For a positive ion, for each charge subtract one electron, *for instance, Na⁺ has 1-1 = 0, BUT it has 8 valence electrons because it has the same electron configuration as Ne. Just as K⁺ has the same configuration as Ar. Therefore, the alkali metal ions with a single positive charge will have 8 valence electrons.

For a negative ion, add one electron for each charge, for instance, O^2- has 6+2 = 8 valence electrons