How do you count the valence electrons of an atom?

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.

In column 1 all of those elements have 1 valence electron.

In column 2 there are 2 valence electrons. He(Helium) is included here.

Skip columns 3 to 12 because their valence electrons vary.

Column 13 has 3 valence electrons.

Column 14 has 4 valence electrons.

Column 15 has 5 valence electrons.

Column 16 has 6 valence electrons.

Column 17 has 7 valence electrons.

Column 18,except for He(Helium), has 8 valence electrons.