How would you calculate the formal charge of NH3?

One Nitrogen atom = ##1## x ##-3## (nitrogen's charge) = ##-3##

Three hydrogen atoms = ##3## x ##+1## (hydrogen's charge) = ##3##

##-3 + 3 = 0## (net charge of ##NH_3##)

If you refer to a periodic table you'll see columns. The in hydrogen's column have a ##+1## charge. The elements in nitrogen's column have a ##-3## charge.

You can determine the charge of the nitrogen column by starting with the noble gases(charge = ##0##) and counting down by column. Similarly the hydrogen column starts with ##+1## while the next column to the right is ##+2##.

Transition metals are typically ##+3##, although there are a few exceptions.