How do you calculate formal charge of a molecule?
NH3 has one lone pair and three bonds with hydrogen. It is in group 5A, so it has 5 valence electrons.
Thus, 5 - (2 + (1/2)(6) = 0
So, NH3 has a formal charge of 0.
NH4 has no lone pairs and four bonds with hydrogen.
Thus, 5 - (0 + (1/2)(8)) = 1
So, NH4 has a +1 formal charge.
Note: I am getting (1/2)(6) because you do not count both electrons in the N-H bond, only the electrons that are on the side of the element you are calculating the formal charge. So there are a total of 6 electrons in the three N-H bonds, but you only count the electrons on the N side which is 3, or 1/2 of 6.