working out shapes of molecules

im really confused on how to work out shapes and what you do if its a negative/positive ion?

im really confused on how to work out shapes and what you do if its a negative/positive ion?

Example: NH4+

1. Work out the central atom. In this case, it is N.
2. Find the group number which the central atom is in. Using the periodic table, N is in group 5.
3. Add the group number of the central atom to the number of bonds it forms. 5 + 4 = 9.
4. Since there is a +1 charge, deduct 1. ( if it was -2, we add 2). So 9-1 = 8.
5. Divide the number calculated by 2. 8/2 = 4.

So there are 4 bonds in total. Looking at NH4+ all four bonds are N-H bonds so there are no lone pairs of electrons. This gives a tetrahedral shape.

This is the method I use, hopefully it makes some sense. You have to just learn the possible shapes that can be formed and apply your answer to them.

I can post another example if it helps