Berkeley Review Chemistry problem formal charges...

[MCAT guy]

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Ok, this concept is really simple but I don't know why it isn't clear to me.

How do I know when the formal charge on something is +1 or -1 or whatever. I guess I don't know the charges of the attached groups. Like I understand if Cl is -1 and there are 4 Cl ligands then the central atom is +4 but I guess I don't have these memorized.

How do I know NH3 is neutral and Cl is -1, do I need to memorize a list or can I tell from the periodic table?

Here is the exact question I had from a FL, which is one you should probably never miss:

What is the formal charge of platinum in cis-platin?

Platinum in cis-platin has a +2 charge, because the two chlorine atoms have negative charges and the two ammonias are each neutral. For the overall compound to be neutral, platinum must carry a +2 charge, and thus have a formal charge of +2
Does anyone know how I know the group charges, or do I just memorize?

 

[loveoforganic]

Ammonia is a pretty common compound... Even if it wasn't you should know nitrogen makes 3 bonds + that lone pair, so with 3 hydrogens it's a neutral species

 

[angleslam08]

in terms of those transition metal compound, I think it is just memorizing the ligands and knowing which will bring in a how much a minus charge with it which means the metal has to to increase it's oxidation state to be in a stable configuration.

 

[rls303]

Hi,
So in this case oxidation state and transitional metal formal charge implies the same thing essentially?

Since we say that Pt is going to have a formal charge of +2 (due to the two amines being neutral),....it's oxidation state is also gonna be +2?