[morvaread13]

Hello,

I was trying to solve a problem about solubility product but got it wrong because I thought the chemical formula for lead iodide was PbI when it is really PbI2. How was I supposed to know there should be two I instead of just one? Thanks for any help

[mjc123]

It's called "knowing chemistry". It's just one of those things you're expected to know at university level.
Which group is Pb in? What oxidation states are typically exhibited by these elements?

[morvaread13]

mjc123,

but I'm having trouble making the connection between oxidation state and chemical formula. I know Pb's oxidation state is usually -4 but why does that tell me that there are two iodine atoms? My first thought is that there should be 4.

[Borek]

Pb has more than one common oxidation state.

[morvaread13]

Okay so I did some research and now see why lead's oxidation state is usually +2. I was assuming it would react similar to carbon since they are in the same group but because iodine is more electronegative than lead, leads oxidation state will be positive rather than negative. Even with this however, its not quite sticking as to why +2 is more stable than +4.

[Arkcon]

I was assuming it would react similar to carbon since they are in the same group but ...

Also, strictly following periodic trends has caused another problem.  Granted, its in the same group as carbon, but its pure coincidence that lead even has a +4 oxidation state.  Lead and tin are metals, they have d-shell electrons hybridizing to their outer p-shells.  They're in the same column, but their properties are nothing like carbon and silicon.  Tin and lead are not what are called the Transition metals, but they really do behave as if they belonged there.