[Jules18]

I was asked to give the name of the compound CaSi on a practice exam, which I thought was weird because as far as I knew Ca was always +2 and the periodic table kinda suggests that Si would be -4.  So I didn't know how the compound CaSi would exist.

So I just said Calcium (IV) Silicide and handed it in, even though I was pretty sure Calcium didn't have multiple oxidation numbers. 
And then they said the right answer was Calcium Silicide and I didn't even need the (IV), so now I'm really confused. 

How can CaSi exist, and why don't I need the (IV)?
Any help will be really appreciated!     (I have an exam on Friday )

Confused again,
~Jules~

[gregdwulet]

I agree with you. Are you sure the test didn't say CaS? Because that would work out very nicely with the Ca⁺² and S^-2 ion...

I really can't see CaSi occurring, unless Ca was somehow able to form an anion and Si a cation of charge +2 or +4

[renge ishyo]

CaSi certainly exists, but it is a non-stoichiometric crystalline compound where the charges for cation and anion seemingly do not balance because unnamed impurities are present in the crystal that compensate for the unbalanced charges in the molecular formula (see the wiki for more: http://en.wikipedia.org/wiki/Non-stoichiometric_compound). The impurities in CaSi are usually iron (Fe+2, and +3) and aluminium (+3) which both help balance the -4 silicon oxidation state by joining with the calcium (+2) so that the overall compound is neutral.

It is an evil exam question for a beginning chemistry student that is for sure.

[lancenti]

Evil indeed, but general rules for naming which may help in the event of another evil exam:

If your metal is from Group IA or IIA, you typically don't have to put (I) or (II) in front of it because Group IA and IIA typically end up as 1+ or 2+. In other words, if you see a Group IA or IIA, just put down its name immediately.

Halogen+Oxygen = Halate (X) where X is the oxidation state of the Halogen (which will typically be positive since Oxygen always adopts a 2-), or at least as far as I've seen for exams.

Interestingly enough, Calcium Silicide is apparently CaSi₂ according to Wikipedia. (http://en.wikipedia.org/wiki/Calcium_silicide) so you may want to bring that up with your TA, unless you have a textbook that says otherwise.

[renge ishyo]

Calcium Silicide is apparently CaSi2

That's *technically* calcium disilicide. The wiki mentions this, and the last sentence includes a reference to the existence of CaSi as well. In practice these compouns are all called calcium silicide because the compound is non-stoichiometric so the exact molar ratios are not super relevant anyways.

[lancenti]

Technically, but if you're desperate enough for marks you could try that. Chemists are supposed to be precise when identifying compounds, after all.

[renge ishyo]

Yeah, I remember reading in one of Pauling's books that he believes the reason why metal alloys (and other compounds that can have non-stoichiometric relationships) are not studied all that much in formal education is because chemists prefer the quantitative precision of having an exact molecular formula which you get with discrete molecular compounds and pure ionic compounds. He seemed to imply that it was a shame that this was so, but I think this is a good example of the kind of confusion it can create. I still question the teacher for putting this on the test...it is only going to confuse the good students who are trying to check their work as they go along.