# Structure of KC8

Graphite reacts with potassium to produce a compound with the empirical formula $\ce{KC8}$. Of the following, which is the best description of this compound's structure?
(a) $\ce{K+}$ ions close packed with polyhedral $\ce{C8-}$ ions;
(b) $\ce{K-}$ ions close packed with polyhedral $\ce{C8+}$ ions;
(c) $\ce{K+}$ ions packed with $\ce{C2^2-}$ ions;
(d) Negatively charged hexagonal carbon layers with intercalated $\ce{K+}$ ions between them;
(e) An expanded diamond lattice with $\ce{K+}$ ions in tetrahedral holes.

The correct answer is (d). I'm not very strong in inorganic chemistry, so I'm unsure if there is a way to deduce this answer.

The only thing I'm sure of is that it's not (b), since it seems very unlikely that potassium would have a negative charge. But I don't know why (a), (c), or (e) are all false (and why (d) is true).

Any help would be greatly appreciated.

• Do you know the structure of graphite? Knowing this it would be rather obvious – Mithoron Sep 10 '15 at 0:02

This question is kind of "illogical", in that the answer can't quite be derived by logic alone; you either know it, or you don't. Well, of course you are right about (b). Also, it's not (c) either, because if we balance the charges to get a neutral compound, it would be $\ce{K_2C_2}$, and that's not what we want (though this thing does exist, too). Then what?
All that being said, carbon is a surprisingly versatile element (think of organic chemistry), so I'd rather avoid making too general conclusions. Suppose someone brings you a sample; you analyze it and see that it contains just K and C, in molar ratio 1:10, so you may say it's $\ce{KC}_{10}$. What's the structure of it? The problem is almost similar to your original question, except they don't tell you they obtained it from graphite, but it looks kinda like graphite anyway (a fine black powder), so shouldn't the logic be pretty much the same? What would you expect? Another graphite intercalation compound like your (d), only having slightly less potassium? Yeah, sure.