[magdi_gamal]

Hey guys, I've got a chemistry exam tomorrow and I have a massive confusion.

In Ca(OH)₂
When I do the lewis structure for the (OH)₂
H-O-O-H and distribute the 14 valence electron it fits perfectly, so the oxidation state should be 0.

But when I do the math, Oxygen has an oxidation state of (O^2-H⁺)₂ = -2
Which add up to the calcium oxidation state but doesn't fit the lewis structure above! What exactly am I doing wrong?

Same goes with Aluminium oxide Al₂O₃.
Ozone should have an oxidation state of zero, right? The electron distribution fits perfectly in the lewis structure O=O-O so how does it form an ionic bond with aluminium which has a positive 3 oxidation state?

[Corribus]

It's hard to know where to start - you've got some fundamental misunderstandings about basic chemistry. Let's start with: the (OH)₂ in Ca(OH)₂ is not hydrogen peroxide. It is TWO hydroxide ions, OH^-. Likewise, O₃ in Al₂O₃ is nothing even close to ozone (just as the Al₂ is not a pair of covalently bonded aluminum atoms). They are three independent O^2- ions.

[magdi_gamal]

I see, thanks a lot.
So is there way to tell just by the formula whether we're talking about covalently bonded or separate atoms? I used to assume that a sub-number to the right suggests a bonded atom. Is that another misconception I have? Or is it actually true but it's still subnumbered in this case because we have an ionic bond?

[Borek]

Subscripted number tells only how many atoms (groups in parentheses) are there. Nothing more.

[Nitishajack]

(OH)2 means that there are two OH groups present. Each one is separate entity and bears the charge of -1. So we can interpret Ca(OH)2 as Ca2+ ion attracting the 2 OH- ions.
The mistake you made is that you assumed that OH groups are bound together via oxygen, which is not the case for bases. (If that were the case, it would be written as (H2O2) and not as (OH)2).

[Corribus]

So is there way to tell just by the formula whether we're talking about covalently bonded or separate atoms?

Unfortunately, no, not really, because of the reason Borek pointed out. There is some chemical intuition involved, and also familiarity with certain ions. Generally, though, when a set of atoms is grouped together in a formula for a salt, it means they are discrete units, usually complex ions, that are bonded internally. So in the case of Ca(OH)₂, the OH's are discrete complex ions (OH^-) that are not bound covalently together (i.e., one OH is not bound to the other OH in any covalent way), BUT the O and the H within a single OH are bound covalently together. On the other hand, if you had a hypothetical Ca(O₂H₂) substance, then you could say that the O's are likely bound together covalently to form a complex ion like [H-O-O-H]^2-. Thus, the use of parentheses can be your guide.

(These rules don't apply to organic molecules, the formulae of which also use parentheses.)