[Naples]

Determine the oxidation number of the atom in bold.
1. Cr₂O₇^-2  Cr=+7, I'm not sure if this is right because on my oxidation states table says the possible oxidation numbers for Cr are +1 to +6
2. Fe₂O₃ Fe=+3
3. NiCl₃ Ni=+3
4. P₄O₆ I have absolutely no idea how to determine the oxidation number of an atom in a covalent bond...


Draw the structure, state the geometry and determine the polarity of the compound.
C₂H₄O
I have the structure (below) and I know that the compound is polar, I'm just not sure of the geometry.
   H  H
    l   l
H-C-C=O
    l
   H

Any help is greatly appreciated!

[Ida Isotope]

Does it specify the geometry of a certain element? Or does it just ask the geometry of the whole molecule?

[Naples]

Draw the structure, state the geometry and determine the polarity of the compound.

That's exactly what it says on the paper, so I believe that it's referring to the geometry of the whole molecule.

[UG]

Determine the oxidation number of the atom in bold.
1. Cr₂O₇^-2  Cr=+7, I'm not sure if this is right because on my oxidation states table says the possible oxidation numbers for Cr are +1 to +6
2. Fe₂O₃ Fe=+3
3. NiCl₃ Ni=+3
4. P₄O₆ I have absolutely no idea how to determine the oxidation number of an atom in a covalent bond...

1. The oxidation number of oxygen in any compound (except peroxides) is -2.
7 x -2 = -14 so the oxidation number of the 2 chromium atoms must equal +12 (since the ion Cr₂O₇^2- has a 2- charge). Therefore, each chromium atom has O.N +6
2. Correct
3. Yep
4. The O.N of oxygen is -2, the compound is neutral, you should get it now 

[Naples]

1. The oxidation number of oxygen in any compound (except peroxides) is -2.
7 x -2 = -14 so the oxidation number of the 2 chromium atoms must equal +12 (since the ion Cr₂O₇^2- has a 2- charge). Therefore, each chromium atom has O.N +6
2. Correct
3. Yep
4. The O.N of oxygen is -2, the compound is neutral, you should get it now 

Thank you for the detailed explanation, it was very helpful.

Now I just need someone to help me with the second problem.

[Schrödinger]

Since the diagram in demand is a 3-D one, it's going to be hard for me to post it. Anyway, i can help you with it.

First, consider the hybridization of each Carbon atom in the molecule and then decide the geometry with respect to each of the 2 carbon atoms.

I hope you can visualise the structure.

The C=O carbon is sp² hybridized and the second carbon will be in the same plane as the first. The second Carbon, Oxygen and hydrogen on the first carbon will lie in the same plane.(Trigonal planar).

With respect to the second carbon atom(the one that isn't attached to Oxygen) which is sp³ hybridized, the structure is tetrahedral.

I don't think there is any distortion in the usual bond angles.
120⁰ for the planar structure and 109.5⁰ for the tetrahedron.

[MrTeo]

Here is my try to provide a visual aid to Schrödinger's explanation...
See what you can get from it 

[UG]

Here's my visual aid