How do you draw molecular formations?

Question

I do not understand this at all and I have an exam tomorrow...I've been trying to figure out MO formations for awhile but I just don't understand them. Any help would be greatly appreciated! This is the question from the practice exam...if you could explain how to do it to me, that would be great!

Acetylene C2H2 has a tendency to lose two protons and form the carbide anion (C2 2-). The carbide ions can be found in compounds such as CaC2 and MgC2. Construst a fully labelled molecular orbital diagram for the formatio nof the carbide anion C2 2-.

Answer 1

I will try to explain it:

when you're doing MO diagrams its kinda like doing an orbital spin diagram with the up and down arrows except you're counting the Atomic Numbers in the Compound.

In your problem it only uses the S and P orbitals so your diagram wouldn't have any D or F orbitals: (the * shows if it is antibonding)

2p* (sigma) ______
2p* (Pi) _______ _______
2p (Pi) _______ _______
2p (sigma) ______
2s* (sigma) ______
2s (sigma) ______

The Lines show where you would put the up and down arrows. The Pi and Sigma represent the axis of the compound. the Pi would be the z and y axis. and the sigma would be the x axis. (not to sure which is which on the axis). But thats why there are two lines for Pi and one for Sigma.

For your practice question it wants u to construct C2 -2, well i would count the Atomic number of Carbon twice. Which is 6.

6(2) = 12
Then the add the anion (-2)

12+2: 14 so there would be a total of 14 up and down arrows you will put on the diagram.

When you are putting the arrows you start from the bottom up. I can't type in arrows that point up and down. Just remember when your are filling up a the Pi area put the arrows pointing up first on each line then if any down arrows are left add those.

I dont know if I explained it well.