how do i figure out when is something paramagnetic?

Question

like in this ex:
what is the magnetism and # of unpaired electrons of B2?
what i got from the solution is it is paramagnetic and 3 but i dont know how

Answer 1

If you have electrons in a partly filled subshell, they will tend to line up with their spins pointing in the same direction. This is because electrons with the same spin keep out of each other's way more effectively. Notice that this can only happen when the subshell contains more than one orbital, because of the exclusion principle.

In your case, you have two electrons in the pi-bonding subshell. So you can put each electron in a different pi-orbital (because there are two of them) and they will have their spins pointing in the same direction.

So B2 is paramagnetic and the number of unpaired electrons is 2.

I am not sure where your 3 came from. There is a number called the "multiplicity", which is 3 for B2; perhaps that is what you were thinking of.