[Nicos]

According to my book the electron configuration of carbon should be: 1s² 2s² 2p_x¹ 2p_y¹ 2p_z⁰, because "you have to put the two electrons of 2p into different p orbitals with the same spin". But why are 2p_x and 2p_y of the same spin?

Thanks in advance.

[fledarmus]

It isn't the p-orbitals that have the same spin, it is the electrons. Rather than putting, say, one spin-up and one spin-down in the 2p_x orbital, you would put one spin-up in 2p_x and one spin-up in 2p_y

[Nicos]

Thanks, but could the electron configuration of carbon be written as

1s² 2s² 2p_x¹ 2p_y⁰ 2p_z¹

rather than

1s² 2s² 2p_x¹ 2p_y¹ 2p_z⁰?

I don't see why not. But perhaps I'm wrong, so if not, why not then?

[fledarmus]

Yes, all of the p orbitals in the same level have the same energy. It is just a convention to label them x, y, and z, and fill them in the order x, y, and z. The choice of axes is totally arbitrary.

On the other hand, it is a convention and conventions are usually there for a reason. It makes it easier to recognize familiar conformations at first glance. Although there may be a good reason for showing electrons in p_x and p_z, especially if you are trying to make specific points dealing with the geometry of the binding, to do it at random just to be different only reduces the precision of your communication. And science is all about precision.