Electron Promotion of Carbon [ENDORSED]

[Shannon Wasley 2J]

In the book, it says that an electron is promoted in carbon, in order for carbon to have four valence electrons. Why do we write the electron configuration as [He] 2s^2 2p^2, when for most carbons in a molecule, we promote an electron so it is [He] 2s^1 2p^3? In other words why don't we just write the electron configuration as [He] 2s^1 2p^3

[Chem_Mod]

Two things going on here which I will discuss in Monday's class.

One writes the electron ground-state for an atom which assists in figuring out valence electrons, drawing Lewis structures, etc. This we have already discussed in detail.

The bonding model, hybridization, we will discuss tomorrow, and here new (hybrid) orbitals are formed from the atomic orbitals during bond formation. This is not the electron ground-state for an atom.

You have not written the hybrid orbitals, which is okay, as we will discuss them in detail.

[Sohini Halder 1G]

We write it the first way when asked for the ground state configuration of just the carbon atom itself. However, you are right, in most cases carbon forms 4 bonds and when it is in these molecules, one electron is promoted to the 2p orbital, leaving 4 unpaired electrons to form 4 bonds. Since the bonds are usually the same energy, we hybridize, so carbon has sp3 hybridization.