# If I have two atoms that come in with one half-filled orbital each, do they make a #sigma# bond?

##### 1 Answer

Well, yeah. If you have two atoms each with a singly-occupied atomic orbital, if they come together and have ideal orbital overlap to form molecular orbitals, the number of electrons is conserved and the number of orbitals is conserved.

If **each** atom comes in with **one** electron in an atomic orbital, then the molecule must have a total of **two** electrons contributed *in total* from those two atoms (and **two** molecular orbitals result, one bonding and one antibonding).

(Here, the bonding orbital is the

#sigma_(1s)# , and the antibonding,#sigma_(1s)^"*"# .)

If **two** electrons, **one** in *each* atomic orbital, start off *unpaired*, they *become paired* (if and only if the degeneracy

And in fact, for the **first covalent bond formed in any molecule**, one in-phase and one out-of-phase **overlap** (head-on overlap) occur, which forms two molecular orbitals each with a degeneracy of

*So, by the definition of degeneracy, there is only one sigma molecular orbital in its own energy level, and thus the two electrons have to go in the same #sigma# molecular orbital before going into the*

*.*

**Footnote:**