Exploring Covalent Bonds: A Deep Dive into the Basics and Complexities

[misko]

I have few questions trying to understand covalent bond in the simplest molecule H₂ of two Hydrogen atoms.

1. Can covalent bond be created between two hydrogen atoms for which electrons have parallel spin? That is, electron in first hydrogen atom has spin UP and electron in the second hydrogen atom has also spin UP.

2. What determines whether covalent bond will be created with bonding or antibonding state?

3. Since antibonding state has higher potential energy than the two individual neutral hydrogen atoms, then how it can ever be created?

4. This final question is about covalent bond in multielectron atoms... Say we have nitrogen molecule N₂ which has 3 covalent bonds. I understand that 3 pairs of electrons in the outer (valence) shell form three covalent bonds but are inner shell involved in covalent bonding as well? That is, 1s and 2s orbitals (nitrogen electron configuration is: 1s² 2s² 2p³).
I am confused about this because at first I thought that only valence electrons form covalent bonds but in my book there is a graph for this molecule that shows that beside valence bonds there are also two sigma states (one bonding marked as σ1s and one antibonding marked as σ*1s).

 

[DrDu]

There is no such thing as an anti bonding state

 

[misko]

https://en.wikipedia.org/wiki/Antibonding_molecular_orbital

 

[DrDu]

What I am saying is that you have to distinguish between antibonding orbitals (which are a theoretical construct) and whether the molecules can be in an antibonding state (which is not the case).

 

[misko]

Ok I acknowledge that. Do you know answers to my questions?

 

[DrDu]

1. Can covalent bond be created between two hydrogen atoms for which electrons have parallel spin? That is, electron in first hydrogen atom has spin UP and electron in the second hydrogen atom has also spin UP.

4. This final question is about covalent bond in multielectron atoms... Say we have nitrogen molecule N₂ which has 3 covalent bonds. I understand that 3 pairs of electrons in the outer (valence) shell form three covalent bonds but are inner shell involved in covalent bonding as well? That is, 1s and 2s orbitals (nitrogen electron configuration is: 1s² 2s² 2p³).
I am confused about this because at first I thought that only valence electrons form covalent bonds but in my book there is a graph for this molecule that shows that beside valence bonds there are also two sigma states (one bonding marked as σ1s and one antibonding marked as σ*1s).

I think I already answered your questions 2 and 3. Sorry if my answer was a little bit condensed, but I answered from my phone.

Concerning 1, two hydrogens with anti-parallel spin will always repell each other. Hence there is no covalent bonding.

Question 4 is the most difficult to answer. The two 1s states will repell each other, but, as they are more compact than the valence orbitals, this repulsion can be overcome by the bonding interaction of the valence electrons. The repulsion of the 1s functions is due to the fact that the antibonding orbitals are so to say more antibonding than the bonding orbitals are bonding, so that a net repulsion remains even if both are filled.
There is also another point: If both the bonding and anti-bonding orbitals formed from a set of atomic orbitals (e.g. the 1s orbitals) are completely filled, then the molecular wavefunction can also alternatively be described as being made up of the atomic orbitals without combining them first into molecular orbitals. Hence it looks like the electrons in the 1s orbitals are little influenced by the bonding of the valence electrons.