Question

The formation of a covalent bond between two hydrogen atoms results in a more favorable overall energy level. True or false?

Answer 1

For a stable covalent bond to form, the molecular orbital into which the bonding electrons will sit must be at a lower energy level to that of the atomic orbitals of the two atoms from which these electrons originated.

This has got to do with something we call the Linear Combination of Atomic Orbitals (LCAO). It's essentially a way of describing how outer shell electrons of atoms are shared into molecular orbitals in forming a covalent bond. It's rather complicated to explain everything about it now, but here's the main points:



We know that each neutral hydrogen atom has 1 electron, sitting in the lowest available energy orbital, the 1s orbital. Both of these atomic orbitals are at the same energy as each other, because they are identical. The lowest possible/available molecular orbital (the orbital to which the shared electrons will sit in forming the covalent bond) is what we call the "1 sigma g" bonding orbital, and it is accompanied (as with all molecular orbitals) by an anti-bonding "1 sigma star u" orbital. Both of these also hold two electrons, like the 1s atomic orbitals.

The Aufbau Principle states that if more than one energy level was available, the electrons will initially fill the orbital at the lowest energy before moving on to fill the ones with higher energy. So, each H atom will share an electron into the "1 sigma g" bonding orbital, as this is the orbital with the lowest available energy. As you can see, this orbital, and indeed any "bonding" molecular orbital, will have a lower energy than the atomic orbitals which are used to fill it, while the "anti-boding" molecular orbital will always be at a higher energy. In this case, we have no electrons left to share to fill the anti-bonding orbital.

So, as the two H atoms will want to exist at the lowest possible energy level, it will be favorable for them to share their electrons into a more stable molecular orbital, making a single, "sigma" covalent bond favorable to form. Hope that helped you! :)