H₂:
* Hydrogen has only one electron per atom.
* When two hydrogen atoms come together, their atomic orbitals combine to form two molecular orbitals: a bonding orbital and an antibonding orbital.
* The bonding orbital is lower in energy and is filled with two electrons, forming a stable H₂ molecule.
* The antibonding orbital remains empty.
He₂:
* Helium has two electrons per atom.
* When two helium atoms come together, they also form bonding and antibonding orbitals.
* Both the bonding and antibonding orbitals are filled with two electrons each.
* The filled antibonding orbital cancels out the stabilizing effect of the filled bonding orbital, resulting in no net bond formation.
In summary:
* H₂ forms a stable molecule because the bonding orbital is filled with electrons, leading to a net attractive force.
* He₂ does not form a stable molecule because the antibonding orbital is also filled, canceling out the bonding effect.
Other factors:
* Pauli Exclusion Principle: The Pauli Exclusion Principle states that no two electrons in an atom can have the same set of quantum numbers. This principle prevents the formation of He₂ because the two electrons in each atom would need to occupy the same energy level.
* Interatomic repulsion: The repulsion between the positively charged nuclei of the helium atoms also contributes to the instability of He₂.
Therefore, the combination of molecular orbital theory, Pauli Exclusion Principle, and interatomic repulsion explains why H₂ exists but He₂ does not.
