In molecular orbital theory, the 1s orbital in the H2 molecule is combined to form a bonding molecular orbital known as the sigma (σ) orbital. This occurs when the two 1s orbitals from each hydrogen atom overlap constructively, allowing for the sharing of electrons between them. As a result, a new molecular orbital is created that is lower in energy than the original atomic orbitals, leading to increased stability of the molecule.
This bonding interaction allows H2 to exist as a stable diatomic molecule. Additionally, when we consider the two hydrogen atoms, one will contribute its 1s electron to the bonding orbital, while the other will also contribute its single 1s electron, resulting in a filled σ1s molecular orbital. This filling of the bonding molecular orbital is key to the stability of the H2 molecule, as it leads to an attractive interaction that overcomes the repulsive forces between the positively charged nuclei.