The next rung: two hydrogen atoms, now bonding. Their 1s orbitals overlap and combine into two molecular orbitals — a σ bonding orbital that piles electron density between the nuclei, and a σ* antibonding orbital with a node there. Both electrons drop into σ; the pile-up screens the proton–proton repulsion and lowers the energy. That drop is the bond — 436 kJ/mol, released as the atoms settle to 74 pm apart. Slide them together and watch the well.
| bond length | rₑ = 74 pm (0.74 Å) |
| bond energy | D₀ = 436 kJ/mol (4.52 eV) |
| well depth | Dₑ ≈ 458 kJ/mol (zero-point sits above the floor) |
| MOs | σ (bonding, 2 e⁻) · σ* (antibonding, empty) |