Molecular orbitals: LCAO and second-period diatomics

$$\psi_\pm = N_\pm(\psi_A \pm \psi_B)$$ The $+$ sign gives the bonding MO: more density between the nuclei, energy lower than the AOs. The $-$ sign gives the antibonding MO (starred, $*$): a nodal plane between the nuclei, energy higher. The number of orbitals is conserved: $k$ AOs give $k$ MOs.