A molecular orbital is the region in a molecule where the individual atomic orbitals overlap. This overlap may cause an overall decrease in the energy of the molecule (bonding orbital) or an overall increase in the energy of the molecule (antibonding orbital). This phenomenon is much like the additive nature of two sound waves passing through each other. If the two sound waves are in phase, they produce a third wave whose magnitude is the sum of the magnitudes of the two original waves. When discussing the phases of atomic orbitals the mathematical operators '-' and '+' are often used. If two atomic orbitals are in phase, a bonding molecular orbital forms. If the two atomic orbitals are of opposite phase, they cancel to produce an antibonding molecular orbital.
The overlap of two s subshells forms a sigma (s) bond. Sigma bonds are symmetrical about the axis joining the two nuclei. Like all atomic orbitals coming together to form molecular orbitals, the phase of the two 1s orbitals determines what type of bond will form. If the two 1s orbitals are in phase they will form a bonding molecular orbital. If they are not in phase they will form an antibonding molecular orbital. Antibonding molecular orbitals contain a nodal plane where there is zero probability of finding an electron.
