Write down the chemical formula for the compound. For example the chemical formula for hydrogen, a diatomic element, is H2.
Find each element of the compound on the periodic table and add together the number of electrons. For example, hydrogen is located in the upper left corner. It has one electron because the atomic number of hydrogen is 1. Since there are two hydrogen atoms in the H2 molecule, the total number of electrons is 1 + 1 = 2.
Know the order with which to write the molecular orbital designations. The order for the molecular orbitals is sigma 1s, sigma 1s star, sigma 2s, sigma 2s star, pi 2p, pi 2p star, and sigma 2p star. The s energy level can hold a maximum of 2 electrons, and the p energy level can hold a maximum of 4 electrons. When calculating the molecular orbitals, it is important to follow Hund's Rule, which states that the orbitals of equal energy have parallel spins before pairing. Since diatomic hydrogen has only two electrons, the molecular orbital is just sigma 1s. You can draw a line with an up arrow and a down arrow in order to represent this.
Understand the difference between bonding and antibonding molecular orbitals. The bonding orbitals, which do not have stars in their notation, represent constructive overlap. The antibonding orbitals, which have the stars in their notation, represent destructive overlap. When destructive overlap occurs, a nodal plane is formed. For example, in the representation for the oxygen diatomic molecule, the orbitals sigma 1s, sigma 1s star, sigma 2s, sigma 2s star, pi 2s, and pi 2s star are represented. Therefore, this molecule contains both constructive and destructive interference.
Use the molecular orbital notation to calculate the bond order of the molecule. If the bond order is zero, then the molecule or compound is not stable. The higher the value of the bond order is, the more stable that the molecule or compound is. The formula for the bond order is 1 / 2 (bonding electrons - antibonding electrons). For example, in the oxygen molecule, there are 10 bonding electrons and 6 antibonding electrons. Therefore, the bonding order is (10 - 6) / 2 = 2. Oxygen is stable.