The diagram we have studied shows the most stable ground state of a hydrogen molecule, in which the electrons have the lowest possible energy. But what happens if an electron is pro moted up from the lowest energy level, the bonding MO, to the next lowest energy level, the antibonding MO? Again, an energy level diagram helps.

Now the electron in the antibonding orbital cancels out the bonding of the electron in the bonding orbital. Since there is no overall bonding holding the two atoms together, they can drift apart as two separate atoms with their electrons in 1s AOs. In other words, promoting an electron from the bonding MO to the antibonding MO breaks the chemical bond. This is difficult to do with hydrogen molecules but easy with, say, bromine molecules. Shining light on Br2 causes it to break up into bromine atoms.