The octet rule relates to how electrons rearrange themselves in sublevels to become more stable.

It states that atoms with sublevels that are full or half full are more stable than those that are not.

This means that in groups 6 and 11, the electron configurations are not how you would think they are. One electron from the s sublevel with the highest principal quantum number will jump to the d sublevel with the highest principal quantum number. This means that, in the case of group 6, the outer s sublevel will have 1 of 2 possible electrons, and the outer d sublevel will have 5 of 10 possible electrons. In group 11, the s sublevel would have (1/2), and the d would have (10/10).

This also means that elements in groups 7, 12, and 15 are more stable than would be presumed.