Lewis diagrams, octet rule

In A-level chemistry you are taught that second-row atoms tend to complete their outer shell of electrons when bonding. The second-row shell can fit 8 electrons, and this method is often called "the octet rule". You would also have learnt how to draw Lewis diagrams:

A single line can also represent a shared pair of electrons:

The octet rule is great way to feel like a knowledgeable chemist, until someone asks you to draw the structure of carbon monoxide. You could draw this:

But the carbon is still two electrons away from an octet. If that structure is allowed, then why aren't these?

The structures are allowed, but they would react vigorously when in contact with other atoms to produce states which satisfy the octet rule - lower energy states.

Carbon monoxide can be burnt to produce CO₂ - a lower energy state which satisfies the octet rule for both atoms - but it doesn't happen immediately. And there are no obvious ways to tell what less-stable states can still exist at RTP (room temperature and pressure) or not.

In conclusion, the octet is useful but not infallible. The sure method of knowing what bonds an atom will form and what their shape are requires using computers to calculate wave equations. The results of these are drawn using molecular orbital diagrams, described later.