The Octet rule is not valid for which molecule - 

Concept: 

Octet theory:

• Kössel and Lewis in 1916 developed this theory.
• This theory of chemical combination between atoms known as the electronic theory of chemical bonding.
• According to this, atoms can combine either by transfer of valence electrons from one atom to another (gaining or losing) or by sharing valence electrons in order to have an octet in their valence shells.
• This is known as the octet rule.
• The elements tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas.

Explanation:

H2O

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• In water, the center atom of oxygen is sp3 hybridized.
• There are two bond pairs and two lone pairs and thus the oxygen atom has a total of eight electrons. Thus, its octet is satisfied.

CO2

• The central atom Carbon has four bonds which means 4 × 2 = 8, electrons.
• Hence, in CO₂, the octet rule is followed.

O₂:

• In Oxygen, there exist two bonds between each O atom, and they have two lone pairs each, so there are 4 pairs of electrons, which means 4 × 2 = 8 electrons on each O atom.
• Hence, the oxygen molecule satisfies the octet rule.

CO:

• In CO molecule, the carbon atom contains only six valence electrons whereas eight electrons are required to fulfill its octet.
• Hence, CO molecule does not have an octet.

Hence, the Octet rule is not valid for CO molecule.