Let's look at CO₂. The Lewis structure is determined by trial and error.

• First thing to do is check the group numbers and count valence electrons
• Carbon is in group 4A. It has 4 valence electrons. Oxygen is in group 6A. It has 6 valence electrons. There are two oxygens The total valence electrons is 4 + 6 + 6 = 16. There is an even number of valence electrons. This means the octet rule should be met.
• Divide sixteen by 2 and get the number of electron pairs total, 8.

• Draw the symbols for the C as the central atom with oxygen atoms on the "outside".

• Place electron pairs between the atom symbols. This uses only four electrons, two pairs.

• Place electron pairs around the atom symbols. This uses all the electrons, but there is no octet on oxygen.

• Move unshared pairs from the carbon to create multiple bonds between the carbon and oxygens.

Count the electrons on each atom to check for an octet on each. The carbon has an octet as do the oxygen atoms. The double bonds do the job.

MAJOR IDEA:

Combinations of atoms with the same number of atoms and electrons will have the same type of Lewis structures. Carbon monoxide and nitrogen both have ten valence electrons and two atoms. They both have a triple bond. Do you expect CN^1- cyanide ion with 10 valence electrons to have a triple bond between the C and the N?