Unshared electrons refer to outer (valence) electrons not part of a covalent bond. Shared electrons are those participating in a bond. Subtract the number of shared electrons (bonds x 2) from the number of valence electrons to discover number of unshared electrons.
Shared and unshared electrons are in the valence electron shell. Valence electrons form the “outside” of an atom and participate in bonding. It is important that shared and unshared electrons add up to the proper number of valence electrons.
Each bond represents two shared electrons. Salisbury University’s “Drawing Lewis Structures” illustrates this method. A molecule like NO2 is written as O=N-O and O-N=O. Each dash corresponds to a bond—a shared electron pair. O-N=O has a nitrogen (N) atom with six shared electrons, two from each bond.
Subtract Shared from Valence
For each atom, subtract shared electrons from number of valence electrons. Oxygen (O) has eight valence electrons. The left oxygen in O-N=O has 8 – 2 = 6 unshared electrons. The right oxygen has 8 - 2(2) = 8 – 4 = 4 unshared electrons. Nitrogen also has eight total valence electrons. In NO2 (O-N=O), the central nitrogen atom has 8 – 3(2) = 8 – 6 = 2 unshared electrons.