Certain steps must be followed in assigning a Lewis structure to a molecule. These steps are
outlined below, using HCN as an example.
Write the skeletal structure of the compound (i.e., the arrangement of atoms). In general, the
least electronegative atom is the central atom. Hydrogen (always) and the halogens F, Cl, Br,
and I (usually) occupy the end position.
In HCN, H must occupy an end position. Of the remaining two atoms, C is the least
electronegative, and therefore occupies the central position. The skeletal structure is as
follows:H C N
Count all the valence electrons of the atoms. The number of valence electrons of the molecule
is the sum of the valence electrons of all atoms present:H has one valence electron;C has four valence electrons;N has five valence electrons; therefore,HCN has a total of ten valence electrons.Draw single bonds between the central atom and the atoms surrounding it. Place an electron
pair in each bond (bonding electron pair).H : C : N
Each bond has two electrons, so 10 – 4 = 6 valence electrons remain.Complete the octets (eight valence electrons) of all atoms bonded to the central atom, using
the remaining valence electrons still to be assigned. (Recall that H is an exception to the octet
rule since it can have only two valence electrons.) In this example H already has two valence
electrons in its bond with C.