LEWIS DOT STRUCTURE
The steps below outline general guidelines for drawing the Lewis dot structures for covalent compounds and polyatomic
ions with a central atom. This theory of bonding will become a powerful tool for understanding how chemical
substances behave physically and chemically. Three representative examples are provided as reference.
1. Count Valence Electrons: Using the molecular formula, find the total number of valence electrons:
Add up valence electrons for all atoms. We will most typically work with main groups elements, with valence
electrons in the outer most s & p orbitals.
For each negative (-) charge, add one electron to the total.
For each positive (+) charge, subtract one electron from the total.
HINT: For the vast majority of stable molecules or ions we encounter, the total number of valence electrons will be EVEN.
While molecules with odd numbers of electrons are found, they often are highly reactive radicals with unpaired electrons.
CH3CH2OH
1 Carbon: 2 x (4 valence electrons)
6 Hydrogens: 6 x (1 valence electron)
1 Oxygens: 1 x (6 valence electrons)
Total: 8 + 6 + 6 = 20 electrons
NO3-
1 Nitrogen: 1 x (5 valence electrons)
3 Oxygens: 3 x (6 valence electrons)
Monoanion: 1 extra electron
Total: 5 + 18 + 1 = 24 electrons
NO2-
1 Nitrogen: 1 x (5 valence electrons)
2 Oxygens: 2 x (6 valence electrons)
Monoanion: 1 extra electron
Total: 5 + 12 + 1 = 18 electrons
2. Determine Connectivity: Decide how the atoms are connected
together. In some cases this will be given, as in CH3CH2OH. Here the formula
suggests the three hydrogens are associated with one carbon, which is
attached to another carbon with two bonded hydrogens, which is then
connected to an oxygen which has an associated hydrogen.
When the connections are not provided based on the formula, use the general rules below:
Place the LEAST electronegative element in the center, surrounded
by the other atoms. With a weaker pull on electrons, the less
electronegative element can afford to share more electrons than
elements with higher electronegativities (which would be placed on
the outside of the molecule or ion).
Hydrogen will never be in the center. It can only form a duet of
electrons maximum (1s2), and therefore can only form 1 bond with
another element.
3. Connect the central atoms to the outer atoms with single bonds.
Each single line represents the sharing of 2 electrons; without sharing electrons, the atoms will not be associated with
one another in a covalent molecule.
Each single line corresponds to a bond order of 1. All else being equal, the higher the bond order, the strong the
covalent bond and the short the bond length. A bond order to 2 would represent the sharing of 2 electron pairs (or 4
electrons total). A bond order of 3 would represent the sharing of 3 electron pairs (or 6 electrons total). Each of the
bonds above currently have a bond order of 1. Proceed to the next steps to learn how higher bond orders are possible in
Lewis dot structures.
