Drawing Lewis structures can be a straightforward process if the proper steps are followed. There are several different strategies to constructing Lewis structures. These instructions outline the Kelter strategy to draw Lewis structures for molecules.
Step 1: Find the total number of valence electrons.
In this step, add up the total number of valence electrons from all the atoms in the molecule.
Step 2: Find the number of electrons needed to make the atoms "happy".
An atom is considered "happy" if the atom's outer electron shell is filled.
Elements up to period four on the periodic table need eight electrons to fill their outer electron shell. This property is often known as the "octet rule".
Step 3: Determine the number of bonds in the molecule.
Covalent bonds are formed when one electron from each atom forms an electron pair. Step 2 tells how many electrons are needed and Step 1 is how many electrons you have. Subtracting the number in Step 1 from the number in Step 2 gives you the number of electrons needed to complete the octets. Each bond formed requires two electrons, so the number of bonds is half the number of electrons needed, or
(Step 2 - Step 1)/2
Step 4: Choose a central atom.
The central atom of a molecule is usually the least electronegative atom or the atom with the highest valence. Hydrogen and halogen atoms tend to appear on the outside of the molecule and are rarely the central atom.
Step 5: Draw a skeletal structure.
Connect the atoms to the central atom with a straight line representing a bond between the two atoms. The central atom can have up to four other atoms connected to it.
Step 6: Place electrons around outside atoms.
Complete the octets around each of the outer atoms. If there are not enough electrons to complete the octets, the skeletal structure from step 5 is incorrect. Try a different arrangement.
Step 7: Place remaining electrons around the central atom.
Complete the octet for the central atom with the remaining electrons. If there are any bonds left over from Step 3, create double bonds with lone pairs on outside atoms. If there are more than eight electrons on the central atom and the atom is not one of the exceptions to the octet rule, the number of valence atoms in Step 1 may have been counted incorrectly.
This will complete the Lewis dot structure for the molecule. Check out Draw a Lewis Structure of Formaldehyde for an example problem using this process.