What are lewis structures?
In 1916, American chemist, Gilbert N. Lewis, introduced bond lines to electron dot
structures. These structures, also known as lewis structures or electron dot structures, are
drawings that visually demonstrate how electrons are shared and arranged around atoms.
The electrons denoted as dots are called lone pairs and belong to an individual atom.
Electrons denoted as lines are bonds and show the sharing of two electrons between two
atoms.
How to create a lewis dot structure
1. Find the total number of valence electrons.
To draw the lewis structure, you will need to know the total number of valence electrons
present. The number of valence electrons in an individual atom can be found based on the
atom’s group number in the periodic table. Below is a table that coordinates the group
number to the valence electrons. Groups 3-12, which are the transition metals, will only have
two valence electrons due to the 4s orbital shell being satisfied. Group 18, which is the
noble gases, will have a full octet.
Group Number
1
2
3-12
13
14
15
16
17
18
Number of Valence Electrons
1
2
2
3
4
5
6
7
8
If you are drawing the lewis structure for more than one atom, you will have to add them up.
For example:
1. O3 = 6×3= 18
​ Oxygen contributes 6 valence electrons per atom. Since there are three
oxygen atoms, there are 18 valence electrons present.
2. NaCl = 1+7= 8
​ Sodium contributes 1 electron.
​ Chlorine contributes 7 electrons.
3. NH4+= 5+ (1×4)- (1)
​ Nitrogen contributes 5 valence electrons.
​ Hydrogen contributes 4 valence electrons because there are 4 atoms
and each gives 1 electron.
​ Finally, since there is a positive charge, we will subtract 1 valence
electron. If it was negative, we would add 1.
2. Determine the central atom
Once you know the number of valence electrons present, you will now need to determine the
central atom. The central atom has a few key characteristics. Firstly, it is the atom that is
able to make the most bonds and therefore is ideal for being in the center. . Typically, it is
also the atom there is only one of, and is the least electronegative atom. You can find the
periodic trend for electronegativity here. One would never see fluorine as the central atom.
3. Form single bonds between the atoms and then place the remaining electrons on the atoms
as lone pairs.
Once we know the number of valence electrons present, as well as the position of the
atoms relative to the others, we can begin to form single bonds. After each atom has a
single bond, we will place the remaining number of valence electrons on the atoms.
Remember each dot is an electron and each line represents two electrons. It is vital to draw
the correct number of electrons. To make sure you have the correct amount, count the
number of electrons drawn and compare it to the total valence electrons that were
calculated prior.
4. Move electrons around to adhere to the octet rule.
Now that we have the correct number of electrons placed on the atoms, we will move them
around to form double or triple bonds. This is done because each atom wants to have its
valence shell filled with eight electrons. This is called the octet rule. Moreover, some atoms
have a set number of bonds they like to form. Below is a table demonstrating this.