In this tutorial, you will learn how to draw the correct lewis dot structures for elements and compounds. In addition, you will learn about valence electrons and the octet rule.
Topics Covered in other Articles
- Valence Electrons: Electrons in the outermost shell of an atom.
- Octet Rule: Elements prefer to have 8 valence electrons and will bond accordingly.
- Group Numbers: The numbers that run along the top of the periodic and contain atoms with the same chemical reactivity.
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.
|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:
- O3 = 6×3= 18
- Oxygen contributes 6 valence electrons per atom. Since there are three oxygen atoms, there are 18 valence electrons present.
- NaCl = 1+7= 8
- Sodium contributes 1 electron.
- Chlorine contributes 7 electrons.
- 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.
|Number of Bonds||4||3||2||1|
|Number of Lone Pairs||0||1||2||3|
Practice Examples: How to Draw a Lewis Structure
What is the lewis dot structure of boron?
When placing the dots, start by drawing them on separate sides until all four spots are filled. After they are all filled, then you can add more dots to the same side. Since boron has 3 valence electrons, there will be 3 dots all placed on differing sides
What is the lewis dot structure for hydrogen bromide (HBr)?
The total number of electrons would 8. There is a single bond connecting hydrogen and bromine. Since hydrogen is satisfied by only two electrons, the rest of the lone pairs will end up on bromine.
What is the lewis dot structure of carbon tetrachloride (CCl4)?
There are 32 valence electrons total in carbon tetrachloride. Moreover, carbon would be the central atom. There will be four single bonds connecting carbon to the chlorines. Since carbon has four bonds, it is satisfied by the octet rule. The remaining electrons will end up on the chlorine atoms.
Need more examples? Here’s a worksheet