Valence electrons are the electrons on the outer shell of an atom that are responsible for the chemical properties of the atom. So knowing the Knowing the number of valence electrons of an element is of crucial importance to draw Lewis diagrams.
To know how many valence electrons does an atom have we need to look at the periodic table. Remember that in any given column, all elements have the same number of valence electrons. To get that number, all we do is count columns start from the left. Skip the transition metals and remember that the only exception to this is helium, which has 2 valence electrons, not 8.
For example, to find the number of valence electrons for the chlorine atom, start by finding it in the periodic table. Cl is in the column 17, so it has 7 valence electrons.
Lewis diagram of an atom
Dots are used to represent valence electrons. So for example the Lewis diagram for chlorine is the symbol Cl with 7 points around it because this atom has 7 valence electrons. But to put these dots in a correct way imagine a box around the element’s icon. Points must be plotted precisely on all four sides of the square with no more than two dots on either side. Where you put the points doesn’t really matter as long as you draw it precisely along the sides of the imaginary square and don’t place more than two points on one side.
Hydrogen has only one valence electron, Carbon has four valence electrons, and Oxygen has 6 valence electrons.
Lewis diagram of a molecule
In a covalent bond, atoms share valence electrons in order to get a full eight or a duet of electrons, that’s every non-metallic element that wants 8 valences electrons, except for hydrogen, which only wants two valence electrons.
The simplest possible molecule is from two hydrogen atoms which is H2. A hydrogen atom contains one valence electron, but he wants to have two. So, in order to fulfil his desire for another electron, two hydrogen atoms will share electrons with each other. The crazy thing is that the shared electrons are counted as owned by both atoms. This means that both hydrogen atoms are stable because they both have 2 electrons so the first shell is full.
Now, to draw Lewis structure of a molecule, lines are used to represent shared electrons and points or (dots) are used only for non bonding electrons. Lines separate the symbols of different atoms in the molecules to show how electrons are shared. For the hydrogen molecule, the two hydrogen atoms are separated with one line showing two shared electrons, there is no dots because in case of H2 molecule there no non bonding electrons.
Lewis Diagram for H2 molecule : H – H
Types of bonds
Single Covalent bond
After the example of H2 molecule, we can say that one line means one bond and it represents two shared electrons.
Double covalent bond
In the same way, 2 lines between atoms will be a double bond and 4 electrons will be shared. Example of a double bond molecule it is the oxygen molecule.
Lewis Structure of the O2 molecule is represented in the following photo
Note that all of the oxygen atoms have 8 valence electrons, 4 of the double bond and 4 of the lone pairs of electrons*.
*Lone pairs of electrons: 2 points together.
Triple covalent bond
Some molecules contain triple bonds, which is represented using 3 lines between the symbols of the atoms. A triple covalent bond means that there is 6 shared electrons. The Nitrogen molecule is an example of a triple bond molecule. Again, note that both Nitrogen atoms have 8 valence electrons, 6 of the triple bond and 2 of the lone pairs of electrons.
Practice – How do you actually draw a Lewis diagram of a molecule?
Let’s start with water, H2O. There are 5 important steps you need to trace when drawing a Lewis diagram of a molecule.
Step 1: Count all the valence electrons in the molecule. For water, hydrogen has one electron and we multiply that because there are two hydrogens in the molecule. Oxygen has 6 valence electrons, add it all up, and we have a total of 8 valence electrons in a water molecule.
Step 2: Determine the central atom. The central atom is that each of other atoms will be linked. It is usually the only element that is present only one time. In the case of H2O, because there are 2 hydrogen atoms and the oxygen is only one, we choose the oxygen a central atom and write it in the middle.
Step 3, draw the individual bonds of the central atom. A single covalent bond is drawn between the atoms. One single bond is between the Oxygen atom and the first Hydrogen, the second single covalent bond is between the Oxygen atom and the second Hydrogen atom.
Step 4, put all remaining valence electrons in the atoms are like lone pairs. For H2O, we started with 8 valence electrons; We have used four electrons for the two bonds, so that leaves another 4 electrons, so we put all 4 of those remaining electrons on oxygen instead of hydrogen because hydrogen is already happy with 2 valence electrons.
Step 5, convert lone pairs to double or triple bonds to give each atom eight or (hydrogen duet). Because all hydrogen has 2 electrons and Oxygen has 8 electrons, there is no need to double or triple
bond, which means that our Lewis diagram of water is now complete.
More Lewis Diagrams for different molecules