How to draw a Lewis diagram using formula

Note: drawing a Lewis diagram of all molecules is not applicable to this set of rules listed below. But, it works well with all molecules, having their total valence electron as an even number, i.e. the total number of valence electron must be divisible by 2. Hence, the total valence of electron must be equal to 2, 4, 6, 8, 10…. In other to follow this rules. Now, let look at the Lewis diagram of NF3 as an example in this post.

Step 1: Find the total number of valence electron

To calculate total valence electron, you have to look at the valency state of individual atom or element on the periodic table. Then sum it up to give the total valence electron.
Total valence electron of NF3 = 26

Step 2: Calculating the lone pair

To calculate lone pair you need to use this formula
Where n = no of terminal atoms
Tv = total valence electron
Once that is done let move to the next stage.

Step 3: Select a central atom

To choose or select the central atom of a molecule. We have to consider it electronegativity values or use the periodic table follow the rules, i.e. electronegativity decrease down the group on the periodic table and increase across the period. For the case of NF3, Fluorine with electronegative value of 4.0 have the highest electronegative value. Hence nitrogen is the least electronegative atom between fluorine and nitrogen. Making nitrogen the central atom, i.e. it have the lowest electronegativity value of 3.0.

Step 4: Drawing a sketch

Once you have your lone pair, no of terminal atom, set your central atom. Then you can start the drawing by placing each lone pair where it belongs and draw a straight line indicating the bond between the two atoms.

Step 4: Balancing electron

Complete the octet around the terminal and central atoms, i.e. each atom must have an octet configuration. Hence, we say fluorine is equal to 8; nitrogen is equal to 8. Making an octet configuration. But if the electrons are not balanced then you have to rearrange. Note, drawing a Lewis diagram involves some trials and error. But the more you are familiar with it; the more you know it.