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Complete Guide To Drawing Lewis Dot Structures and Formal Charges

Lewis dot structure shows the bonds between atoms in a molecule as well as positions of lone pairs of electrons.


Lewis Dot Structures

Mayya's Tip:

  1. Calculate the total number of valence electrons in a molecule (group number = number of valence electrons)

  2. Draw out your molecule and put lines between atoms (bonds)

  3. Subtract electrons used in bonds (2 electrons per bond) from the total number of valence electrons

  4. Use the remaining electrons to make outer atoms happy (octet). Keep in mind hydrogen only wants 2 electrons.

  5. If there are any electrons left, place them on the central atom.

  6. Check if everyone is happy(octet). If not use neighboring atom's lone pairs to make double and triple bonds.


Lewis Dot Structure for CO2


how to draw CO2 lewis dot structure
how to draw CO2 lewis dot structure

Carbon is in group 14 and has 4 valence electrons. Oxygen is in group 16 and has 6 valence electrons. Altogether, we get 4+6*2 = 12 total valence electrons. Then we draw out carbon attached to two oxygens. Two bonds were used. Each bond is 2 electrons, which means we used 4 electrons. We can subtract 4 from 16 to get 12 electrons left. Now, we need to take care of the outer atoms (oxygens) which want an octet (8 electrons). We need to give each oxygen 6 more electrons to be happy. Now, there are no electrons left. However, carbon has two bonds only which is 4 electrons and it wants an octet (8electrons). We need to take a lone pair from each oxygen and make a double bond on each side for the carbon to have 8 electrons.


Lewis Dot Structure for COCl2


Lewis Dot Structure for COCl2
Lewis Dot Structure for COCl2

Lewis Dot Structure for Ions (charged molecules)

The rules for drawing Lewis dot structures for ions are the same as before except:

  • If the ion has a negative charge, add the number of electrons equal to the negative charge to the total number of valence electrons. For example if the charge is -1, add 1 electron. If the charge is -2, add 2 electrons and so on.

  • If the ion has a positive charge, subtract the number of electrons equal to the positive charge from the total number of valence electrons. For example, if the charge is +1, subtract 1 electron. If the charge is +2, subtract 2 electrons and so on.


Lewis Dot Structure for BF4-

Lewis Dot Structure for BF4-
Lewis Dot Structure for BF4-

Exceptions to the octet rule: Expanded Octet

Atoms of elements in period 3 on lower (4,5...) on the Periodic Table can have more than eight electrons. These include but are not limited to: P, S, Xe...

Let's take a look at an example:

Lewis Dot Structure for XeF4

Lewis Dot Structure for XeF4
Lewis Dot Structure for XeF4


Using Formal Charge to Determine the Best Lewis Dot Formula

Formal charge can be calculated using the following formula:

Formal Charge = number of valence electrons - (number of bonds + number of lone electrons).

If the central atom can have an extended octet, determine its formal charge once you wrote out the Lewis dot structure of the molecule. If there is a formal charge, use the lone pairs of attached atoms to add double bonds until all the formal charges are zeros.


Lewis Dot Structure for H2SO4

Lewis Dot Structure for H2SO4
Lewis Dot Structure for H2SO4

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