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How to draw Lewis structures.

How to draw Lewis structure of compounds.

Lewis structure is a branch of organic chemistry that describes the graphical representation of how atoms are connected in a given molecule. Therefore, knowing how to draw Lewis structures is very important for a chemistry student.

History of Lewis’s structure.

 Lewis structure was first introduced by sir Gilbert N. Lewis in 1916. He was a nominee for the Nobel prize but did not win any. He discovered how molecules are connected via covalent bonds. Knowing how to draw Lewis structures can be very straightforward if you master the steps we are going to discuss below.

Importance of knowing how to draw Lewis structures.

As mentioned before, knowing how to draw the Lewis structures of the molecules has many benefits, including;

  • Molecular geometry- Molecular geometry is the shape of the molecule without considering the lone pair of electrons. By drawing the Lewis structure of molecules, you can easily predict the molecular geometry of a molecule.
  • Electron geometry- Electron geometry is the shape the electrons take in a molecule. One can also determine the electron geometry by looking at the Lewis structure of the molecule.
  • Intermolecular forces: The intermolecular forces of a molecule can best be described by the Lewis structures.
  • Orbital diagrams- drawing of orbitals around a molecule can only be done by knowing Lewis structures.
  • Determining the oxidation state of each atom in a molecule.
  • Polarity- Lewis structure helps to determine the polarity of a given compound.

Steps On How To Draw Lewis Structures;

Find the total number of valence electrons in the molecule.

In this step, all you have to do is add up the total valence electrons of atoms in a given molecule. Let’s use H2O to show how the steps can be useful in determining the Lewis structures. 

H2O has a total of 8 valence electrons

Whereby

Oxygen has 6 valence electrons, while each hydrogen has 1 valence electron.

Find the number of electrons needed to complete the octet (stable) in each atom.

In this step, you have to ensure that each atom has 8 electrons around it. This is called an octet. However, there is an exception for hydrogen because it needs only 2 electrons to be stable.

Now, in our case, we have water. So hydrogen will have 2 electrons while oxygen will require 8 electrons.

Determine the central atom.

The central atom is the atom that forms the most bonds in the molecule. For example, in H2O, Oxygen forms 2 bonds while each hydrogen has 1 bond.

Another way to know the central atom is that it is usually the least electronegative atom. An example is CCl4, where C is the central atom.

Draw the skeletal structure

Now, we know the central atom and how the other atoms are connected to it. Use a straight line to represent covalent bonds. In our case for water, the central atom is oxygen, while the two hydrogens are each connected to it.

Place the lone pairs around the atoms.

Now, this is the last and crucial part. You have to place the lone pairs around the atoms, whether it is the central atom of the atoms bonded to the central atom. In our case, oxygen has already used two electrons to form a bond.

A single covalent bond is formed by 2 electrons. One electron is from Hydrogen, and the other is from oxygen. Since they are two of them, it means that Oxygen has used its 2 electrons. Therefore, water will have 2 lone pairs (4 electrons) around the central atom while hydrogen will not have any.

In the case of CCl4, the central atom is C and will not have any lone pairs, but each Cl will have 3 lone pairs around it.

Below are examples of common Lewis structures of different compounds.

  • Lewis structure of water, H2O
  • Total valence electrons of H2O: 8 electrons
  • O + H + H = 6e- + 1e- + 1e- = 8e-
  • The central atom is Oxygen (because it forms most bonds)
Lewis structure of  water
  • Lewis structure of Carbon dioxide, CO2
  • Total valence electrons of CO2: 16 electrons
  • C + O + O = 4e- + 6e- + 6e- = 16e-
  • The central atom is Carbon
Lewis structure of  carbon dioxide
  • Lewis structure of Ammonia, NH3
  • Total valence electrons of NH3: 8 electrons
  • N + 3H = 5e- + (3 x 1e-) = 8e-
  • The central atom is N
Lewis structure of  ammonia
  • Lewis structure of Oxygen, O2
  • Total valence electrons of O2: 12 electrons
  • The central atom of O2, it can be any oxygen atom.
Lewis structure of  oxygen
  • Lewis structure of Ozone, O3
  • Total valence electrons of O3:18 electrons
  • O + O + O = 6e- + 6e- + 6e- = 18e
  • The central atom of O3=one of the O atoms
  • O3 has two resonance structures, as shown below.
Lewis structure of  ozone
  • Lewis structure of Nitrate ion, NO3
  • Total valence electrons of NO3: 23 electrons
  • N + O + O + O = 5e- + 6e- + 6e- + 6e- = 23e-
  • Since there is a negative charge which implies one extra electron, we add one electron to the total valence electrons obtained, that is, 23e + 1e = 24e. Therefore, the total valence electrons in NO3- become 24e
  • The central atom is Nitrogen, N.
  • NO3- has three resonance structures, as shown below
Lewis structure of  Nitrate ion (NO3-)
  • Lewis structure of Methane, CH4
  • Total valence electrons of CH4: 8 electrons
  • C + 4H = 4e- + (4x1e-) = 8e
  • The central atom is Carbon, C
Lewis structure of  methane
  • Lewis structure of Difluoro sulfide, SF2
  • Total valence electrons of SF2: 20 electrons
  • S + F + F = 6e- + 7e- + 7e- = 20e
  • The central atom is sulfur, S
Lewis structure of  SF2-sulfur difluoride.
  • Lewis structure of Methyl Chloride or Chloromethane, CH3Cl
  • Total valence electrons CH3Cl: 14 electrons
  • C + 3H + Cl = 4e- + 3e- + 7e- = 14e
  • The central atom is Carbon, C
Lewis structure of  chloromethane
  • Lewis structure of Nitrogen, N2
  • Total valence electrons of N2: 10 electrons
  • N + N = 5e- + 5e- = 10e
  • The central atom of N2=any since its linear
Lewis structure of  nitrogen (N2)
  • Lewis structure of Sulphur trioxide, SO2
  • Total valence electrons of SO2: 18 electrons
  • S + O + O + O = 6e- + 6e- + 6e- + 6e- = 24e
  • The central atom is S
  • Sulfur trioxide, SO2 has three resonance structures, as shown below
Lewis structure of sulfur dioxide.
  • Lewis structure of XeF4
  • Central atom xenon tetrafluoride, XeF4=Xe
  • Total valence electrons XeF4 :36 electrons
  • Xe + 4(F)= 8e- + 4(7e-)=36e-
Lewis structure of XeF4
  • Lewis structure of Boron trifluoride, BF3
  • Total valence electrons of BF3 : 24electrons
  • B+ 3(F) = 3e- + 3(7e-)=24e-
  • The central atom of BF3=B
Lewis structure of BF3
  • Lewis structure of acetic acid(CH3COOH)
  • The central atom of acetic acid (CH3COOH)=C with double bond Oxygen
  • Total valence electrons of acetic acid(CH3COOH): 24 electrons
  • 4(H) + 2(C) + 2(O)= 4(1e-)+2(4e-)+2(6e-)=24e-
Lewis structure of carbxylic acid
  • Lewis structure of nitrogen dioxide, NO2
  • The central atom of NO2=N
  • Total valence electrons of NO2: 17 electrons
  • N+ 2(O)= 5+ 2(6e-)=17e-
  • Nitrogen dioxide has 2 resonance structures, as shown below
Lewis structure of NO2
  • Lewis structure of ethanol [ CH3CH2OH]
  • The central atom of ethanol CH3CH2OH =C with OH
  • Total valence electrons of ethanol CH3CH2OH: 20 electrons
  • 6(H) + 2(C) + 1(O)= 6(e-)+2(4e-)+1(6e-)=20 e-
Lewis structure of ethanol
  • Lewis structure of NH4+(ammonium ion)
  • The central atom of NH4+(ammonium ion)=N
  • Total valence electrons of NH4+(ammonium ion): 9 electrons (the positive charge indicates that it has gained an extra electron)
  • N+ 4(H)=5e-+4(1e-)=9e-
Lewis structure of NH4+
  • Lewis structure of H2SO4(sulfuric acid)
  • The central atom of H2SO4(sulfuric acid) =S with two double bond Oxygen.
  • Total valence electrons of H2SO4(sulfuric acid): 32
  • 2(H) + 4(O)+S=2(1e-)+4(6e-)+6e-=32e-
  • Sulfuric acid has three resonance structures
Lewis structure of H2SO4-sulfuric acid
  • Lewis structure of hydrogen bromide, HBr
  • Central atom=linear
  • Total valence electrons of hydrogen bromide, HBr: 8 electrons
  • H+Br=1e-+7e-=8e-
Lewis structure of HBr
  • Lewis structure of hydrogen fluoride, HF
  • Central atom=Linear
  • Total valence electrons of hydrogen fluoride: 8 electrons
  • H+F=1e-+7e-=8e-
Lewis structure of HF
  • Lewis structure of hydrogen chloride, HCl
  • Central atom=Linear
  • Total valence electrons of hydrogen chloride: 8 electrons
  • H+Cl=1e-+7e-=8e-
Lewis structure of HCl
  • Lewis structure of hydrogen cyanide, HCN
  • Central atom=Linear
  • Total valence electrons of hydrogen cyanide: 10 electrons
  • H+C+N=1e-+4e- + 5e-=10e-
Lewis structure of HCN
  • Lewis structure of carbonate ionsCO32-
  • Central atom=C
  • Total valence electrons of the carbonate ion: 22 electrons
  • C+ 3(O)=4e-+3(6e-)=22e-
  • Carbonate ion CO32- has three resonance structures, as shown below.
Lewis structure of CO32- carboxylate ion

In a nutshell.

Knowing how to draw lewis’s structure is important in passing organic chemistry in college. It will help you identify a nucleophile from an electrophile when doing mechanisms. If you have problems withdrawing lewis structures, we have chemistry tutors who can help you in solving chemistry questions. Click on the link below and place your order.

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