Lewis Dot Structures & Octet Rule: A Simple Method to write Lewis Structures

Why are Lewis structures important?

Lewis structures constitute an overwhelming majority of the structures that students encounter in courses in general chemistry, organic chemistry, or biochemistry. Hence, the writing of Lewis structures corresponding to a given molecular formula is an important exercise for them. The ability of students to write these structures correctly and then to calculate the formal charges of the atoms involved, is a prerequisite to understanding such topics as:

  • Resonance
  • Reπactivity
  • Acidity and basicity
  • Bond and molecular polarities
  • Inter- and intra-molecular bonding
  • Isomerism and molecular geometry.

Since many chemistry textbooks do not give easy procedures for writing such structures we present a set of instructions for writing Lewis structures

But at first let’s see the following:

Which are the essential propositions of the Lewis theory for bonding?

In 1916, G.N. Lewis published a paper on the chemical bond in non-ionic substances, entitled “The Atom and the Molecule”.G.N. Lewis using essentially empirical concepts along with his chemical intuition combined observations at the time about chemical bonding. The theory he proposed not only was essential in understanding how elements bonded but also provided a visual representation for them the so called Lewis symbols and the Lewis dot structures.

Lewis symbols:

Lewis structures

Which are the types of bonds proposed by Lewis?
There are two basic types of bonding proposed in the Lewis theory:

Ionic bonding: This type of bonding occurs when outer electron(s) are transferred from one atom to an other so that both of them attain an octet of electrons in their outermost shell. The two atoms are held together by electrostatic forces

Covalent bonding: This type of bonding forms when two non-metal atoms share a pair of outer electrons so that both of them attain an octet of electrons in their outermost shell.

A Simple Method to write Lewis Structures

For molecules, before we draw their Lewis structures we have to learn Duplet rule and Octet rule.

Duplet rule: Duplet rule is defined as a stable electronic configuration when an element gets two electrons in its valence shell.

Octet rule: Octet rule states that in forming compounds, atoms gain, lose or share electrons to
give a stable electron configuration characterized by eight valence electrons. This rule
is applied to the main-group elements of the second period

All elements tend to become stable. But with incomplete atomic electron configurations, atoms cannot be stable; thus, they become very reactive in order to fill the shells with electrons by gaining, losing or sharing electrons. The atoms, ions and molecules which obey the octet or duplet rule are stable. The key difference between octet and duplet is that octet has eight electrons in its outermost orbital whereas duplet has two electrons in its orbital.

Draw Lewis structures step by step:

  1. Choose a center atom.
  2. Count all valence electrons. V
  3. Draw the molecular skeleton, draw a single bond from each surrounding atom to the central atom (subtract 2 valence electrons for each bond).
  4. Calculate the number of electrons which must be shared through π bonds (pi bonding), P in the molecule:   P = 6n + 2 – V                                                                                                              [Where (i) n  is the number of atoms in the molecule excluding the hydrogen atoms and (ii) V is the total number of valence electrons in the molecule = ( sum of group numbers of  the atoms in the molecule) – charge
  5. If there are any electrons left, distribute them as lone electron pairs to maximize the number of the octets.
  6. If some of the atoms lack octet structures, change as many lone electron pairs into shared electron pairs as required to complete the octet shells.
  7. Calculate the formal charges of the atoms in the molecule as follows: (i) Any atom having its normal valency number of bonds will be electrically neutral; any atom which does not have its normal valency number of bonds will have a formal charge  (ii) The formal charge may be calculated by subtracting the number of electrons “owned” by the element from its group number; Formal Charge of Element = Group Number – Total number of electrons “owned” by the atom concerned 
Example of Lewis dot Structure:

Key Terms

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  1. G.N. Lewis, J.A.C.S, 38, 762-785, (1916)
  2. Chang, R., 2010. Chemistry. 10th ed. New York: McGraw-Hill.
  3. A.B.P. Lever, J. Chem. Educ., 49, 819-821, (1972)

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