Lewis Dot Structures

The developments in the field of atomic structure provided foundations for various theories of valency. The initial contributions in this field came from W. Kossel and G. N. Lewis in 1916.They were the first to provide some logical explanation of valency which was based on the inertness of noble gases. This view, later on came to be known as Octet rule.

Octet Rule: This rule can be considered as the first rational explanation for the cause of combination of atoms. It is based on the electronic configuration of noble gases. The noble gases like neon, argon etc., have 8 electrons in their outermost shell. They do not have any tendency to take part in chemical combination. The chemical inertness of noble gases was related to the presence of octet of electrons. Other elements which were chemically reactive had less than 8 electrons in their outermost shell. These observations led Kossel and Lewis to put forward a generalisation known as Octet rule.

The rule states that atoms of various elements enter into chemical combination so as to attain the configuration of eight electrons in their outermost shell. The octet rule is quite useful in explaining the normal valencies of large number of elements and in understanding the formation of various bonds.

The Lewis dot structure provides a picture of bonding in molecules and ions in terms of the shared pairs of electrons and the octet rule. While such an illustration may not explain the bonding and behaviour of a molecule completely, it does help in understanding the formation and properties of a molecule to a large extent. Writing of Lewis dot structures is therefore, very useful.

Steps for drawing Lewis dot structure:

• The total number of electrons required for writing the structure is obtained by adding the valence electrons of the combining atoms. For example: in the CH4 molecule there are eight valence electrons available for bonding (4 from carbon and 4 from the four hydrogen atoms.)
• For anions, each negative charge would mean addition of one electron. For cations, each positive charge would result in subtraction of one electron from the total number of valence electrons. For example, for the CO₃^2-ion, the two negative charges indicate that there are two additional electrons than those provided by the neutral atoms.
• Knowing the chemical symbols of the combining atoms and having knowledge of the skeletal structure of the compound, it is easy to distribute the total number of electrons as bonding shared pairs between the atoms in proportion to the total bonds.
• In general, the least electronegative atom occupies the central position in the molecule/ion. For example in NF3 and CO₃^2-, nitrogen and carbon are the central atoms whereas fluorine and oxygen occupy the terminal positions.
• After accounting for the shared pairs of electrons for single bonds, the remaining electron pairs are either utilized for multiple bonding or remain as the lone pairs. The basic requirement being that each bonded atom gets an octet of electrons.

Lewis Dot structure of CO₂ molecule:

Step 1: Count the total number of valence electrons of carbon and oxygen atoms. The outer (valence) shell configurations of carbon and oxygen atoms are 2s² 2p² and 2(2s² 2p⁴⁾ respectively (as there are 2 oxygen atoms).

The total number of valence electrons: 4+12 = 16

Step 2: Skeletal structure of CO₂ is written as:

Step 3: Now draw a single bond (one shared electron pair) between C and O, distribute the valence electrons around the atoms until each atom has a complete the octet.

Step: 4 As all 16 electrons have been used, it is necessary to check if the central atom (C) has a complete octet.  The C only has 4 surrounding electrons with the two single bonds on both sides.  Now, remove one pair of non-bonding electrons from one of the O atoms in order to form a double bond between the C and that O atom.

Step 5: Carbon atom now has 6 electrons, so now remove another pair of non-bonding electrons from the other O atom in order to form another double bond between the C and the other O atom.

In the above structure, atom (C) has eight electrons and the O atoms also have eight electrons, which completed the Lewis Structure and the formal charge on both C and O is zero.