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Lewis symbol is a symbol in which the electrons in the valence shell of an atom or simple ion are represented by dots placed around the letter symbol of the element. Each dot represents one electron.

Hydrogen
Oxygen
Chlorine
Chloride ion

A covalent bond is a chemical bond formed by the sharing of a pair of electrons between two atoms.

The Lewis structure of a covalent compound or polyatomic ion shows how the valence electrons are arranged among the atoms in the molecule to show the connectivity of the atoms.

Instead of using two dots to indicate the two electrons that comprise the covalent bond, a line is substituted for the two dots that represent the two electrons.

Below is shown the Lewis structure for water. Two hydrogens (H) are separately covalently bonded to the central oxygen (O) atom. The bonding electrons are indicated by the dashes between the oxygen (O) and each hydrogen (H) and the other two pairs of electrons that constitute oxygens octet, are called non-bonding electrons as they are not involved in a covalent bond.

The first step in drawing Lewis structures is to determine the number of electrons to be used to connect the atoms. This is done by simply adding up the number of valence electrons of the atoms in the molecule.

Consider carbon dioxide CO₂

carbon (C) has four valence electrons x 1 carbon = 4 e^-
oxygen (O) has six valence electrons x 2 oxygens = 12 e^-
There are a total of 16 e^- to be placed in the Lewis structure.

Connect the central atom to the other atoms in the molecule with single bonds.

Carbon is the central atom, the two oxygens are bound to it and electrons are added to fulfill the octets of the outer atoms.

Complete the valence shell of the outer atoms in the molecule.

Place any remaining electrons on the central atom.

There are no more electrons available in this example.

If the valence shell of the central atom is complete, you have drawn an acceptable Lewis structure.

Carbon is electron deficient - it only has four electrons around it. This is not an acceptable Lewis structure.

If the valence shell of the central atom is not complete, use a lone pair on one of the outer atoms to form a double bond between that outer atom and the central atom. Continue this process of making multiple bonds between the outer atoms and the central atom until the valence shell of the central atom is complete.

<>	becomes
The central atom is still electron deficient, so share another pair.
<>	becomes

Double check to make sure that you have used the correct number of electrons in the Lewis structure and that no atom that cannot exceed its valence shell, does not.
The best Lewis structure that can be drawn for carbon dioxide is:

Bond Order and Bond Length

The bond order is equal to the number of bonds between two atoms.

The bond length is the distance between those two atoms. The greater the number of electrons between two atoms, the closer the atoms can be brought towards one another, and the shorter the bond.

The BO is an indication of the bond length, the greater the bond order, the shorter the bond.

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