Electrons present in the valence shell which remain unshared during a bonding process are better known as non-bonding electrons.
The Lewis structures showing the diatomic molecules clearly depicts what the bonding and non-bonding electrons mean. For any atom the sole criteria is to get to nearest octet configuration and hence the sharing of electrons. The sharing takes place for only those electrons which eventually form bond with neighbouring like elements which either tries to reach duplet or octet.
The electrons get shared and once the octet is achieved the rest of electrons do not undergo any more sharing as the primary objective of reaching octet is complete. This leaves a few of the electrons in valence cloud without getting a paring. These unshared set of electrons are referred as non-bonding electrons. A bonding pair of electrons would define as a region in which the electrons are most likely to be found.
Similarly a non-bonding pair or the lone pair of electrons
is defined as an electron domain which is located principally on one atom. Let us take up the case of ammonia. In ammonia Lewis structure of NH3
it has a total of four electron domains around the central atom of nitrogen. The three hydrogen around nitrogen bond in three of these four electron domains while the fourth domain remains unshared. That becomes the lone pair of ammonia molecules.Let us take another molecule of ozone.
In ozone O3
which has three electron domains around the central oxygen atom. Two other oxygen atoms combine with the central oxygen in two of these domains, once with a double bond and another with single bond. That leaves one domain without a pairing and hence aptly termed as non-bonding pair of electrons.
Each non-bonding pair produces either single or multiple bond on these electron domains around the central atom.
With the focus on non-bonding electrons the structures of molecules also get affected due to these non-bonding electrons. The progressive replacement of bonding electron pairs by non-bonding pairs affects the shape of molecule. The structures either take axial, symmetrical, or distorted octahedral shapes.
The molecule of water has an angular or bent shape. The H-O-H bond angle in a molecule of water is 105 degree, an angle that is also quite close to bond angle of methane of about 109.5 degree.
Let us find out the total non-bonding electrons in water. Oxygen has six electrons in the valence shell cloud. So overall three electron pair domains in oxygen atom. Out of these two electron domains gets a pairing with Hydrogen atom on either side.
Since the two Hydrogen atoms makes use of only two electrons, four more electrons remain without taking part in bonding in the cloud. That finally gives us two pairs of non-bonding electrons or lone pairs. Due to the repulsion of two lone pairs the shape of water is bent. The general tetrahedral structure for the electron pairs of a molecule of water if we place the two non-bonding electron pairs at corners of the tetrahedron.
A tetrahedral arrangement of the electron pairs accounts for the angular arrangement of the three atoms. The bond angle is less than 109.5 degree as the non-bonding pairs are effectively bigger than the bonding pairs and hence the structure is not a perfect tetrahedral.
Methanol has a molecular formula of CH4
O. If we draw the Lewis formula the centrally placed carbon will have four bonds with Hydrogen one bond and Oxygen two bonds. The Oxygen atom trying to achieve inert gas octet will bond with Carbon on one end and Hydrogen atom on another end. This arrangement shows four electrons without pairing or two pairs of non-bonding. These electrons although are in valence shell but do not take part in bonding.
The hydroxy group is placed to the right of carbon atom. Methanol could also be represented on a two dimensional surface by other structures. Carbon gets connected by single electron pair bonds to three Hydrogen atoms and one hydroxyl group but only the arrangement keeps changing because of Carbon’s typical catenation property.Lewis dot structure of methanol
If we look at the formula of methyl alcohol, we can predict the atomic arrangement where three hydrogen atoms and the oxygen atom with another hydrogen atom bonded to oxygen atom.The total number of valence electron space available for this molecule = 24
In order, to find a Non- Bonding Electron follow the steps below:
- Carbon has eight, Hydrogen around Carbon has six, Hydrogen beside Oxygen has two and Oxygen has eight.
- The overall number of valence electrons is 14.
- The total number of bonding is 24 – 14 = 10
- The number of electrons which did not take part in bonding: 14 – 10 = 4
- Total number of lone pair: 2
- The number of neutral atom bonds is equivalent to the subtracted total number of electrons present in a complete valence shell and valence shell electrons.
- The total number of bonds present = complete valence shell – number of valence electrons
- Total number non-bonding electrons is equivalent to the difference between complete valence shell and twice the number of bonds present.
- The same method could be applied to get the calculation for number of electrons that are not talking part in bonding.
- The number of non-bonding electrons is now equivalent to the difference between complete valence shell and electron numbers which takes part in chemical bonding.