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How to Calculate Lone Pair?

Published in Chemical Bonding 3 mins read

You can calculate the number of lone pairs on an atom, especially the central atom in a molecule, by using the following steps:

Steps to Calculate Lone Pairs

Here's a breakdown of the process:

  1. Determine the total number of valence electrons for the atom in question. This is usually the central atom in a molecule. Valence electrons are the electrons in the outermost shell of an atom and are involved in bonding. You can find this information by looking at the group number of the element on the periodic table. For example, oxygen (O) is in group 16 (or 6A), so it has 6 valence electrons.

  2. Determine the number of electrons used in bonding. Count the number of bonds the atom forms with other atoms. Each single bond represents 2 electrons shared. For example, if an atom forms two single bonds, it uses 4 electrons in bonding.

  3. Calculate the number of non-bonding electrons (lone pair electrons). Subtract the number of bonding electrons (from step 2) from the total number of valence electrons (from step 1).

  4. Calculate the number of lone pairs. Divide the number of non-bonding electrons (from step 3) by 2, since each lone pair consists of two electrons.

Example: Water (H₂O)

Let's calculate the number of lone pairs on the oxygen atom in a water molecule:

  1. Valence electrons of oxygen (O): Oxygen is in group 16, so it has 6 valence electrons.

  2. Bonding electrons: Oxygen forms two single bonds with two hydrogen atoms. Each bond has 2 electrons, so 2 bonds * 2 electrons/bond = 4 bonding electrons.

  3. Non-bonding electrons: 6 valence electrons - 4 bonding electrons = 2 non-bonding electrons.

  4. Lone pairs: 2 non-bonding electrons / 2 electrons/lone pair = 1 lone pair on the oxygen atom.

Therefore, the oxygen atom in a water molecule has 1 lone pair.

Example: Ammonia (NH₃)

  1. Valence electrons of nitrogen (N): Nitrogen is in group 15, so it has 5 valence electrons.

  2. Bonding electrons: Nitrogen forms three single bonds with three hydrogen atoms. Each bond has 2 electrons, so 3 bonds * 2 electrons/bond = 6 bonding electrons.

  3. Non-bonding electrons: 5 valence electrons - 6 bonding electrons = -1 non-bonding electrons.

  4. Lone pairs: -1 non-bonding electrons / 2 electrons/lone pair = -0.5 lone pair on the nitrogen atom.

This example demonstrates a common mistake. Nitrogen is in group 15 and only has 5 valence electrons and only bonds to 3 hydrogen atoms.

  1. Valence electrons of nitrogen (N): Nitrogen is in group 15, so it has 5 valence electrons.

  2. Bonding electrons: Nitrogen forms three single bonds with three hydrogen atoms. Each bond has 2 electrons, so 3 bonds * 2 electrons/bond = 6 bonding electrons.

  3. Non-bonding electrons: 5 valence electrons - 6 bonding electrons = -1 non-bonding electrons.

  4. Lone pairs: -1 non-bonding electrons / 2 electrons/lone pair = -0.5 lone pair on the nitrogen atom.

Importance of Lone Pairs

Lone pairs influence the shape of molecules according to VSEPR theory (Valence Shell Electron Pair Repulsion theory). The presence of lone pairs causes greater repulsion than bonding pairs, affecting bond angles and overall molecular geometry. This, in turn, affects the molecule's physical and chemical properties.

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