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How to Find Lone Pairs?

Published in Chemical Bonding 3 mins read

You can find lone pairs on an atom, particularly the central atom in a molecule, by following a few key steps that involve understanding valence electrons and chemical bonding. Here's a breakdown:

Steps to Determine Lone Pairs:

  1. Determine the Total Number of Valence Electrons: Add up the valence electrons of all atoms in the molecule or ion. Remember, valence electrons are the electrons in the outermost shell of an atom and are responsible for bonding.

    • You can usually determine the number of valence electrons an element has based on its group number in the periodic table. For example, oxygen (Group 16/6A) has 6 valence electrons.

  2. Draw the Lewis Structure: This involves connecting the atoms with single bonds first. Usually, the least electronegative atom occupies the central position.

  3. Complete Octets (or Duets for Hydrogen): Distribute the remaining valence electrons as lone pairs around the atoms to fulfill the octet rule (8 electrons) for each atom, or the duet rule (2 electrons) for hydrogen. Start with the most electronegative atoms first.

  4. Calculate Remaining Electrons: After forming bonds and completing octets (or duets), count how many valence electrons were used. Subtract this from the total number of valence electrons you calculated in Step 1.

  5. Place Remaining Electrons on the Central Atom: If there are remaining electrons, place them on the central atom as lone pairs, even if it exceeds the octet rule (this is common for elements in period 3 and beyond).

  6. Count Lone Pairs on the Central Atom: Once the Lewis structure is complete, count the number of lone pairs specifically located on the central atom. This is your answer.

Example: Water (H₂O)

  1. Total Valence Electrons:

    • Hydrogen (H): 1 valence electron x 2 atoms = 2
    • Oxygen (O): 6 valence electrons x 1 atom = 6
    • Total = 2 + 6 = 8 valence electrons

  2. Lewis Structure (initial): H-O-H

  3. Complete Octets/Duets: Place remaining electrons on oxygen to fulfill its octet.

  4. Used Electrons: Each bond is 2 electrons. The initial Lewis Structure above requires 4 electrons. After placing lone pairs on the oxygen to fill its octet, 4 more electrons are used (two lone pairs). 8 electrons are used in total.

  5. Remaining Electrons: 8 (total valence electrons) - 8 (used electrons) = 0

  6. Lone Pairs on Central Atom: In the completed Lewis structure of water, the central oxygen atom has two lone pairs.

Table Summary

Step Description
1. Valence Electrons Sum the valence electrons of all atoms.
2. Lewis Structure Connect atoms with single bonds.
3. Complete Octets/Duets Distribute remaining electrons to fulfill octets (or duets for H).
4. Calculate Remaining Electrons Subtract the number of electrons already assigned from the total valence electrons.
5. Place Remaining Electrons on Central Atom Add remaining electrons as lone pairs to the central atom.
6. Count Lone Pairs Count the number of lone pairs on the central atom.

By following these steps, you can accurately determine the number of lone pairs present on an atom in a molecule.

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