What is VSEPR theory and how to explain structures

VSEPR theory or valence shell electron pair repulsion theory explain the shapes of molecules. Here, we will study about the geometry of the molecules with different examples.

What is VSEPR theory

The basic assumptions of theory are:

  1. Electron pairs surrounding the central atom repel one another.
  2. The electron pairs occupy the position at maximum possible distance.
  3. There are no further repulsions in the molecule.
  4. Lone pair-Lone pair > Bond pair-Lone pair > Bond pair-Bond pair ( The magnitude of repulsions )
  5. If there are no lone pairs, the. shape is symmetrical or regular otherwise irregular

What are the shapes of molecules (VSEPR theory)

A represents the central atom, B as bond pair, L as lone pair.

  1. AB2 molecule 2 Bond Pairs + 0 Lone pair linear molecule CO2, BeH2, BeCl2
  2. AB3 molecule 3 Bond Pairs + 0 Lone pair Triangular planar BF3, AlCl3, SO3
  3. AB2L molecule 2 Bond Pairs + 1 Lone pair Bent shape NO2, SO2, O3
  4. AB4 molecule 4 Bond Pairs + 0 Lone Pair Tetrahedral NH4^+, SiF4
  5. AB3L molecule 3 Bond Pairs + 1 Lone pair Trigonal pyramidal NH3, PCl3, NCl3, NF3,PH3, H3O^+
  6. AB2L2 molecule 2 Bond Pairs + 2 Lone pairs Bent shape H2O, H2S, NH2^-
  7. AB5 molecule 5 Bond Pairs + 0 Lone pair Trigonal bipyramidal PCl5, PF5, SbCl5, AsF5
  8. AB4L molecule 4 Bond Pairs + 1 Lone Pair See saw SF4
  9. AB3L2 molecule 3 Bond Pairs + 2 Lone pairs T-shaped ClF3
  10. AB2L3 molecule 2 Bond Pairs + 3 Lone pairs Linear XeF2
  11. AB6 molecule 6 Bond pairs + 0 Lone pairs Octahedral SF6
  12. AB5L molecule 5 Bond pairs + 1 Lone pair Square pyramidal ClF5
  13. AB4L2 molecule 4 Bond pairs + 2 Lone pairs Square planar XeF4

Predicting the shape of the molecule

  1. Calculate the total number of electron pairs around the central atom = 1/2 ( Valence e’s of central atom + No. of atoms linked to central atom by single bonds )
  2. No of bond pairs = No. of atoms linked to central atom by single bonds.
  3. No. of lone pairs = Total no. of electron pairs – No of bond pairs

What is the shape of ClF3 molecule ?

No. of valence electrons of the central atom that is Cl = 7 Total no. of electron pairs around Cl = ( 7 + 3 ) /2 = 5

No. of bond pairs = 3 No of lone pairs = 5 – 3 = 2

The molecule is AB3L2 type, T shaped

Check out the post of how to calculate the formal charge here

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