Numford
Hybridisation

Hybridisation of SO₃

S is sp² hybridised (hybridization by the VSEPR count): 3 sigma bonds and no lone pairs give a trigonal planar shape, bond angle 120°.

OSOO
  • S
  • O

SO₃Sulphur trioxide

Trigonal planar

AB₃: 3 bond pairs, no lone pairs

  • Hybridisationsp²
  • Bond angle120°
  • Polaritynon-polar

Why this shape

  1. S has 6 valence electrons. It uses six in three double bonds to O, so none are left over.
  2. Steric number = three sigma bonds + no lone pairs = 3, so S is sp² hybridised. A double or triple bond counts once: only its sigma bond uses a hybrid orbital.
  3. Three electron pairs point to the corners of a flat triangle (trigonal planar electron geometry). With no lone pairs the atoms take the same shape: trigonal planar.
  4. All the bond pairs repel each other equally, so the angle is the ideal 120°.
  5. The bond dipoles cancel in the symmetric trigonal planar shape, so SO₃ is non-polar.

Lewis structure

24 valence electrons
i

Drawn with three S=O bonds, an expanded octet with no formal charges, the way NCERT draws H2SO4.

SOOO

Formal charges: none

Working

Why this hybridisation and shape

  1. S has 6 valence electrons. It uses six in three double bonds to O, so none are left over.
  2. Steric number = three sigma bonds + no lone pairs = 3, so S is sp² hybridised. A double or triple bond counts once: only its sigma bond uses a hybrid orbital.
  3. Three electron pairs point to the corners of a flat triangle (trigonal planar electron geometry). With no lone pairs the atoms take the same shape: trigonal planar.
  4. All the bond pairs repel each other equally, so the angle is the ideal 120°.
  5. The bond dipoles cancel in the symmetric trigonal planar shape, so SO₃ is non-polar.

Class 11, Chapter 4 Chemical Bonding and Molecular Structure.

Questions

Hybridisation of SO3: quick answers

What is the hybridisation of SO3?

S has 6 valence electrons. It uses six in three double bonds to O, so none are left over. Steric number = three sigma bonds + no lone pairs = 3, so S is sp² hybridised. A double or triple bond counts once: only its sigma bond uses a hybrid orbital.

What is the shape of SO3 and its bond angle?

Trigonal planar, with a bond angle of 120 degrees. Three electron pairs point to the corners of a flat triangle (trigonal planar electron geometry). With no lone pairs the atoms take the same shape: trigonal planar. All the bond pairs repel each other equally, so the angle is the ideal 120°.

Is SO3 polar or non-polar?

Non-polar. The bond dipoles cancel in the symmetric trigonal planar shape, so SO₃ is non-polar.

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