Shapes of Molecules (Cambridge International A-Level Chemistry)

VSEPR Theory

Spec point — 3.5.1

• Bonding and non-bonding electron pairs around a central atom behave like negatively charged clouds that repel each other

• To minimise repulsion, these electron pairs arrange themselves as far apart as possible in three-dimensional space

• VSEPR theory follows three key rules:

  1. All electron pairs (bonding and lone pairs) spread out as far as possible
  2. Lone pairs repel more strongly than bonding pairs
  3. Multiple bonds behave like a single bond when determining shape

• Using the valence shell electron pair repulsion theory (VSEPR), this allows us to predict:

  • The shape of the molecule
  • The angles between the bonds

• Each region of electron density around the central atom is called an electron domain

  • A domain may contain one, two, or three pairs of electrons

Repulsion between different types of electron pairs

Different types of electron pairs have different repulsive forces

• Molecules can adapt the following shapes and bond angles:

Bond shapes and bond angles

Molecules of different shapes can adapt with their corresponding bond angles

Examples of molecules with different shapes and bond angles

Each different shape has a specific name and specific bond angle(s)

Worked Example VSEPR & shapes of molecules

Draw the shape of the following molecules and compounds:

1. Phosphorous(V) chloride
2. N(CH₃)₃
3. CCl₄

Answer 1

• Phosphorous has a +5 charge so 5 Cl^– ions are needed to neutralise the charge

  • Therefore, the phosphorous(V) chloride is PCl₅

• P is in Group 5 and has 5 valence / outer electrons

• Cl is in Group 7 (17) and has 7 valence / outer electrons

• All 5 electrons of phosphorous are used to form single covalent bonds to the 5 chlorines

  • There are no lone pairs

• So, phosphorous(V) chloride has a trigonal bipyramidal shape

Answer 2

• Nitrogen is in Group 5 and has 5 valence / outer electrons

• The carbon in the -CH₃ groups is in Group 4 and has 4 valence / outer electrons

  • 3 of these electrons are already used in covalent bonds with hydrogen

• 3 of N's valence / outer electrons are involved in bonding pairs with the carbon from the -CH₃ groups

  • This leaves one pair of electrons as a lone pair

• So, N(CH₃)₃ has a pyramidal shape

Answer 3

• Carbon is in Group 4 and has 4 valence / outer electrons

• Cl is in Group 7 (17) and has 7 valence / outer electrons

• All 4 valence / outer electrons of carbon are used as bonding pairs with the 4 chlorines

  • There are no lone pairs

• So, the shape of CCl₄ is tetrahedral