Shapes of Molecules (DP IB Chemistry): Revision Note

Shapes of molecules

What is valence shell electron pair repulsion (VSEPR) theory?

• 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

Two electron domains

• If there are two electron domains around the central atom, they arrange themselves on opposite sides of the atom to minimise repulsion

  • This results in a bond angle of 180°

• Molecules with this shape are described as linear

• Examples of linear molecules include:

  • BeCl₂

  • CO₂

  • HC≡CH (ethyne)

Three electron domains

• If there are three electron domains around the central atom, they arrange themselves as far apart as possible in a flat triangle

  • This gives a bond angle of 120°

• The electron domain geometry is called trigonal planar

Trigonal planar examples (no lone pairs)

• When all three domains are bonding pairs, the molecular shape is also trigonal planar

• Examples include:

  • BF₃

  • CH₂=CH₂ (ethene)

  • CH₂O (methanal)

Trigonal planar with one lone pair

• If one of the three domains is a lone pair, it exerts stronger repulsion than bonding pairs

  • This pushes the bonding domains slightly closer together

  • The bond angle is reduced to approximately 118°

• The molecular shape is no longer trigonal planar, it is described as bent

Example: sulfur dioxide (SO₂)

• SO₂ has two bonding pairs and one lone pair around the central sulfur atom

  • It also contains a double bond

  • But, VSEPR treats multiple bonds as a single domain

• SO₂ is an example of an expanded octet

  • The sulfur has 10 electrons in its valence shell

• The shape is best described as bent

Four electron domains

• If there are four electron domains around the central atom, they arrange themselves as far apart as possible in a tetrahedron

  • The ideal bond angle is approximately 109.5°

• The electron domain geometry is called tetrahedral

Tetrahedral shape (no lone pairs)

• If all four domains are bonding pairs, the molecular shape is also tetrahedral

• Examples include:

  • CH₄ (methane)

  • NH₄⁺ (ammonium ion)

Trigonal pyramidal shape (one lone pair)

• If one domain is a lone pair, it exerts stronger repulsion than bonding pairs

  • This slightly reduces the bond angle to around 107°

• The molecular shape is trigonal pyramidal

• Example:

  • NH₃ (ammonia)

Bent shape (two lone pairs)

• If two of the four domains are lone pairs, the bond angle is further reduced due to lone pair–lone pair repulsion

  • The bond angle is approximately 104.5°

• The molecular shape is described as bent, angular, or v-shaped

• Example:

  • H₂O (water)

Electron pair repulsion hierarchy

• Lone pairs are held closer to the nucleus than bonding pairs

• As a result, they repel more strongly

• This affects molecular geometry by reducing bond angles more than bonding pairs alone

Summary table of electron domains and molecular shapes

• These are the domains and molecular geometries you need to know for Standard Level: