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Shapes of Molecules (Cambridge (CIE) A Level Chemistry): Revision Note

Exam code: 9701

Richard Boole

Last updated

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

Repulsion between different types of electron pairs

Diagram of water molecule showing 104.5° H-O-H angle, greatest repulsion between lone pairs, and least repulsion between bonding pairs.
Different types of electron pairs have different repulsive forces

  • Molecules can adapt the following shapes and bond angles:

 Bond shapes and bond angles

Diagram of molecular geometries: linear, trigonal planar, bent, pyramidal, tetrahedral, trigonal bipyramidal, octahedral, with bond angles.
Molecules of different shapes can adapt with their corresponding bond angles

Examples of molecules with different shapes and bond angles

Diagram comparing trigonal planar BF3 and linear CO2 molecular shapes, showing electron arrangement, bond angles, and electron repulsion explanations.
Diagram comparing CH₄ tetrahedral structure with NH₃ pyramidal structure, highlighting differences in bond angles due to electron pair repulsions.
Diagram showing water with 2 lone pairs causing 104.5° angle, and SF6 with octahedral shape, 90° bond angles due to equal electron pair repulsion.
Diagram of PF5 in a trigonal bipyramidal shape with bond angles of 120° and 90°, showing electron pairs and fluorine atom positioning.
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 

Diagram shows phosphorus pentachloride (PCl5) structure; on the left, electron distribution and on the right, 3D bond angles of 90° and 120°.

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

Chemical diagram showing nitrogen bonded to three methyl groups in a trigonal pyramidal shape with a bond angle of 107 degrees.

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

Molecule diagrams: left shows carbon bonded to four chlorine atoms with electron dots; right depicts 3D tetrahedral shape with 109.5° bond angle.

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