Shapes of Simple Molecules & Ions (Oxford AQA International A Level Chemistry)

Valence Shell Electron Pair Repulsion

• The valence shell electron pair repulsion theory (VSEPR) predicts the shape and bond angles of molecules

• Electrons are negatively charged and will repel other electrons when close to each other

• In a molecule, the bonding pairs of electrons will repel other electrons around the central atom forcing the molecule to adopt a shape in which these repulsive forces are minimised

• Different types of electron pairs have different repulsive forces:

  • Lone pairs of electrons have a more concentrated electron charge cloud than bonding pairs of electrons

  • The cloud charges are wider and closer to the central atom’s nucleus

  • The order of repulsion is therefore: lone pair – lone pair > lone pair – bond pair > bond pair – bond pair

• The shape of the molecule is dependent on the number of pair of electrons surrounding the central atom

Shapes & Bond Angles

BeCl₂

• Beryllium is in Group 2, so has 2 outer electrons

• Both electrons are used to form covalent bonds with chlorine and there are no lone pairs around the central atom

• Accommodating less than 8 electrons in the outer shell means than the central atom is ‘electron deficient’

• This gives a linear shape with bond angles of 180°

BCl₃

• Boron is in Group 13, so has 3 outer electrons

• All 3 electrons are used to form covalent bonds with chlorine and there are no lone pairs around the central atom

• Accommodating less than 8 electrons in the outer shell means than the central atom is ‘electron deficient’

• This gives a trigonal planar shape with bond angles of 120°

CH₄

• Carbon is in Group 14, so has 4 outer electrons

• All 4 electrons from carbon are used to form covalent bonds with hydrogen

• This gives a tetrahedral arrangement with a bond angle of 109.5°

NH₄⁺

• Nitrogen is in Group 15, so has 5 valence electrons

• Only 3 electrons from nitrogen are used to form covalent bonds with hydrogen and 2 are used to form a dative covalent bond

• This gives a tetrahedral arrangement 

• With a bond angle of 109.5° (similar to CH₄)

PCl₅

• Phosphorus is in Group 15, so has 5 outer electrons

• All 5 electrons are used to form covalent bonds with Cl and there are no lone pairs around the central atom

• This gives a trigonal (or triangular) bipyramidal shape

• With bond angles of 120° and 90°
  

SF₆

• Sulfur is in Group 16, so has 6 outer electron

• 12 electrons are used to form 6 covalent bonds so there are no lone pairs around the central atom

• This gives an octahedral arrangement with a bond angle of 90°

Lone pairs

• Some molecules have lone pairs of electrons around the central atom

• Given that lone pair-lone pair repulsion is greater than lone pair-bond pair repulsion the geometry of the molecule will change if present

NH₃

• Nitrogen is in Group 15, so has 5 outer electrons

• Only 3 electrons are used to form covalent bonds with hydrogen and 2 are unbonded as a lone pair

• This gives a trigonal pyramid arrangement with a bond angle of 107°

H₂O

• Oxygen is in Group 16, so has 6 outer electrons

• Only 2 electrons from oxygen are used to form covalent bonds with hydrogen

• 4 electrons are unbonded as two lone pairs

• This gives a V-shaped arrangement with a bond angle of 104.5°

ClF₄^-

• Chlorine is in Group 17, so has 7 outer electrons

• Only 4 electrons from chlorine are used to form covalent bonds with fluorine

• 4 electrons are unbonded as two lone pairs (one lone pair contains an electron that has been donated to it)

• This gives a square planar arrangement with a bond angle of 90°