1.7.6 Predicting Shapes & Angles

• You will need to know and explain the shapes of the following molecules:
  • BeCl₂
  • BCl₃
  • CH₄
  • NH₃
  • NH₄⁺
  • H₂O
  • CO₂
  • PCl₅ (g)
  • SF₆
  • C₂H₄

• In order to write the correct shapes and structures of the molecules listed we need to consider the number of valence electrons

BeCl₂

• Beryllium is in group 2, so has 2 valence electrons; Cl is in group 17, so has 7 valence electrons
• Both electrons are used to form covalent bonds with Cl and there are no lone pairs
• 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 valence electrons; Cl is in group 17, so has 7 valence electrons
• All 3 electrons are used to form covalent bonds with Cl and there are no lone pairs
• 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 & NH₃

• Carbon is in group 14, so has 4 valence electrons; H is in group 1, so has 1 valence electron
• All 4 electrons are used to form covalent bonds with H
• This gives a tetrahedral arrangement with a bond angle of 109.5°
• Nitrogen is in group 15, so has 5 valence electrons
• Only 3 electrons are used to form covalent bonds with H and 2 are unbonded as a lone pair
• This gives a trigonal pyramid arrangement with a bond angle of 107°

NH₄⁺

• Nitrogen is in group 15, so has 5 valence electrons; H is in group 1, so has 1 valence electron
• Only 3 electrons are used to form covalent bonds with H 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)

The NH₄⁺ ion 

Other examples

PCl₅ (g)

• • Phosphorus is in group 15, so has 5 valence electrons; Cl is in group 17, so has 7 valence electrons
  • All 5 electrons are used to form covalent bonds with Cl and there are no lone pairs
  • This gives a trigonal (or triangular) bipyramidal shape
  • With bond angles of 120° and 90°

Phosphorus pentachloride molecule 

C₂H₄

• Each carbon atom is in group 14, so has 4 valence electrons, each hydrogen atom is in group 1 so has 1 valence electron
• This gives a total of 12 electrons
• There are 4 C-H bonds which use 8 electrons 
• This leaves 4 electrons which are involved in the C=C
• Therefore the overall shape is trigonal planar as there are three regions of electron density for each carbon atom

Ethene molecule