Predicting Shapes & Angles (Edexcel International AS Chemistry): Revision Note

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 7, 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 that the central atom is ‘electron deficient’

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

BCl₃

• Boron is in Group 3, so has 3 valence electrons; Cl is in Group 7, 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 that the central atom is ‘electron deficient’

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

CH₄ & NH₃

• Carbon is in Group 4, 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 5, 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 5, 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

CO₂

• Carbon is in Group 4, so has 4 valence electrons; each oxygen has 6

• Carbon forms two C=O double bonds (4 shared electrons in each) and has no lone pairs

• Two regions of electron density give a linear shape with a bond angle of 180°

H₂O

• Oxygen has 6 valence electrons: 2 are used in bonds to hydrogen and 4 form two lone pairs

• Four regions of electron density, with lone pair-lone pair repulsion greatest, give a non-linear (bent) shape with a bond angle of 104.5°

SF₆

• Sulfur has 6 valence electrons, all used to form six bonds to fluorine, with no lone pairs

• Six equal bonding pairs give an octahedral shape with bond angles of 90°

PCl₅ (g)

• Phosphorus is in Group 5, so has 5 valence electrons; Cl is in Group 7, 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 4, 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