Cambridge International AS Level Chemistry
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Ammonia has three bonding pairs of electrons and one lone pair. As lone pair–bond pair repulsion is greater than bond pair–bond pair repulsion, the bonding pairs of electrons are pushed closer together. This gives the ammonia molecule a triangular pyramidal shape. The H N H bond angle is about 107°. Water has two bonding pairs of electrons and two lone pairs. The greatest electron pair repulsion is between the two lone pairs. This results in the bonds being pushed even closer together. The shape of the water molecule is a nonlinear V shape. The H O H bond angle is 104.5°.
QUESTION 6 a Predict the shapes of the following molecules, which you drew in question 2 on page 52: i
tetrachloromethane, CCl4
ii beryllium chloride, BeCl2 iii phosphorus(III) chloride. b Draw dot-and-cross diagrams for the following molecules and then predict their shapes: i
hydrogen sulfide, H2S
ii phosphine, PH3.
O
C
O
O
C
O
Figure 4.19 Carbon dioxide.
Carbon dioxide
Carbon dioxide has two carbon–oxygen double bonds and no lone pairs. The four electrons in each double bond repel other electrons in a similar way to the two electrons in a single bond (Figure 4.19). So, the O C O bond angle is 180°. We describe the shape of the carbon dioxide molecule as linear.
Phosphorus pentafluoride
Phosphorus pentafluoride has five bonding pairs of electrons and no lone pairs. The repulsion between the electron pairs results in the most stable structure being a trigonal bipyramid (Figure 4.20). Three of the fluorine atoms lie in the same plane as the phosphorus atom. The bond angles FPF within this plane are 120°. Two of the fluorine atoms lie above and below this plane at 90° to it.
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F F
More molecular shapes
F
F
We can work out the shapes of other molecules by following the rules for electron-pair repulsion.
P F
Boron trifluoride is an electron-deficient molecule. It has only six electrons in its outer shell. The three bonding pairs of electrons repel each other equally, so the F B F bond angles are 120° (Figure 4.18). We describe the shape of the molecule as trigonal planar. ‘Trigonal’ means ‘having three angles’.
F B
B
F F
F
F
F
Figure 4.20 Phosphorus pentafluoride.
Sulfur hexafluoride
Sulfur hexafluoride has six bonding pairs of electrons and no lone pairs. The equal repulsion between the electron pairs results in the structure shown in Figure 4.21. All F S F bond angles are 90°. We describe the shape as octahedral.
F
F F
F
F
F
F
F
F S
S
Figure 4.18 Boron trifluoride.
F P 120° F
Boron trifluoride
F
90°
F
F
Figure 4.21 Sulfur hexafluoride.
F
F
F