Working out the shape of a molecule or a bond angle always starts with a dot and cross diagram, even if the exam question doesn’t ask (common trick!). If we know how many pairs of electrons (bonding pairs or lone pairs) surround the central atom in the molecule, then working out the shape is simple.
- tetrahedral (bond angle is 109.5°)
By starting with a dot / cross diagram we can see how many pairs of electrons we have surrounding our central atom, in this case C. We also need to know whether these pairs are bond / bonded / bonding* or lone pairs as lone pairs have a greater repulsive effect than bonding pairs of electrons.
* check your text book and some recent mark schemes to figure out the terminology you need to use (bond pairs = OCR ‘B’; bonded pairs = OCR ‘A’; bonding pairs = AQA and Edexcel last time I checked!)
Pay very close attention to the language used (highlighted in red above). In this example, we might write that there are four bond pairs of electrons surrounding the central carbon atom which repel each other. The bond pairs move to minimise repulsion between them in 3D space OR we could say that the bond pairs move as far apart as possible, hence the bond angle is 109.5°.
- pyramidal / triangular pyramid (bond angle is 107°)
- bent / v-shaped / non-linear (bond angle is 104.5°)
- linear (bond angle is 180°)
- trigonal planar / planar (bond angle is 120°)
- square planar (bond angle is 90°)
This is quite an unusual shape except for transition metal complex ions – it is best thought of as an octahedron with the top and bottom removed! Other examples are the tetrafluorochloride ion, ClF4–, and xenon tetrafluoride, XeF4.
- octahedral (bond angle is 90°)
- trigonal bipyramid / bipyramidal (bond angles are 90° and 120°)
Practice questions
- Predict and explain the Cl-S-S bond angle in the molecule Cl-S-S-Cl.
- Draw the 3D structure of ammonia and name its shape.
- Draw the shape of krypton difluoride (KrF2), including any lone pairs that influence its shape. Name the shape and suggest a bond angle.
- There are two lone pairs on the oxygen atom in a molecule of oxygen difluoride, OF2. Explain how the lone pairs influence the bond angle in OF2, and include a diagram in your answer.
- Sodium reacts with ammonia to form NaNH2. Draw the shape of the NH2– ion and include any lone pairs that influence the shape. Predict the bond angle and justify your prediction.
- Which of the following molecules has one or more bond angles of 90°? CH4, NH4+, ClF4–, AlCl4–
- Ammonia reacts with aluminium chloride to form the molecule H3NAlCl3.
(i) Draw diagrams to show the shape of ammonia and aluminium chloride molecules and includeany lone pairs that would influence the shape.
(ii) State the bond angle in each molecule.
(iii) Name the type of bond between N and Al in the H3NAlCl3 molecule and explain how it isformed.
(iv) Explain how the Cl-Al-Cl bond angle changes, if at all, on formation of the H3NAlCl3 molecule.
8. Draw a dot and cross diagram for phosgene, showing outer electrons only. Name the shape of the molecule and explain why it has this shape.
9. The H-O-H bond angle in ice is about 109° and in gaseous water it is 104.5°. Explain this difference.
10. State the shape and bond angle around the carbon atom in the alkyl group of propanoic acid. Explain the shape. Finally, suggest a value for the C-O-H bond angle.
11. Which of the following molecules has the largest bond angle? BF3, CF4, NF3, OF2
12. Gallium forms an anion with chlorine, GaCl4–, forming 3 covalent bonds with chlorine atoms and making a dative covalent bond to a chloride ion. Draw a dot and cross diagram to show the bonding and predict the bond angle and shape of this ion.
13. Complete the dot and cross diagram for ethyl ethanoate. State and explain the bond angle around the ester group carbon (in bold).
14. Draw a dot and cross diagram for carbon dioxide and use it to name the shape of the molecule.
Answers
- Bond angle is 104-105° . There are 4 pairs of electrons surrounding the central S atom, made up of 2 bonding pairs and 2 lone pairs which move to minimise repulsion / as far apart as possible. Lone pairs repel more strongly as they are pulled closer into the nucleus than bonding pairs of electrons, hence a smaller angle than 109.5° (tetrahedral).
2. Wedge and dashed bonds shown, trigonal pyramidal / pyramidal.
3. Linear, 180° (there are 5 pairs of electrons surrounding the Kr atom, 3 lone pairs and 2 bonded pairs, which would suggest a trigonal bipyramid structure, then if the 3 lone pairs are in the same plane, that leaves the 2 Cl atoms above and below the plane = linear).
4. OF2 will have a very similar structure to H2O – there are 4 pairs of electrons surrounding the central O atom, made up of 2 bonding pairs and 2 lone pairs which move to minimise repulsion / as far apart as possible. Lone pairs repel more strongly as they are pulled closer into the nucleus than bonding pairs of electrons, hence a smaller angle than 109.5° (tetrahedral), probably 103-106°.
5. There are 4 pairs of electrons surrounding the central N atom, made up of 2 bonding pairs and 2 lone pairs (draw a dot and cross to prove it, remember that N has 5 electrons and then it has picked up an extra electron to make it an anion), which move to minimise repulsion / as far apart as possible. Lone pairs repel more strongly as they are pulled closer into the nucleus than bonding pairs of electrons, hence a bond angle of 104.5° (shape would be v-shaped / bent / non-linear).
6. ClF4– (you can draw a dot and cross to find how pairs of electrons surround the Cl atom).
7. Bond angle for ammonia is 107°; bond angle for aluminium chloride is 120°
The bond between N and Al is a dative covalent / coordinate bond; N donates its lone pair of electrons which are accepted by Al.
On formation of the H3NAlCl3 molecule, Al is now surrounded by 4 regions of electron density / 4 bonded pairs of electrons so the bond angle decreases from 120° to 107-111°.
8. Trigonal planar / planar. There are 3 regions of electron density surrounding the central C atom, which move to minimise repulsion / as far apart as possible.
9. In gaseous water, there are 4 pairs of electrons surrounding the central O atom, made up of 2 bonding pairs and 2 lone pairs which move to minimise repulsion / as far apart as possible. Lone pairs repel more strongly as they are pulled closer into the nucleus than bonding pairs of electrons, hence the bond angle of 104.5°. In ice, these lone pairs form hydrogen bonds with neighbouring molecules. Because of the solid structure this essentially means that in ice there are 4 bonded pairs of electrons, hence the tetrahedral bond angle of about 109°.
10. Tetrahedral, 109.5°. There are 4 bonding pairs of electrons surrounding the central C atom which move to minimise repulsion / as far apart as possible. The bond angle for C- O-H will be 104.5° (2 bonded pairs, 2 lone pairs).
11. BF3
12. Bond angle 109 – 111°, tetrahedral
13. 120°; there are 3 regions of electron density surrounding the central C atom which move to minimise repulsion and move as far apart as possible.
14. Linear
