2. There are two groups of electrons — in this case, two bonding pairs around the central beryllium atom in beryllium chloride, BeCl 2.

3. They repel each other equally.

4. The bond angle in beryllium chloride is 180°. It is a linear molecule.

5. Carbon dioxide also has two groups of electrons. In this case, there are two groups of two bonding pairs of electrons around the central carbon atom. There are two double bonds.

6. The groups of electrons repel each other equally, so carbon dioxide is also a linear molecule.

7. Ethyne, C 2 H 2, is also a linear molecule with bond angles of 180°.

8. There are three groups of bonding electrons — in this case, there are three single covalent bonds around the central boron atom in boron trifluoride, BF 3.There are three bonding pairs of electrons.

9. They repel each other equally.

10. The bond angle in boron trifluoride is 120°. It is a triangular molecule.

11. It is also a flat, planar molecule.

12. So it is a planar triangular molecule.

17. This is a displayed formula for methane, CH 4.

18. There are four groups of bonding electrons — in this case, there are four pairs of bonding electrons around the central carbon atom.

19. They are negatively charged, and repel each other equally to form a molecule that is not flat.

20. The bond angle is approximately 109°, not 90° as you might have expected from the displayed formula.

21. We can draw lines between three of the hydrogen atoms.

22. They form the corners of a triangle.

23. The carbon atom is at the centre of a regular four-sided tetrahedron.

24. Methane is a molecule with a tetrahedral shape.

25. This is a displayed formula for ammonia, NH 3.

26. There are four groups of bonding electrons — in this case, there are three pairs of bonding electrons and one lone pair of electrons around the central nitrogen atom.

27. The pairs of electrons form a tetrahedral shape. The bonding pair of electrons forms a pyramidal shape.

28. This is a displayed formula for water, H 2 O.

29. There are four groups of electrons around the oxygen — in this case, two single covalent bonds and two lone pairs of electrons.

30. The pairs of electrons form a tetrahedral shape. The bonding pairs of electrons form a pyramidal shape.

31. This is a dot-cross diagram for the bonding in sulphur(VI) fluoride, SF 6. There are six pairs of bonding electrons.

32. This is the corresponding displayed formula of SF 6. But the molecules are not flat.

33. They form a shape like two Egyptian pyramids on top of each other.

34. The two pyramids are joined to form a regular eight-sided object, called an octahedron.

35. The two pyramids are joined to form a regular eight-sided object, called an octahedron.

36. The two pyramids are joined to form a regular eight-sided object, called an octahedron.

37. The two pyramids are joined to form a regular eight-sided object, called an octahedron.

38. There are two axial fluorine atoms.

39. There are four equatorial fluorine atoms.

40. Now see what the molecule looks like in three dimensions.

41. SF 6 is an octahedral molecule. There are no lone pairs of electrons around the central sulphur atom.

42. What happens if there is a lone pair of electrons around the central atom?

43. This is IF 5. The lone pair of electrons is arranged in axial position, giving a square pyramidal molecule.

44. What happens when there are two lone pairs of electrons around the central atom?

45. This is ICl 4 –. Its two lone pairs of electrons are both in axial positions, giving a square planar ion.

46. Linear molecules have two pairs of electrons, or two groups of electrons, around the central atom.

47. Planar triangular molecules have three pairs of electrons, or three groups of electrons, around the central atom.

48. Tetrahedral molecules have four pairs of electrons around the central atom.

49. Octahedral molecules have six pairs of electrons around the central atom.