2. For a simple molecular substance to boil, energy is needed to overcome the
intermolecular forces.

3. Large molecules have stronger instantaneous dipole–induced dipole attractions
than small molecules.

4. Compounds with large molecules tend to have higher boiling points than those
with small molecules.

5. The instantaneous dipole–induced dipole attractions of these hydrides increase
as their relative formula masses increase.

6. So their melting points also increase as their relative formula masses increase.

7. The same thing happens with these hydrides.

8. But something very different happens with water, H2O.

9. It has a much higher boiling point than expected. Why?

10. It has a much higher boiling point than expected. Why?

11. A water molecule contains two hydrogen atoms and an oxygen atom.

12. Each hydrogen atom is bonded to the oxygen atom by a single covalent bond.

13. Oxygen is more electronegative than hydrogen, so it gains a partial
negative charge.

14. The two hydrogen atoms gain a partial positive charge.

15. There are permanent dipole–permanent dipole attractions between water
molecules.

16. They occur in a particular set of circumstances in water, and are called hydrogen
bonds.

17. Oxygen has six electrons in its outer shell.

18. Two of these form bonding pairs with the electrons from the two hydrogen atoms.

19. This leaves four electrons that are not involved in bonding
This leaves four electrons that are not involved in bonding. They form two
lone pairs of electrons.

20. There is a force of attraction…

21. … between a lone pair of electrons on an oxygen atom and the δ+ charge on
a hydrogen atom.

22. This is a hydrogen bond.

23. Water molecules have two lone pairs of electrons, so ...

24. … each water molecule can form more than one hydrogen bond.

25. … each water molecule can form more than one hydrogen bond.

26. … each water molecule can form more than one hydrogen bond.

27. … each water molecule can form more than one hydrogen bond.

28. Hydrogen bonds are stronger than other types of intermolecular forces.

29. Molecules with hydrogen bonding have higher boiling points than their relative
formula masses alone would suggest.

30. Nitrogen, oxygen and fluorine are very electronegative and small enough for
hydrogen bonding to occur.

31. Ammonia (NH3), water (H2O) and hydrogen fluoride (HF) have hydrogen bonding.

32. Methane (CH4) does not have hydrogen bonding, even though its molecules
contain hydrogen atoms.

33. Methane and ammonia have similar relative formula masses
Methane and ammonia have similar relative formula masses. Their instantaneous
dipole–induced dipole attractions are similar.

34. But their boiling points are very different because ammonia also has
hydrogen bonding.