The boiling point of helium is 4.22 K (−268.93 °C). The boiling point of hydrogen is 20.28 K (−252.87 °C). However, the attractive force between two hydrogen atoms 100 pm apart is almost 5000 times stronger than the attractive force between two helium atoms 100 pm apart.
Think about helium and hydrogen at the atomic scale. Then write in your notebook an explanation for the fact that both helium and hydrogen have very low boiling points but hydrogen’s is higher.
A substance made of molecules is called a covalent molecular substance. An important point in the activity you just completed is this: unlike metals or noble gases, where boiling involves freeing atoms from each other, boiling a covalent molecular substance involves freeing molecules from each other. No covalent bonds are broken during the boiling process and the same molecules are present in the gas phase as were present in the liquid phase. The same reasoning applies to melting: covalent bonds between atoms within molecules are not broken, but forces between the molecules must be partially overcome.
Because there are many different kinds of nonmetal atoms that can form covalent bonds, and because molecules can consist of anywhere from two to many thousands of atoms, the range of properties of covalent molecular substances is much broader than for ionic compounds or metals. Many covalent molecular substances are liquids or gases, in other words, they melt and some boil, below room temperature or not too far above. Covalent molecular substances do not conduct electricity well as solids or liquids, the solids may be weak and brittle or soft and waxy, and many are insoluble in water. We will begin to explore this broad range of molecules and properties in Unit 2. For now, we consider a single class of covalent molecular substances: hydrocarbons.