D8.1 Covalent Molecular Substances
Applying Core Ideas: Comparing Hydrogen Molecules and Helium Atoms
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.
Think. Write in your notebook. Then left-click here for an explanation.
In the case of hydrogen, because two H atoms form molecular orbitals where the MO electron configuration has two electrons in the bonding MO, two H atoms are much lower in energy when close together. Therefore the H2 molecule has a very strong bond. At the atomic scale hydrogen consists of hydrogen molecules, H2.
In the case of helium, two He atoms do not form a bond. Two He atoms have four electrons: two occupy the bonding and two occupy the antibonding MO. There is no lowering of energy, no covalent bond, and no He2. At the atomic scale helium consists of helium atoms, He.
In order to boil hydrogen, H2 molecules must be completely separated from each other. In order to boil helium, He atoms must be completely separated from each other. Thus the attractive forces, that is, London dispersion forces, between H2 molecules should be compared with LDFs between He atoms.
For both H2 and He, there are two electrons per atomic-scale particle: H2 has a single, electron-pair bond with the two electrons spread out over two nuclei (protons) that are 74 pm apart; He has two electrons in the 1s orbital attracted by two protons in the He-atom nucleus. (The two neutrons in a typical He nucleus carry no electric charge and do not influence the surrounding electrons.)
In H2 the electrons are spread out over a larger volume, both because a H atom is bigger than a He atom and because the electrons are spread over both H nuclei. Therefore, it is easier to distort the electron density in a H2 molecule than to distort the electron density in a He atom. LDFs depend on both the number of electrons and the ease with which an electron density distribution can be distorted. Thus, LDFs are greater for H2 than for He and H2 has the higher boiling point. The difference is not large, however. Both boiling points are very low.
LDFs affect molecules as well as atoms, and for both atoms and molecules, the size of the LDFs can be predicted by comparing number of electrons and ease of distortion of an electron density distribution.
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.
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