Question

Which of the substances in each of the following sets would be expected to have the lowest melting point? Explain why. a. \(\mathrm{H}_{2}, \mathrm{N}_{2}, \mathrm{O}_{2}\) b. Xe, NaCl, C (diamond) c. \(C l_{2}, B r_{2}, I_{2}\)

Short answer

a. \(\mathrm{H}_{2}\) has the lowest melting point due to the weakest London dispersion forces among these substances, caused by its least number of electrons. b. Xe has the lowest melting point due to weak dispersion forces, which are weaker than the ionic bonds in NaCl and the covalent network in diamond. c. \(Cl_{2}\) has the lowest melting point due to the smallest atoms and weakest London dispersion forces among these substances.

Step-by-step solution

Identify the type of bonding in each substance

Comparing the type of bonding in each substance will give us insight into their melting points. In general, ionic and covalent compounds have high melting points, while substances with weak intermolecular forces have lower melting points. a. \(\mathrm{H}_{2}, \mathrm{N}_{2}, \mathrm{O}_{2}\) - All three are simple molecular substances with nonpolar covalent bonds. b. Xe - Noble gas with weak dispersion forces; NaCl - Ionic compound; C (diamond) - Covalent network. c. \(Cl_{2}, Br_{2}, I_{2}\) - All three are molecular substances with nonpolar covalent bonds.

Compare the strength of the forces in each set

Now that we know the type of bonding in each substance, we can compare the relative strength of the forces within each set to predict which substances will have the lowest melting points. a. \(\mathrm{H}_{2}, \mathrm{N}_{2}, \mathrm{O}_{2}\) - All three have weak London dispersion forces, but \(\mathrm{H}_{2}\) has the least number of electrons making its dispersion forces the weakest. b. Xe - weak dispersion forces; NaCl - strong ionic bonds; C (diamond) - strong covalent network. Out of these, Xe has the weakest forces. c. \(Cl_{2}, Br_{2}, I_{2}\) - All three have dispersion forces, but the strength of dispersion forces increases with the size/mass of the atoms. In this case, \(Cl_{2}\) has the smallest atoms and the weakest dispersion forces.

Determine the substance with the lowest melting point and explain why

a. \(\mathrm{H}_{2}\) has the lowest melting point because it has the least number of electrons and the weakest London dispersion forces. b. Xe has the lowest melting point because it has weak dispersion forces, which are much weaker than the ionic bonds in NaCl and the covalent network in diamond. c. \(Cl_{2}\) has the lowest melting point because it has the smallest atoms and therefore the weakest London dispersion forces among the three substances.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces that hold molecules together and are critical in determining the physical properties of a substance. They vary in strength and can be substantially weaker than the bonds that hold atoms together within a molecule, known as intramolecular forces.

There are several types of intermolecular forces, and these include:

• London Dispersion Forces: These are weak forces that arise from temporary fluctuations in the electron distribution within molecules or atoms, leading to a temporary dipole moment. They are present in all substances and are particularly significant in nonpolar molecules.
• Dipole-Dipole Interactions: These occur between polar molecules where the positive end of one molecule is attracted to the negative end of another molecule.
• Hydrogen Bonds: This is a special type of dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine.

Substances with larger molecules or those that are more polarizable (easily forming temporary dipoles) tend to have stronger dispersion forces and, therefore, higher melting points. This concept explains why, in the exercise, H₂ has the lowest melting point in its set - its molecules are smaller and less polarizable than those of N₂ or O₂.

Chemical Bonding

Chemical bonding pertains to the connection between atoms within a molecule or compound. The main types of chemical bonds include:

• Ionic Bonds: These form when electrons are transferred from one atom to another, leading to the creation of oppositely charged ions that attract each other. Ionic bonds are typically strong and result in high melting points for ionic compounds like NaCl.
• Covalent Bonds: These involve the sharing of electrons between atoms and can be either polar or nonpolar. Nonpolar covalent bonds occur between atoms of the same element or when there is equal sharing of electrons. Polar covalent bonds happen when electrons are shared unequally.
• Metallic Bonds: Metals share their electrons in a 'sea' of electrons, which gives them unique properties like conductivity and malleability.
• Covalent Network Bonds: These are directional bonds that create large networks or chains, featuring in materials such as diamond or silica, and contribute to their high melting points and hardness.

An understanding of these bonds allows us to predict melting point trends as seen in the exercise. For instance, NaCl, with its strong ionic bonds, has a much higher melting point than Xe, which relies only on weak dispersion forces for intermolecular interactions.

Molecular Substances

Molecular substances consist of molecules bound together by intermolecular forces rather than chemical bonds. The properties of these substances, including their melting points, are largely influenced by the nature of these intermolecular forces. Molecular substances often exist as gases, liquids, or low-melting-point solids at room temperature, due to the relative weakness of intermolecular forces compared to ionic or covalent bonds.

For example, the exercise mentions Cl₂, Br₂, and I₂ - all halogens that exist as molecular substances with nonpolar covalent intramolecular bonding and London dispersion forces between their molecules. The size and electronic characteristics of the halogen molecules affect the strength of these dispersion forces: the larger and more electron-rich the molecule, the stronger the intermolecular attractions. Thus, Cl₂ has the lowest melting point in its group because it has the smallest, least polarizable molecules, resulting in weak dispersion forces and hence a lower melting point.