1. a
Both CCl4 and CBr4 are nonpolar. This means that London dispersion forces hold the molecules together in the liquid state. To boil the substances, the LDF must be broken. Since CBr4 has more electrons than CCl4, a stronger momentary dipole can be formed when its electrons shift. Therefore, the molecules are more attracted to each other and more energy is required to break those attractive LD forces.

2. b
When nonpolar iodine and nonpolar bromine boil, London dispersion forces are being broken. [Bond energy is the energy required to break the covalent bond within the Br2 and I2 molecule. It’s not relevant to boiling for these two substances.] Since iodine is a larger molecule with MORE ELECTRONS, more energy is required to break its London dispersion forces.

3. c
SbCl3 is trigonal pyramidal in shape while SbCl5 is trigonal bipyramidal. The lone pair of electrons on SbCl3 creates an asymmetrical molecule with the three chlorine atoms on one side of the molecule. Therefore, it is polar and has a dipole moment. The trigonal bipyramidal shape has a symmetrical arrangement of atoms and is nonpolar (has no dipole moment).

4. d
Fluorine and iodine are both nonpolar molecules that would be held together by London dispersion forces. Since iodine has a large number of electrons, the LDFs are strong enough to keep the atoms attracted to each other in the solid state. Since fluorine is small, there are not enough electrons to create a strong enough dipole moment to allow the molecules to stick together, so it’s a gas at 25C.

5. e
There are two lone pairs of electrons on the central atom in ICl4-. Since there are no lone pairs of electrons on BF4-, it’s electrons spread out into the shape of a tetrahedral.

6. f
Ammonia has LDF, DD, and Hydrogen attractions. Phosphine has LDF and DD. The extreme polarity of ammonia (because of the large difference in electronegativity between hydrogen and nitrogen) causes it to be more attracted to the water molecules than phosphine, making it more soluble.

7. g
Because the ions in NaF are smaller than the ions in CsCl, they can get closer to each other. According to Coulomb’s law, having a smaller distance between ions (r2) increases the force of attraction and causes more energy to be required to break the electrostatic attractions. Hence, NaF has a higher melting point than CsCl.

8. h
Solid K is a metallic solid. The delocalized sea of electrons are able to move current throughout the solid. Potassium nitrate in an ionic solid. The electrons are trapped on to the ions and cannot move; therefore, the solid doesn’t conduct electricity.