Comparing melting points

NaCl and CCl4

NaCl has a higher m.p because NaCl is a giant ionic compound, it is held together by strong electrostatic forces in a giant ionic lattice. CCl4 is a weak covalent compound held together by london forces. The oppositely charged ions in NaCl requires much more energy to break than london forces.

PCl5 and SiCl4

Both PCl5 and SiCl4 are simple molecules, non-polar, and rely on london forces. however, PCl5 has a higher m.p because it is a larger molecule, it contains more electrons, which means the molecule is highly polarizable, creating stronger temporary and induced dipoles. Because of this, it's london forces are stronger than that of SiCl4. Requiring more energy to break compared to SiCl4.

SO2 and SiO2

SO2 is a simple molecule and relies on london forces. however SiO2 is a giant covalent structure experiencing strong intermolecular covalent bonds. A large amount of energy would be needed in order to break apart the lattices.

Na and Mg

Both are metals. However, Mg forms a 2+ ionic charge and has twice as many electrons per atom compared to Na. Mg is also smaller than Na, and has higher charge density. This causes stronger E.A. between the ions and the delocalized electrons. Hence higher m/b.p.

H2O and H2S

H2O has a higher m/b.p because H2O is able to form hydrogen bonds, which requires lots of energy to break. H2S does not form hydrogen bonds, instead, it relies on london forces which are significantly weaker than hydrogen bonds. Even though H2S has stronger london forces than H2O, this isn't nearly as strong as the hydrogen bonds.

ICl and Br2

has polar covalent bonds as Cl is very electronegative and Iodine is not. This causes a net dipole moment between molecules on top of LF which leads to ICl having permanent dipole-dipole forces. Br2 however, relies only on london forces in between molecules. Because permanent dipole forces are stronger than london forces, ICl has a higher m/b.p than Br2.

Si and C(diamond)

Both form giant covalent lattices. However Si-Si bonds are weaker than C-C bonds. From Si o , nuclear charge increases, shielding increases, effective nuclear charge stays the same, however atomic radius increases and the outer electrons are further away experiencing less FA between its nucleus and outer electrons. Requiring less energy to break them.

S8 and P4

Both are nonmetals, and rely on london forces between molecules. However S8 experiences stronger london forces as it is a larger molecule containing more electrons. Which results in the molecule having higher chances of polarizability and therefore creates stronger temporary and induced dipoles between neighboring molecules. Requiring more energy to break. Hence, a higher m/b.p.