Final answer:
Among the given molecules, HCl and HCN have polar covalent bonds. H2 and N2 are nonpolar as they consist of identical atoms. CO2 has polar bonds but is overall a nonpolar molecule due to its linear shape causing dipole cancellation.
Step-by-step explanation:
To determine if the molecules H2, N2, HCl, HCN, and CO2 possess polar covalent bonds, we need to consider the electronegativity difference between the atoms within each molecule. Molecules like H2 and N2 are nonpolar because they consist of two identical atoms with the same electronegativity, hence no difference, and thus no polarity in their bond. On the other hand, molecules such as HCl and HCN exhibit polar covalent bonds due to significant electronegativity differences between the hydrogen (H) and other atoms (Cl in HCl and C or N in HCN).
Carbon dioxide (CO2) also has polar covalent bonds due to the difference in electronegativity between carbon (C) and oxygen (O), but it is important to note that the molecule overall is nonpolar because of its linear geometry which allows the dipole moments to cancel each other out.