Which gases approach ideal behaviour due to weak intermolecular forces?

• A. Diatomic molecules like O2 and N2
• B. Noble gases such as helium and neon
• C. Halogens like Cl2
• D. Water vapour

Answer: (B)

Ideal gas behavior arises when intermolecular forces are negligible and the actual molecular size is tiny compared with the container. The gas that best fits this is the one whose particles interact very weakly because they are monoatomic and nonpolar with minimal polarizability. Noble gases fit this perfectly: they don’t form bonds, their atoms are spherical, and the dispersion forces between them are extremely small. Because of these negligible attractions, their collisions are almost perfectly elastic and the volume of the atoms themselves is insignificant, so the gas behaves very closely to an ideal gas (PV ≈ nRT) over a wide range of conditions.

In contrast, diatomic gases like O2 and N2 do have weak London dispersion forces, but these are not as tiny as in noble gases. Halogens like Cl2 have stronger dispersion forces due to more electrons and greater polarizability. Water vapour experiences strong hydrogen bonding, which greatly pulls molecules together and causes large deviations from ideal behavior.