Give reasons for each of the following observations I. Only higher members of the group 18 of the periodic table are expected to form compounds. II. NO2 readily forms a dimer whereas ClO2 doesn’t.

Let's delve into the observations you've mentioned regarding group 18 elements and the behavior of nitrogen dioxide (NO2) and chlorine dioxide (ClO2). Each observation has its own underlying reasons based on the properties of the elements involved.

Group 18 Elements and Compound Formation

Group 18 of the periodic table, also known as the noble gases, includes helium, neon, argon, krypton, xenon, and radon. The observation that only the heavier members of this group are expected to form compounds can be explained through several key factors:

• Inert Nature: The lighter noble gases, such as helium and neon, have a complete valence shell with a stable electronic configuration. This stability makes them largely unreactive, or inert, under normal conditions.
• Atomic Size and Polarizability: As we move down the group, the atomic size increases, and so does the polarizability of the electron cloud. Heavier noble gases like xenon and radon can form compounds because their larger electron clouds can be distorted more easily, allowing them to interact with other elements.
• Energy Considerations: The energy required to form bonds decreases for heavier noble gases due to their larger atomic radius and lower ionization energies compared to lighter noble gases. This makes it energetically favorable for them to engage in chemical bonding.

In summary, the combination of inertness in lighter noble gases and the increasing reactivity of heavier ones due to atomic size and polarizability explains why only the higher members of group 18 form compounds.

Behavior of NO2 and ClO2

Now, let’s examine why nitrogen dioxide (NO2) readily forms a dimer while chlorine dioxide (ClO2) does not. This observation can be understood through the following points:

• Molecular Structure: NO2 is a bent molecule with an unpaired electron, making it a free radical. This unpaired electron allows NO2 molecules to easily react with each other to form dinitrogen tetroxide (N2O4), a dimer. The dimerization process stabilizes the radical by pairing the unpaired electrons.
• Stability of the Dimer: The dimer N2O4 is more stable than two separate NO2 molecules due to the formation of additional bonds, which lowers the overall energy of the system. This thermodynamic favorability drives the reaction towards dimerization.
• ClO2's Structure and Stability: In contrast, ClO2 is also a radical but has a different electronic structure. The presence of two unpaired electrons in ClO2 leads to a repulsion between the molecules, making dimerization less favorable. Instead of forming a stable dimer, ClO2 tends to exist as individual molecules.

To summarize, the propensity of NO2 to form a dimer is due to its radical nature and the stability of the resulting dimer, while ClO2's structure and the repulsive interactions between its unpaired electrons prevent it from doing so.