Group 1 Elements

The question asks for the correct order of hydration enthalpies of alkali metal ions. In order to determine this, we need to understand the concept of hydration enthalpy and how it varies with alkali metals.

Hydration enthalpy is the energy released when ions are surrounded by water molecules. For alkali metal ions, hydration enthalpy is influenced by the size of the ion; smaller ions have higher charge density and attract water molecules more strongly, resulting in higher hydration enthalpy.

Alkali metals in the periodic table are arranged as follows based on increasing atomic number: Li, Na, K, Rb, Cs.

The size of the ion increases as we move down the group from Li⁺ to Cs⁺. Thus, the charge density decreases, meaning the hydration enthalpy also decreases.

Based on this understanding, the correct order of hydration enthalpies from highest to lowest is:

This order corresponds to the first option provided in the question, which is:

Therefore, the correct answer is the first option: .

Other options are incorrect as they do not follow the proper trend based on ionic size and charge density. Remember, in general, smaller ions release more energy upon hydration due to their greater attraction to the polar water molecules.

Beryllium Chloride Structure Analysis

exhibits different structures depending on the phase and temperature:

Solid State:

In the solid state, exists as a polymeric structure. Beryllium has a tendency to form coordinate covalent bonds due to its small size and relatively high charge density. In the polymeric structure, each beryllium atom is bonded to four chlorine atoms, and each chlorine atom bridges between two beryllium atoms, forming a chain-like structure.

Vapour Phase:

In the vapour phase, below 1200 K, primarily exists as a dimer ( ). The dimeric structure is formed through chloro-bridging, where two chlorine atoms bridge between two beryllium atoms. This satisfies the beryllium's desire for a coordination number of four.

Very High Temperature (above 1200 K):

At very high temperatures (above 1200 K), the thermal energy is sufficient to break the chloro bridges in the dimer, leading to the formation of monomeric units. The monomeric form has a linear structure with beryllium having a coordination number of two.

Conclusion:

Therefore, the structures of are polymeric in the solid state, dimeric in the vapour phase, and monomeric at very high temperatures.

Analysis of the Given Compounds:

• NaO₂: This compound does not exist. The correct formula for this compound is Na₂O, which is sodium oxide.
• BeCl₂: This is beryllium chloride, a valid and known chemical compound.
• PbEt₄: This is tetraethyllead, a valid chemical compound used historically as a fuel additive.
• (NH₄)₂BeF₄: This is ammonium beryllium fluoride, which is also a valid chemical compound.

Conclusion:

The compound that does not exist is NaO₂. The correct formula is Na₂O.

Answer: B