First slide

Alkaline earth metals

Question

Which is not the correct variation of the property indicated of chlorides of alkaline earth metals?

Moderate

A

${BeCl}_{2} < {MgCl}_{2} < {CaCl}_{2} < {BaCl}_{2}$ (Covalent nature)
B

${BeCl}_{2} < {MgCl}_{2} < {CaCl}_{2} < {BaCl}_{2}$ (melting point)
C

${BaCl}_{2} < {CaCl}_{2} < {MgCl}_{2} < {BeCl}_{2}$ (Lattice energy)
D

${BaCl}_{2} < {CaCl}_{2} < {MgCl}_{2} < {BeCl}_{2}$ (Thermal stability)

Solution

By Fajan's rule,
smaller the cation,
larger the anion,
larger the charge,
presence of d- and f-electrons,
then larger the polarisation of the cation by anion and vice-versa.
Thus, covalent nature increases, melting point decreases, lattice
energy increases and thermal stability increases
$Size {Be}^{2 +} < {Mg}^{2 +} < {Ca}^{2 +} < {Ba}^{2 +}$ (anion is common)
Thus, BeCI₂, has maximum covalent nature and has least melting point.
BeCl₂has maximum lattice energy and has maximum thermal stability.
Thus (a) is incorrect, (b) is correct, (c) is correct, (d) is correct.

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