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Q.
Which is correct increasing order of their tendency of the given elements to form $M^{3-}$ ion?
Classification of Elements and Periodicity in Properties
Solution:
On moving down the group, the stability of -3 oxidation state decreases. This is due to the following reasons (i) On descending a group the size o f the atom or ion increases. As a result, attraction of the nucleus per newly added electron decreases, (ii) A large anion cannot fit easily into lattice of a small cation, (iii) As the negative charge on the ion increases, it becomes more and more susceptible to polarisation.