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Question

Which is the correct order for ionic radii?


A

Li+ < Al3+ < Mg2+ < K+

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B

Al3+ < Mg2+ < K+ < Li+

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C

Mg2+ < Al3+ < K+ < Li+

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D

None of the above

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Solution

The correct option is A

Li+ < Al3+ < Mg2+ < K+


Li+ 1s2

Al3+ 1s22s22p6

Mg2+ 1s22s22p6

K+ 1s22s22p63s23p6

Principal quantum number (n) is maximum in K+, therefore it is maximum in size.

Down the group, atomic radius increases because the principal quantum number (n) increases. And the valence electrons are farther from the nucleus.

Al3+ and Mg2+ are isoelectronic.

Al3+ < Mg2+

Higher the ionic charge on the atom, more will be the ionic radius. Now, in option (a), the correct order will be Mg2+ > Li+ > Be2+

Because Mg2+ has principal quantum number (n) = 3 and Be2+ gas n = 2

Among Li2+ and Be2+, Li+ > Be2+ because along a period from left to right, atomic radius increases.

Now, in option (b)

H+ < Li+ < H

For H+, there are no electrons, so no atomic radius. In the case of Li+ and H, Li+ < H, because n is least in Li+, i.e., n = 1.

Therefore, it is the smallest in size. So, the correct order is Li+ < Al3+ < Mg2+ < K+


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