In lanthanides, with increase in atomic number atomic radius decreases, except for the element $\underline{X}$. What is $\underline{X}$ ?

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Aicharya · Accepted Answer

Lanthanides show decrease in atomic radius with increase in atomic number since electrons are filled in the $f$-subshell. Since, the $f$-subshell after poor shielding, the atomic size will decrease as the nuclear charge increases. Eu electronic configuration is $4 f^7, 6 s^2$ with half-filled outermost shell so nuclear pull is less and it does not show lanthanide contraction. ~

In lanthanides, with increase in atomic number atomic radius decreases, except for the element…

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