Find the atomic number of the element at whic | vedclass

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(D) The electronic configuration of Lanthanum $(Z=57)$ is $[Xe] 5d^1 6s^2$.The filling of the $4f$ orbital begins with Cerium $(Z=58)$,which has the c

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$65$

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(D) The electronic configuration of Lanthanum $(Z=57)$ is $[Xe] 5d^1 6s^2$.The filling of the $4f$ orbital begins with Cerium $(Z=58)$,which has the configuration $[Xe] 4f^1 5d^1 6s^2$.As we proceed through the Lanthanide series,electrons are added to the $4f$ subshell.Pairing in the $4f$ orbital starts when the $8^{th}$ electron enters the $4f$ subshell,which corresponds to Gadolinium $(Z=64)$.The configuration of Gadolinium is $[Xe] 4f^7 5d^1 6s^2$,and the next element,Terbium $(Z=65)$,has the configuration $[Xe] 4f^9 6s^2$ (or $[Xe] 4f^8 5d^1 6s^2$),where the $8^{th}$ electron forces pairing in the $4f$ orbital.

List-$I$ (Element/Compound)	List-$II$ (Use)
$A$. Lanthanoid oxide	$1$. Production of iron alloys
$B$. Lanthanoid	$2$. Television screens
$C$. Mischmetal	$3$. Petroleum cracking
$D$. Magnesium based alloy	$4$. Lanthanoid metal $+$ iron
$E$. Mixed lanthanoid oxides	$5$. Bullet tips
	$6$. $X$-ray screens

Find the atomic number of the element at which pairing in the $4f$ orbital starts.

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