Total number of ions from the following with noble gas configuration is ____. \[ \begin{aligned} & \mathrm{Sr}^{2+}(\mathrm{Z}=38), \mathrm{Cs}^{+}(\mathrm{Z}=55), \mathrm{La}^{2+}(\mathrm{Z}=57) \mathrm{Pb}^{2+} \\ & (\mathrm{Z}=82), \mathrm{Yb}^{2+}(\mathrm{Z}=70) \text { and } \mathrm{Fe}^{2+}(\mathrm{Z}=26) \end{aligned} \]

JEE Main · 2024 · 27 Jan 2024 · Shift 2 · Atomic Structure

Total number of ions from the following with noble gas configuration is ____.

Sr^{2 +} (Z = 38), Cs^{+} (Z = 55), La^{2 +} (Z = 57) Pb^{2 +} (Z = 82), Yb^{2 +} (Z = 70) and Fe^{2 +} (Z = 26)

Answer: 2

A noble-gas configuration ends in a filled

n s^{2} n p^{6}

shell with no partially filled d or f subshell.

S r^{2 +}

(Z=38): losing 2 electrons from

[K r] 5 s^{2}

gives

[K r]

— noble gas.

C s^{+}

(Z=55): losing 1 electron from

[X e] 6 s^{1}

gives

[X e]

— noble gas.

L a^{2 +}

(Z=57): gives

[X e] 5 d^{1}

— a partially filled d subshell, not noble gas.

P b^{2 +}

(Z=82): gives

[X e] 4 f^{14} 5 d^{10} 6 s^{2}

— the

6 s^{2}

pair (inert-pair effect) remains, not noble gas.

Y b^{2 +}

(Z=70): gives

[X e] 4 f^{14}

— a filled but still occupied f subshell, not the immediately preceding noble gas.

F e^{2 +}

(Z=26): gives

[A r] 3 d^{6}

— partially filled d subshell, not noble gas.

Only

S r^{2 +}

and

C s^{+}

attain a true noble-gas configuration — a count of 2.

JEE Main · 2024 · 27 Jan 2024 · Shift 2 · Integer answer