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  4. An ion has 18 electrons in the outermost shell, it is

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Q. An ion has $18$ electrons in the outermost shell, it is

AIPMTAIPMT 1990Structure of Atom

Solution:

The electronic configuration of $Cu , Th , Cs$ and $K$ are
$Cu (29)=2,8,18,1$
$C u^{+}=2,8,18$
$Th(90)=2,8,18,32,18,10,2 $
$T h^{4+}=2,8,18,32,18,8 $
$Cs(55)=2,8,18,18,8,1 $
$C s^{+}=2,8,18,18,8 $
$ K (19)=2,8,8,1$
$K^{+}=2,8,8$
Thus, $C u^{+}$has $18$ electrons in the outermost shell.