Question

$X^{-},Y^{2-}$ and $Z^{3-}$ are isotonic and isoelectronic. Thus, increasing order of atomic number of $X,Y$ and $Z$ is :

• A. $X<Y<Z$
• B. $Z<Y<X$
• C. $X=Y=Z$
• D. $Z<X<Y$

Answer 1

Answer: (B)

$Z<Y<X$

$X^{-},Y^{2-}$ and $Z^{3-}$ are isotonic and isoelectronic.
So, we can assume they all have same number of $n$ electrons.
$X^{-}$ have $n-1$ protons.
$Y^{2-}$ have $n-2$ protons.
$Z^{3-}$ have $n-3$ protons.
Therefore, the order of atomic number (proton) is $Z<Y<X$.