Question

Compound formed from the combination of an element X (atomic number 20) with the element Y (atomic number 17) is $X_P{Y}_Q$. Find the value of P+Q.

• A. 4
• B. 2
• C. 7
• D. 3

Answer 1

The correct option is D 3

The atomic number of X is 20. So, its electronic configuration is 2, 8, 8, 2. It has two electrons in its outermost shell. Thus, X atom transfers its two valence electrons to two Y atoms (2, 8, 7) and changes into a stable X ion $\left(X^{+2}\right)$.
$X\to X+2+2e^{-}$
2,8,8,2 2,8,8
$2[:\underset{..}{\ddot{Y}}:]+2e^{-}\to [:\underset{..}{\ddot{Y}}:{]}^{-}$
These ions are now held together by electrostatic forces of attraction :
$X^{+}+2[:\underset{..}{\ddot{Y}}:{]}^{-}\to X^{+2}2Y^{-}$
or $X^{+2}+2Y^{-}\to X{Y}_2$