Question

Chemical compounds in which anions and cations are held together by electrostatic force of attraction are known as ionic compounds. Cations and anions are charged species which are formed by losing and gaining electron(s) respectively to attain the nearest noble gas configuration or stability. Usually metals lose their valence electrons to form cations while non-metals accept electrons and form anions. Being held by a strong electrostatic force of attraction between ions, ionic compounds have high melting and boiling points.

(iv) The electronic configuration of elements Q and R as follows:
Q: 2, 8, 2
R: 2, 8, 7
Which of the following correctly represents the formula of ionic compound formed by combination of Q and R?

• A. QR
• B. ${\text{Q}}_{2}R$
• C. ${\text{Q}}_2{R}_2$
• D. $\text{Q}{R}_2$

Answer 1

The correct option is D $\text{Q}{R}_2$

The electronic configuration of elements Q and R as follows:
Q: 2, 8, 2 (Atomic number = 12)
R: 2, 8, 7 (Atomic number = 12)
Therefore, Q (Atomic number = 12) is magnesium and R (Atomic number = 17) is chlorine.

To attain its nearest noble gas configuration, magnesium (Q) has to lose two electrons while chlorine (R) has to gain one.
If both Q and R are going to form an ionic compound, magnesium must give its valence electrons to two different atoms of chlorine hence will result in the formation of compound having $\text{Q}{R}_2$ as its chemical formula.

$\text{Q}{R}_2$ is magnesium chloride $\text{Mg}{Cl}_2$.