Question

Which of the following pair of an atomic numbers represents s-block elements?

• A. $7,15$
• B. $6,12$
• C. $9,17$
• D. $3,20$

Answer 1

The correct option is D

$3,20$

Explanation for correct option:

(D) $3,20$

• s-block elements are present in group 1 and 2 of periodic table.
• Group-1 contains Alkali metals while Group-2 contains Alkaline earth metals. Group-1 contains one electron in its outermost shell while Group-2 contains 2 electrons in its valence shell.

Lithium $\left(\mathrm{Li}\right)$ has atomic number $3$, it belongs to group 1 i.e. Alkali metals while calcium $\left(\mathrm{Ca}\right)$ has atomic number $20$ and it belongs to group 2 i.e. Alkaline earth metals.

Hence, the correct option is (D) $3,20$.

Explanation for incorrect options:

(A) Nitrogen $\left(\mathrm{N}\right)$ has atomic number $7$ while phosphorus $\left(\mathrm{P}\right)$ has atomic number $15$. These elements belong to p-block elements.

(B) Oxygen $\left(\mathrm{O}\right)$ has atomic number $8$ while magnesium $\left(\mathrm{Mg}\right)$ has atomic number $12$ . Oxygen belongs to p-block while magnesium belongs to s-block.

(C) Fluorine $\left(\mathrm{F}\right)$ has atomic number $9$ and belongs to p-block element. Chlorine $\left(\mathrm{Cl}\right)$ has atomic number $17$ and it belongs to p-block elements.

Therefore, the correct option is (D) $3,20$ since the given pair of atomic numbers represents s-block elements.