Q. Which of the following pairs of atomic numbers represents $s$ -block elements?

1486 210 Bihar CECEBihar CECE 2009Classification of Elements and Periodicity in Properties Report Error

A

7, 15

0%

B

6, 12

0%

C

9, 17

0%

D

3, 11

100%

Solution:

s-block elements contain either 1 or 2 electrons in their outer most shell.
$3 = 1 s^{2}, 2 s^{1}$
$11 = 1 s^{2}, 2 s^{2}, 2 p^{6}, 3 s^{1}$
From the above electronic configuration, it is clear that elements having atomic number 3 and 11 belong to $s$ -block. Since, both contains one electron in outer most shell, these belong to alkali metals.