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Q.
Which of the following pairs of atomic numbers represents $s$-block elements?
Bihar CECEBihar CECE 2009Classification of Elements and Periodicity in Properties
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.