Question

Which pair of atomic numbers represent s-block elements?
A) 7,15
B) 6,14
C) 9,17
D) 4,12

Answer 1

Hint: From the atomic number of each element, write its electronic configuration. From the electronic configuration, determine the block to which the element belongs.

Complete answer:
In the modern periodic table, the elements are arranged in the increasing order of the atomic number. In the modern periodic table, there are seven periods (horizontal rows) and 18 groups (vertical columns). s-block elements include alkali metals and alkaline earth metals. Alkali metals include lithium, sodium potassium, rubidium cesium and francium. Alkaline earth metals include beryllium, magnesium, calcium, strontium, barium and radium.
S-block elements are highly electropositive metals with low ionization energy values. They have general electron configuration of $ns^(1-2)$. Alkali metals have one valence electron whereas alkaline earth metals have two valence electrons.

Atomic number	Electronic configuration	Number of valence electrons	block
4	$1s^2,2s^2$	2	S
6	$1s^2,2s^2,2p^2$	4	P
7	$1s^2,2s^2,2p^3$	5	P
9	$1s^2,2s^2,2p^5$	7	P
12	$1s^2,2s^2,2p^6,3s^2$	2	S
14	$1s^2,2s^2,2p^6,3s^2,3p^2$	4	P
15	$1s^2,2s^2,2p^6,3s^2,3p^3$	5	P
17	$1s^2,2s^2,2p^6,3s^2,3p^5$	7	P

Hence, atomic numbers 4,12 represent s-block elements.

Hence, the correct option is the (D) 4,12.

Note: In transition metals (d-block elements), the last electron enters the penultimate d subshell.
In inner transition metals (f-block elements), the last electron enters the re-penultimate f subshell. Lanthanides are d-block elements and actinides are f-block elements.