Question

Atomic number 56 belongs to which block?
A ) s
B ) p
C ) d
D ) f

Answer 1

Hint: Electronic configuration of elements in the periodic table describes how electrons are distributed in its atomic orbitals. From the outer electronic configuration of an element we can predict the Block from which it belongs.

Complete answer:
In the periodic table, there are seven periods and eighteen groups. Eighteen groups are subdivided into four blocks, s,p,d and f.
When the last electron enters the s subshell, the element is in the s block of the periodic table. When the last electron enters the p subshell, the element is in the p block of the periodic table. When the last electron enters the d subshell, the element is in the d block of the periodic table. When the last electron enters the f subshell, the element is in the f block of the periodic table.

The atomic number of xenon is 54. The electronic configuration of xenon is \[{\rm{2}},{\rm{8}},{\rm{ 18}},{\rm{ 18}},{\rm{ 8}}\]. It can also be written as \[1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}3{d^{10}}4{s^2}4{p^6}4{d^{10}}5{s^2}5{p^6}\]. The electronic configuration of element having atomic number \[56\] is \[{\rm{2}},{\rm{8}},{\rm{ 18}},{\rm{ 18}},{\rm{ 8}},{\rm{2}}\]. It can also be written as \[1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}3{d^{10}}4{s^2}4{p^6}4{d^{10}}5{s^2}5{p^6}6{s^2}\] or \[\left[ {{\rm{Xe}}} \right]6{s^2}\].

For the element with atomic number 56, the nearest noble gas is xenon with atomic number 54. Thus, the element with atomic number 56 has two additional electrons when compared to xenon. These two additional electrons are in the 6s subshell. Hence, the element with atomic number 56 belongs to the s block in the periodic table. It is alkaline earth metal. Its electronic configuration is \[\left[ {{\rm{Xe}}} \right]6{s^2}\].

Hence, the option A ) s block is the correct answer.

Note: While writing electronic configuration of an element, the electrons should be filled into the atomic orbitals, in the order as predicted by the Aufbau principle. Also Pauli exclusion principle should also be considered.