Question

What is the group number, period and block of the element with atomic number 40?

Answer 1

Try to draw the electronic configuration of the element according to Aufbau principle and you will get the group number, period and block of that element from the configuration itself.

Complete step by step answer:
Aufbau principle states that, in the ground state of an atom or ion, electrons are to be first filled in the atomic orbitals having lowest available energy levels before occupying higher levels. For example, the $1s$subshell is to be filled before the $2s$ subshell is occupied.
As per the given question, the atomic number of the element is 40.
So, the electronic configuration of the element with atomic number 40 will be,
$1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}},3{{p}^{6}},4{{s}^{2}},3{{d}^{10}},4{{p}^{6}},5{{s}^{2}},4{{d}^{2}}$

Since, from the configuration, it is seen that the last electrons are present in the d-subshell, thus the element will belong to the d-block.
And it is also seen that the outermost shell of the element is fifth that is $5s$, so the period in which the element will belong is 5.
As we know that, the group number of the element of d-block is equal to the sum of the electrons present in $(n-1)d$ subshell (i.e. the penultimate shell) and the outermost shell (i.e. the valence shell).
That means,
Group Number = Number Of Electrons in (n-1)d shell + Number Of Electrons In The Valence Shell
Here, n is the valence shell i.e. 5.

So, the $(n-1)d$ shell will be $(5-1)d$ shell i.e. $4d$ shell.
And from the configuration we can see that two electrons are present in the $4d$ shell and two electrons are present in the valence shell.
Therefore, the group number will be,
$Group Number = 2 + 2 = 4$
Hence, the element with the atomic number 40 belongs to the d-block in the fifth period with group number 4.

Note: Do not get confused with the penultimate shell to be considered. Here, the penultimate shell to be taken is $4d$ not $3d$ to find out the group number.