Question

An element has an atomic number 79. Predict the group and period in which the element is placed.
(A)2nd group,7th period
(B)11th group,6th period
(C)13th group,6th period
(D)12th group,6th period

Answer 1

Write down the electronic configuration for the element with atomic number(Z)= 79.After that find the highest principal quantum number to find period. To find the group, find the subshell in which the last electron goes.

Complete step by step solution:
We will now write down the electronic configuration for the element with atomic number(Z)=79.
Electronic configuration: $$$$$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}^{10}},5{{p}^{6}},6{{s}^{1}},4{{f}^{14}},5{{d}^{10}}$
Always remember to write the electronic configuration in accordance with the Aufbau Principle to avoid errors.
To find the period in which the element is present,
We need to find the greatest principal quantum number(n) in which the electron infilled. For the element with atomic number(Z)=79, the highest principal quantum number is n=6.
Therefore, the element with Z=79 belongs to the 6th Period.
Now we need to determine the group in which the element belongs. To do so we need to again refer to the electronic configuration for the element.
As suggested in the hint above we need to see the subshell in which the last electron exists. From the above configuration, we find that the last subshell is the d subshell.
In the periodic table, d blocks exist from group number 3 to 12.
Since 9 electrons are there in the d subshell, the element with Z=79 will exist in the ninth group of the d block, i.e. in group number 11.
Alternatively, we can find the group number using a simple formula.
For d block elements, the group number is equal to the number of electrons in the (n-1)d subshell + electrons in the valence shell.
Based on the above formula, the group number is 10 + 1 = 11.

Therefore, the correct answer is option (B).

Note: Do not get confused in the methods to find the period and the group. To avoid this confusion first find the period and then the group. Write the electronic configuration for the element by following the principle of Aufbau to avoid confusion and errors.