Question

Which of the following can be represented by the configuration \[\left[ {{\rm{Kr}}} \right]{\rm{5}}{{\rm{s}}^{\rm{2}}}\]?
A. Ca
B. Sr
C. Ba
D. Ra

Answer 1

Hint: Beryllium, Magnesium, Calcium, Strontium, Barium, and Radium have similar properties and hence are placed in group 2 of the periodic table. These metals are known as alkaline earth metals. Their common outer electronic configuration is \[{\rm{n}}{{\rm{s}}^{\rm{2}}}\].

Complete Step by Step Answer:
The elements in which the final electron joins the s-orbital are organised in an s-block.
In this block, there are two groups, namely group-1 and group-2.
Group 1 metals involve Lithium, Sodium, Potassium, Rubidium, Cesium and Francium.
Group-2 holds Beryllium, Magnesium, Calcium, Strontium, Barium, and Radium.
These elements are the alkaline earth metals as they show alkalinity and are found in the earth.
All the given elements are alkaline earth metals belonging to group-2.
Their common outer electronic configuration is \[{\rm{n}}{{\rm{s}}^{\rm{2}}}\].

We have to find out which of the following options alkaline earth metals have configuration \[\left[ {{\rm{Kr}}} \right]{\rm{5}}{{\rm{s}}^{\rm{2}}}\].
A. Ca
Its atomic number is 20.
The electronic configuration is \[\left[ {{\rm{Ar}}} \right]{\rm{5}}{{\rm{s}}^{\rm{2}}}\].
So, A is incorrect.

B. Sr
Its atomic number is 38.
The electronic configuration is \[\left[ {{\rm{Kr}}} \right]{\rm{5}}{{\rm{s}}^{\rm{2}}}\].
So, B is correct.

C. Ba
Its atomic number is 56.
The electronic configuration is \[\left[ {{\rm{Xe}}} \right]{\rm{5}}{{\rm{s}}^{\rm{2}}}\].
So, C is incorrect.

D. Ra
Its atomic number is 88.
The electronic configuration is \[\left[ {{\rm{Rn}}} \right]{\rm{7}}{{\rm{s}}^{\rm{2}}}\].
So, D is incorrect.

So, option B is correct.

Note: The electronic configuration of alkaline earth metals is represented in terms of noble gas configuration. So, to find out the metal which has the given configuration we have to remember the atomic number of alkaline earth metals and noble gases. The atomic number of Kr is 36. The atomic number of Sr is 38. These two atomic numbers are close. The addition of two electrons to the 5s subshell of Kr will give the configuration of Sr.