The alkaline earth metals Ba, Sr, Ca, and Mg may be arranged in the order of their decreasing first ionisation potential as
A. Mg, Ca, Sr, Ba
B. Ca, Sr, Ba, Mg
C. Sr, Ba, Mg, Ca
D. Ba, Mg, Ca, Sr

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Hint: Ba, Sr, Ca, and Mg are alkaline earth metals and belong to the second group of the periodic table. Ionisation potential or ionisation energy is the amount of energy required to eliminate an electron from the outermost shell of a gaseous isolated atom.

Complete Step by Step Solution:
The atomic radius of an element is interpreted as the space between the centre of the nucleus and the outermost valence electron. On moving down a group, atomic radii increase.

The valence electrons are packed in higher energy levels. The valence electrons are now far away from the nucleus with the increase in the energy level. Electrons are no longer near to the nucleus and are as a result, not under the impact of the nucleus. Hence, atomic radii increase.

We know that ionisation energy is the amount of energy required to eliminate the outermost electron. So, the elimination of electrons becomes easier down the group. Metals at the top for their small size will require a higher amount of energy for the process while for the metals lying below the group, less energy is required for the process. The order of decreasing first ionisation potential is Mg, Ca, Sr, Ba.
So, option A is correct.

Note: The ionisation energy is defined as the amount of energy required to remove the most loosely bound electron from an isolated gaseous atom to form a gaseous positive ion. The most loosely bound electron must lie in the outermost shell. Removal of an electron from an atom of Mg will be difficult for its small size and high intermolecular force of attraction between the nucleus and the valence electrons.