Question

What is the least reactive alkaline earth metal?

Answer 1

The reactivity of an element depends upon the readiness of it to lose or gain electrons. Alkaline earth metals have 2 electrons in their outermost orbit, so they are very reactive, but also the attraction of electrons to its nucleus affects the readiness of the atom to lose the electrons.

Complete answer:
The correct answer to this question is beryllium ( $Be$ ).
Alkaline earth metals are those elements which reside in group two, they are very much reactive as they have two free electrons. Thus, all alkaline earth metals are reactive, but the least reactive is beryllium ( $Be$ ).
As we go down in the group, more and more electrons are added in atoms of those elements, thus more and more orbits are also added. Now, as more orbits are added, the electrons in the outermost orbit will be far away from the nucleus.
Thus, these electrons feel less attraction towards its nucleus and can easily give up their electrons. While in the case for elements on the top of the group, they have less electrons and thus less orbits. Therefore, the electrons in their outermost orbit will be comparatively nearer to their nucleus.
So, those electrons will feel more attraction towards their nucleus. And therefore, they will be comparatively less reactive than the elements at the bottom.
Beryllium ( $Be$ ) is at the top of the second group, that is alkaline earth metals, so it is the least reactive alkaline earth metal. On the contrary, radium will be the most reactive alkaline earth metal as it is at the bottom of the group.

Note:
The principle behind this is the same that shows how radium ( $Ra$ ) has the most metallic character. The conclusion that can be drawn from this is that the reactivity of elements increases as we go down the group, same as metallic character. Also, beryllium is the only element that does not react with water or steam.