Question

Among the alkali metals, Cs is the most reactive because:
(A) its incomplete shell is nearest to the nucleus
(B) it has a single electron in its valence shell
(C) it is the heaviest alkali metal
(D) its outermost electron is more loosely bound than the outermost electron of the other alkali metals

Answer 1

Hint:
Among all the alkali metals, Cs has the highest atomic number and it is situated downwards than all the other alkali metals. As distance between two particles increases, the magnetic attractive force between them also decreases.

Complete answer:
First alkali metal is Li, then respectively Na, K, Rb and Cs come in the periodic table. We know that as numbers of electrons increase with increase in atomic numbers. So, Cs is the biggest alkali metal in size.
- The valence electrons in Cs will be at the highest distance than other alkali metals. So, we can say that the attractive force between the valence electron and the nucleus will be the least in the Cs metal amongst other alkali metals.
- So, the tendency of Cs to donate that valence electron will be the highest. So, it is clear from that evidence that Cs will be the most reactive alkali metal.
So, the correct answer is (D) its outermost electron is more loosely bound than the outermost electron of the other alkali metals.

Additional Information:
Below are the atomic number and electronic configurations of the alkali metals.
Lithium(Li) : 3 (\[1{s^2}2{s^1}\])
Sodium(Na) : 11 (\[1{s^2}2{s^2}2{p^6}3{s^1}\])
Potassium (K) : 19 (\[1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^1}\])
Rubiduim (Rb) : 37 (\[1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^2}3{d^{10}}4{p^6}5{s^1}\])
Caesium (Cs) : 55 (\[1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^2}3{d^{10}}4{p^6}5{s^2}4{d^{10}}5{p^6}6{s^1}\])

Note:
Do not forget that Cs is situated at the bottom of the s-block elements in periodic table. Remember that heaviness of the atoms does not have any relation to its chemical reactivity.