Question

Na, Mg and Al are the elements of ${{\text{3}}^{{\text{rd}}}}$ period of the modern periodic table having group numbers 1,2 and 13 respectively. Which one of these elements has (a) highest valency (b) largest atomic radii and (c) maximum chemical reactivity? Justify your answering starting with the reason.

Answer 1

Valency of the element depends upon the number of valence electrons. The element that has a less effective nuclear charge will have the largest size and the less stable element will be more reactive.

Complete step by step answer:
(a) We can select the metal having the highest valency as follows:
According to the group number, the number of valence electrons in sodium is one, magnesium is two and aluminium is three.
So, aluminium has the highest number of valence electrons, so aluminium can show the valency + 3.
Therefore, aluminium has the highest valency among all three due to the high number of valence electrons.
(b) We can select the metal having the largest atomic radii as follows:
On going left to right in a period, the size of the elements decreases.
The reason for decreases in size is as follows:
Increase in effective nuclear charge: On going left to right in a period, the electrons are added in the same shell and the proton number also increases, so the attraction of the nucleus on the outermost shell increases. So, the size of the atom decreases.
Sodium is the top left (group-1) element and aluminium is the right side (group-13) element so, sodium has the highest atomic radii.
Therefore sodium has the highest atomic radii to a very less effective nuclear charge.
(c) We can select the metal having the highest reactivity as follows:
On going left to right in the periodic table, the effective nuclear charge increases. So, the size decreases, and the stability of the metal increases. So, due to a less effective nuclear charge, sodium metal can donate electrons easily so sodium has the highest reactivity.
Therefore, sodium has the highest reactivity due to a less effective nuclear charge.

Note: Valency shows the number of electrons that can be donated by metal. Group number tells the number of electrons present in the valence shell. All elements belong to period third so the principal quantum number of the valence shell is 3.