Question

Na, Mg and Al are the elements having one, two, and three valence electrons respectively.
Which of these elements (i) has the largest atomic radius, (ii) is least reactive? Justify your answer stating the reason for each.

Answer 1

Hint: The size of the atom depends on the electronic configuration of the atom. The atomic size generally decreases along the period and increases down the group. The reactivity also decreases along the period.

Complete step by step solution:
The electronic configuration of the metas are given by-l
Electronic configuration of Na is 2,8,1
Electronic configuration of Mg is 2,8,2 and
Electronic configuration of Al is 2,8,3
So we can see from this that sodium metal has one valence electron, magnesium metal has two valence electrons and aluminum metal has three valence electrons.
From the valence electrons we can get to know that the atom is in which group and period.
In Na we have one valence electron so it is group 1.
In Mg we have two valence electrons so it is group 2.
In Al we have three valence electrons but above two valence electrons we need to add ten to the number. So aluminum is in the 10+3= 13 group.
We can know the group of the element by the electronic shells. So Na has three electron shells so it is period 3. Mg has three electron shells so it is period 3. Al has three electron shells so it is period 3.
So now we can know the largest atomic size by comparing them in period three.
In periods the atomic size decreases.
It is so because the effective nuclear charge increases as electron shielding remains constant. The nuclear charge is high, so there is the pulling between the electrons and the nucleus. And so the electron cloud gets closer to the nucleus which results in smaller atomic size. So this concludes that lesser the number of valence electrons higher is the atomic size. Hence, we came to the conclusion that sodium (Na) has the largest atomic size.
Now for the reactivity of an element we know that the reactivity of an element decreases along the period. The valence electrons are shielded due to the more number of electron shells. The reason is that as we know that along the period the nuclear charge increases so the attraction between valence electron and nucleus increases due to which the valence electrons do not remove easily, so they are not available for the reaction.
So by this we conclude that more the number of valence electrons, lesser is its reactivity.

Hence, aluminum is the least reactive element.

Note: It is necessary to identify the period and group of the elements before stating their reactivity or atomic size. Atomic size increases down the group as the number of energy levels (n) increases so there is a greater distance between the outermost orbital and nucleus. Across a period, the energy level remains the same. Reactivity of metals is due to the difference in stability of their configuration as atoms and ions. All metals lose electrons to form positive ions called cation.