Question

Which one of the following has the smallest size?
A. $Al$
B. $A{l^ + }$
C. $A{l^{ + 2}}$
D. $A{l^{ + 3}}$

Answer 1

The size of the element is described by its position in the periodic table but when it comes to the different ions of the same element, then the atomic size of those molecules are determined by the number of electrons present in their outer valence shell.

Complete answer:
In this question, we have to find the smallest size among the various ions of the element ‘Aluminium’.
To find the sizes of different given molecules, first we have to find the number of electrons present in their valence shell.
Before that, let’s first know about the position of the Aluminium in the periodic table.
So, Aluminium has an atomic number $13$ and it belongs to the third group of the periodic table.
Therefore, the order of size in the decreasing order is:
Anion $>$ Neutral element > Cation
Now, coming to the options given in the question:
The largest size will be of the anion, and since there is no anion present in the options, so the largest size will be of the neutral element i.e. $Al$ - Option A.
While all the other molecules i.e. $A{l^ + }$, $A{l^{ + 2}}$, $A{l^{ + 3}}$ are cations and are smaller in size than $Al$.
And hence $Al^{+3}$ has the smallest size.
Therefore option D is the correct answer.

Note:
As we discussed above, the number of electrons present in the outer shell of the molecule determines its size, so we know that the anion contains more electrons than its neutral element, so the size of anion must be larger than the neutral atom.
Likewise, the number of electrons present in the cations are less than the number of electrons present in the neutral element, so, the size of the cation is less than the size of the neutral element.