Question

Which one of the following has the largest radius?
A. ${\text{N}}{{\text{a}}^{\text{ + }}}$
B. ${\text{M}}{{\text{g}}^{{\text{2 + }}}}$
C. ${{\text{O}}^{{\text{2 - }}}}$
D. ${\text{A}}{{\text{l}}^{{\text{3 + }}}}$

Answer 1

The radius of the atom is defined as the distance from the nucleus of the atom to the outermost orbital of the electron of the atom. The Heisenberg uncertainty principle can be used to determine the radius of the atom. The different types of radius are ionic radius, metallic radius, covalent radius and Van Der Waals radius.

Complete step by step answer:
To determine the radius of the atom we need to know the configuration of the ion or the element given. Generally the cation with high positive charge has less radius while the anion with high negative charge has more radius.
So the ions given to us are of isoelectronic species as they all have the same number of electrons which is equal to 10. Now the cations are formed by losing the electrons so they will have less electrons and due to high effective nuclear charge the distance between the nucleus and outermost electron will be less. While the anion is formed by the addition of electrons so they will acquire the shell thereby increasing the size due to electron electron repulsion. So the anion will have more size than the cation.

So, the correct answer is Option C.

Note: In the down the group the atomic radius increases because the number of shells increases down the group. While in the period due to the high effective nuclear charge the atomic size decreases from left to right. While in the homoatomic molecules the covalent bond is calculated by calculating the half of the internuclear distance of the two similar bonds which are bonded by a single covalent bond.