Question

When an element forms an anion, what happens to the radius? When an element forms a cation, what happens to the radius? Why? Define the term isoelectronic. When comparing sizes of ions, which ion has the largest radius, and Which ion has the smallest radius in the isoelectronic series? Why?

Short answer

Answer

1. Radius increases when the anion is formed.
2. Radius decreases when cation is formed.
3. In the isoelectronic series, the anion is the largest and the cation is the smallest in radius.

Step-by-step solution

Step 1. The radius of an element after forming an anion

When an element forms an anion, it gains an electron(s) which in terms increases repulsion among its electron and thus radius increases.

Step 2. The radius of an element after forming a cation

When an element releases electron(s), the nucleus attracts more powerfully as there is a decrease in no. of electron(s). Thus, the radius of the element decreases.

Step 3. Defining isoelectronic

When two or more atoms/molecules/ions have the same no. of electron and electronic structure, then they are said to be isoelectronic.

For eg. $C{H}_{\mathit{4}}\mathit{}and\mathit{}N{H}_{\mathit{3}}$are isoelectronic species have 10 electrons each.

Step 4. Size of radius in isoelectronic species

Taking $M{g}^{\mathit{2}\mathit{+}}\mathit{}and\mathit{}{F}^{\mathit{-}}$ as an illustration.

Anionic isoelectronic species will be the biggest in size because less no. of protons will be attracting more no. of electrons.

In a Fluorine ion, 9 protons will be attracting 10 electrons.

Cationic isoelectronic species will be the smallest because more no. of protons will be attracting less no. of electrons.

In the Magnesium ion, 12 protons will be attraction 10 electrons.