Question

Define ionic radius. How does the size of an atom change when it is converted to (a) an anion and (b) a cation?

Short answer

Ionic radius is the size of an ion; it increases when an atom becomes an anion and decreases when it becomes a cation.

Step-by-step solution

Define Ionic Radius

The ionic radius is the measure of an atom's ion in a crystal lattice structure. It is the distance from the ion's nucleus to the outermost edge of its electron cloud when the atom becomes an ion.

Atom to Anion Conversion

When an atom gains one or more electrons to become an anion, its radius generally increases. This is because the added electrons increase electron-electron repulsion in the outer shells, which causes the electron cloud to expand.

Atom to Cation Conversion

When an atom loses one or more electrons to become a cation, its radius generally decreases. The loss of electrons reduces electron-electron repulsion and may cause the ion to lose an entire electron shell, resulting in a smaller radius.

Key concepts

Anion Size Change

When an atom gains extra electrons, it transforms into an anion, which is a negatively charged ion. This gain of electrons significantly affects the size of the atom's radius. When additional electrons are added:

• The electron-electron repulsion increases because there are more electrons crowded into the same space.
• This increased repulsion pushes the electrons further apart from each other, causing the electron cloud around the nucleus to expand.
• As a result, the ionic radius of the anion is larger than its original atomic radius.

The expansion of the electron cloud means that the ion takes up a larger space within a crystal lattice. This makes sense because more space is needed to accommodate the extra electrons and counterbalance the increased repulsive forces.

Cation Size Change

The story is quite different when an atom turns into a cation by losing one or more electrons. A cation has a positive charge due to the loss of its negatively charged electrons:

• When electrons are removed, the electron cloud contracts, as there are fewer electrons contributing to repulsion.
• This contraction often leads to the cation having a smaller ionic radius than the original atom.
• Sometimes, the loss of electrons is significant enough to shed an entire electron shell, further reducing the size.

Thus, cations generally have a smaller radius due to a decrease in repulsive forces, allowing the remaining electrons to be pulled closer to the nucleus. This influences the ion's role and behavior in forming and maintaining crystal structures.

Electron Cloud Expansion

The concept of electron cloud expansion is crucial to understanding how the size of an ion changes. When electrons are added, such as in the formation of an anion, the electron cloud must adjust to fit the additional electrons:

• The electrons redistribute themselves, increasing the space they occupy.
• This expansion results in a larger area surrounding the nucleus and effectively increases the ionic radius.

Conversely, when an atom becomes a cation and loses electrons, the electron cloud contracts:

• With fewer electrons, the remaining electrons can come closer to the nucleus, further concentrating into the available space.
• As such, the contraction of the electron cloud scales down the ionic radius.

Thus, the behavior of the electron cloud—either expanding with more electrons or contracting with fewer—directly affects the size of the ion, playing a critical role in defining the ionic radius.