Question

Which relation represent the correct relationship of the radius of an atom, its cation and its anion? (1) atom = cation \(=\) anion (2) atom \(>\) cation \(>\) anion (3) atom \(>\) cation \(<\) anion (4) atom \(<\) cation \(>\) anion

Short answer

Option (3): atom > cation < anion.

Step-by-step solution

Understand Atomic and Ionic Radii

An atom's radius is the distance from its nucleus to the outermost electron shell. Cations are positively charged ions formed by the loss of electrons, leading to a decrease in the radius. Anions are negatively charged ions formed by the gain of electrons, which increases the radius.

Relate Atomic Radius to Cation Radius

Since a cation is formed by losing one or more electrons, the electron cloud shrinks, reducing the atomic radius. Consequently, the radius of a cation is smaller than that of the neutral atom.

Relate Atomic Radius to Anion Radius

An anion is formed by gaining one or more electrons, causing the electron cloud to expand due to increased electron-electron repulsion. Thus, the radius of an anion is larger than that of the neutral atom.

Compare Radii of Atom, Cation, and Anion

Based on the steps above, we find the relationship: Atom radius is larger than cation radius, which in turn is smaller than anion radius. This implies: atom > cation < anion, matching option (3).

Key concepts

Atomic Radius

The atomic radius is a fundamental concept in chemistry that measures the size of an atom. It is the distance from the nucleus to the outermost electron shell. The larger the atomic radius, the bigger the atom.
Atoms in the same group of the periodic table show an increase in atomic radius down the group. This happens because each successive element has an additional electron shell. This increased distance results in a larger atomic radius.
On the other hand, atoms within a period show a decrease in atomic radius from left to right. As you move across a period, the number of protons in the nucleus increases. This higher positive charge pulls the electron cloud closer to the nucleus, decreasing the atomic radius.

• Summary:
  - Distance from the nucleus to the outermost shell.
  - Increases down a group.
  - Decreases across a period.

Cation Radius

A cation is an ion with a positive charge, formed when an atom loses one or more electrons. This loss of electrons results in a smaller radius compared to the neutral atom. The decrease in radius happens because the removal of electrons reduces electron-electron repulsion in the electron cloud.
Also, the remaining electrons are pulled closer to the nucleus due to the unbalanced positive charge, shrinking the electron cloud even more.
For example, when a neutral sodium atom (Na) loses one electron to form a cation (Na⁺), the atomic radius becomes smaller.

• Summary:
  - Formed by losing electrons.
  - Smaller radius compared to the neutral atom.
  - Reduced electron cloud due to less electron-electron repulsion.

Anion Radius

An anion is an ion with a negative charge, formed when an atom gains one or more electrons. This gain of electrons results in a larger radius compared to the neutral atom. The increase in radius happens because the additional electrons increase electron-electron repulsion in the electron cloud.
The increased repulsion forces the electron cloud to expand, making the anion larger than its neutral counterpart.
For instance, when a neutral chlorine atom (Cl) gains one electron to become an anion (Cl^-), the atomic radius increases.

• Summary:
  - Formed by gaining electrons.
  - Larger radius compared to the neutral atom.
  - Expanded electron cloud due to increased electron-electron repulsion.