Explanation:
In order to understand why the oxidation number of ions composed of only one atom is equal to the charge on the ion, we need to first understand what oxidation number is.

Oxidation number:
Oxidation number is a concept used in chemistry to indicate the number of electrons that an atom gains, loses, or appears to use when joining with other atoms. It is a way to keep track of the distribution of electrons in a compound or molecule.

Ion:
An ion is an atom or a molecule that has gained or lost one or more electrons, giving it a net positive or negative charge. When an atom loses one or more electrons, it becomes a positively charged ion called a cation. When an atom gains one or more electrons, it becomes a negatively charged ion called an anion.

Oxidation number of ions composed of only one atom:
When an ion is composed of only one atom, the oxidation number is equal to the charge on the ion. This is because in a neutral atom, the number of protons is equal to the number of electrons, resulting in a net charge of zero. When an atom loses or gains electrons to form an ion, the number of protons remains the same, but the number of electrons changes, resulting in a net charge.

For example:
- Sodium (Na) has an atomic number of 11, meaning it has 11 protons and 11 electrons. In its neutral state, the oxidation number of sodium is 0 because the number of protons (positive charge) is equal to the number of electrons (negative charge).
- When sodium loses one electron to form a cation, the number of protons remains the same (11), but the number of electrons decreases to 10. As a result, the ion has a net charge of +1, and the oxidation number of the sodium ion (Na+) is +1.

This pattern holds true for all ions composed of only one atom. The oxidation number will always be equal to the charge on the ion because the charge is determined by the gain or loss of electrons, while the number of protons (and thus the atomic number) remains constant.

Summary:
The oxidation number of ions composed of only one atom is equal to the charge on the ion. This is because the gain or loss of electrons changes the net charge of the ion while the number of protons (and thus the atomic number) remains constant.