Which statement correctly describes an ion?

The correct answer is: A charged atom or group of atoms.

Step 1: Define an ion
An ion is an atom or a molecule (a group of atoms) that has lost or gained one or more valence electrons, giving it a net positive or negative electrical charge.

Step 2: Understand how ions are formed
- A neutral atom has an equal number of positively charged protons and negatively charged electrons.
- If an atom loses electrons, it has more protons than electrons, resulting in a positively charged ion called a cation (e.g., Na⁺).
- If an atom gains electrons, it has more electrons than protons, resulting in a negatively charged ion called an anion (e.g., Cl^-).
- Polyatomic ions are groups of covalently bonded atoms that carry an overall charge (e.g., NH₄⁺ or SO₄^2-).

Step 3: Analyze the options
- A molecule containing similar atoms only: This describes a homonuclear molecule (e.g., O₂), not necessarily an ion.
- A charged atom or group of atoms: This correctly describes the fundamental definition of an ion.
- A compound formed by covalent bonds: This describes a covalent/molecular compound, which consists of neutral molecules.
- A neutral atom formed by chemical bonding: An ion cannot be neutral because it explicitly carries an electrical charge.

Therefore, the statement that correctly describes an ion is "A charged atom or group of atoms".