Hydrogen is the first element on the periodic table, and it has several important properties and characteristics. However, one of the statements provided about hydrogen is incorrect.

The incorrect statement is option C: Hydrogen has three isotopes of which tritium is the most common.

Explanation:
1. Isotopes of Hydrogen:
- Hydrogen has three isotopes: protium, deuterium, and tritium.
- Protium is the most common isotope of hydrogen and accounts for more than 99% of naturally occurring hydrogen. It consists of a single proton and does not have any neutrons.
- Deuterium, also known as heavy hydrogen, contains one proton and one neutron. It is less common than protium and is used in various scientific and industrial applications.
- Tritium is the least common isotope of hydrogen. It contains one proton and two neutrons, making it the heaviest isotope. Tritium is radioactive and decays over time, emitting beta particles. It is primarily used in nuclear research and as a tracer in various fields.

2. Hydronium ion, H3O+:
- Hydronium ion, also known as oxonium ion, is formed when a water molecule (H2O) gains an extra proton. The extra proton is attached to one of the oxygen atoms in the water molecule, resulting in the formation of H3O+.
- Hydronium ions exist freely in solution, particularly in acidic solutions, where they are formed through the dissociation of acids. In these solutions, hydronium ions play a crucial role in determining the acidity of the solution.

3. Dihydrogen as a reducing agent:
- Dihydrogen gas, H2, can act as a reducing agent in various chemical reactions.
- It can donate electrons to other substances, leading to the reduction of the other substance and oxidation of hydrogen.
- For example, in the reaction between dihydrogen and oxygen gas (2H2 + O2), dihydrogen acts as a reducing agent, while oxygen is reduced to water (2H2O).

4. Hydrogen as a cation in ionic salts:
- Hydrogen can act as a cation in some ionic salts, particularly those containing polyatomic ions such as ammonium (NH4+).
- For example, ammonium chloride (NH4Cl) consists of ammonium cations (NH4+) and chloride anions (Cl-).
- The ammonium cation is formed by the donation of a proton (H+) from a hydrogen atom to the ammonia molecule (NH3).

In conclusion, the incorrect statement is option C: Hydrogen has three isotopes of which tritium is the most common. The correct statement is that hydrogen has three isotopes (protium, deuterium, and tritium), and protium is the most common isotope of hydrogen.

Dihydrogen acts as a powerful reducing agent and reduces metal oxides such as CuO, ZnO, PbO and Fe₃O₄ to their respective metals.