Hydrogen: molecules, metal or plasma?

We tend to think of hydrogen as a very simple material. The hydrogen atom is most people's introduction to atomic structure and quantum mechanics, and the hydrogen molecule is the simplest and best studied example of a compound structure. As a gas of hydrogen molecules is heated up, it first dissociates and then ionises to form a simple plasma, in accordance with the ideas developed by the Indian physicist Saha almost 70 years ago.