Helium is a light, odorless, tasteless, colorless, inert gas. It is the first noble gas in the periodic table and has the lowest melting point of any element. It is a liquid at normal pressures and a solid at extreme pressures.
It is non-reactive and has low density, a high specific heat, a low boiling point and high thermal conductivity. It is a good shielding gas in robotic arc welding and cryogenic applications, preventing oxidation or corrosion of hot metals.
Unlike most other gases, it cannot form diatomic molecules, although it can react with nitrogen to form nitrates. It is used in laser spectroscopy as a carrier gas and in the semiconductor industry to grow silicon & germanium crystals and produce titanium and zirconium.
He has a hardness of N/A, but it is possible to make it reactive by exchanging an electron in the valence shell for an ion or to promote an electron to a higher energy level (excimer). The atoms that make up an excimer can break apart and decay back to a repulsive ground state, but in the right circumstances it is possible to form a solid combination under suitable pressures.