Hydrogen gas, H₂ and Helium gas (He)

(above) hydrogen image # 1: HYDROGEN GAS: picture showing H₂ molecules. This image is 600 x 600 pixels; the original image is 4096 x 4096 pixels.

HYDROGEN: is the first (in the periodic table) and simplest of the elements and (usually) consists of a proton with one associated electron. Under normal conditions it exists as a gas with molecules made up of two hydrogen atoms joined together (H₂). It is an intensely studied element because its quantum states are simple enough for mathematical modelling. It is also the most abundent of the elements in the cosmos and so is a favourite of astronomers.

HELIUM: is the second (in the periodic table) and exists as an atomic gas under all but the most extreme conditions.