(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.