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.
HYDROGEN FUEL: is a very efficient and clean fuel (burning to water) but it has the disadvantage of being explosive.