Brief description: hydrogen is the lightest element. It is by far the most abundant element in the universe and makes up around 90% of the universe by weight. Hydrogen as water (H2O) is absolutely essential to all life and it is present in all organic compounds. Hydrogen is the lightest gas it was used in lighter-than-air balloons for transport but is far too dangerous because of the fire hazard. It burns in air to form only water as waste product and if hydrogen could be made on sufficient scale from other than fossil fuels then there might be a possibility of a hydrogen economy.
Note that while normally shown at the top of the Group 1 elements in the periodic table, the term "alkaline metal" refers only to Group 1 elements from lithium onwards.
