Hydrogen is a chemical element. It has the symbol H and atomic number 1. The first and foremost element of the periodic table, Hydrogen, is the lightest element known till date. The atomic number of Hydrogen is 1. It has a standard atomic weight of 1.008, meaning it is the lightest element in the periodic table. Hydrogen is the most common chemical element in the Universe, making up 75 percent of all ordinary (baryonic) matter (by mass). The essential function of Hydrogen in the human body is to keep you hydrated. Water is made up of Hydrogen and oxygen and is absorbed by the cells of the body. Therefore, it is a crucial element which is used not in our body but also as a fuel, in military weapons etc. This chapter deals with everything related to Hydrogen, including its action on litmus, combustibility, its action with different substances like oxygen, chlorine gas, metallic oxide, and finally its uses and tests.