The Elements : Hydrogen
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On the Earth, Hydrogen occurs in virtually all Organic Compounds (along with Carbon). It is rare in its elemental form because it reacts explosively with Oxygen. However, its compounds are everywhere.
Water (H20) is one of the most important substances on the Earth. It is a liquid over a wide range of temperatures, it dissolves many other substances, it holds heat well acting like a thermostat for the planet, and it freezes from the top. This last property is unusual for a liquid; wax, for example, solidifies from the bottom. This property means that life can survive under water during cold periods insulated by a layer of ice.
Other substances containing Hydrogen include petroleum (Greek: oil of rock) and its products, acids, sugars, waxes and alcohols.
Hydrogen is a colourless odourless gas, occuring as a diatomic molecule (H2). Its lightness saw it being used in balloons but its flammability made it too dangerous. It becomes a liquid at -253°C.
Outside the Earth, Hydrogen is the most abundant element making up over 80% of the atoms in the Universe. Its nuclear reactions within stars provide the energy that powers the Universe and life on Earth. Nuclear reactions happen between the nuclei of atoms. These require extreme conditions and are not common on the Earth. The Evolution of Stars describes this in more detail.
Chemically, Hydrogen stands apart from other elements in its properties.
Acids are usually compounds of Hydrogen with the non-metallic elements. Examples include Hydrochloric Acid (HCl), Sulphuric Acid (H2SO4), Nitric Acid (HNO3) and Formic Acid (H2CO2). Acids tend to be corrosive and can dissolve metals. The bubbles formed when metals dissolve in acids are free Hydrogen.
Some atoms of Hydrogen contain a proton and a neutron in the nucleus. Chemically this is still Hydrogen as it is the electrons that determine the chemical properties of an atom. When atoms differ in their nuclear composition they are called isotopes. This isotope of Hydrogen is called Deuterium or Heavy Hydrogen.
Isotopes cannot be separated chemically but must be separated using physical differences like their masses.