Hydrogen is the lightest chemical clement. Its mass is the unit of measurement for the masses of other elements.
Atomic weight, or mass, was long considered the most important property of an clement. By weighing the amounts of individual elements making up a chemical compound and calculating the weights of these ratios
to the weight of hydrogen which will combine with the same elements, it is found that the atomic weights of the other elements are almost, but not quite, whole numbers.
During the nineteenth ccntury a committee of chemists was chosen to decide upon a standard of accuracy for atomic weights. The committee set the atomic weight of oxygen at 16.000 in order to make the atomic weights of other elements come out closer to whole numbers. That change of standards gave hydrogen the weight of 1.008.
Hydrogen was first obtained in 1766 by Sir Henry Cavendish in London. He found that he could get the gas by dissolving zinc, iron or tin in diluted vitriolic acid (H2S04) or spirit of salt (HCI). He discovered that a mixture of hydrogen and common air explodes with a long noise, and he was impressed with the lightness of the gas. He named the gas "inflammable air", the name "hydrogen" (water-former) was given by Lavoisier.
Hydrogen exists in three isotopic forms, known as hydrogen (or protium), deuterium and tritium. Tritium is radioactive, with a short half-life. Deuterium is stable, and occurs in a small amount with ordinary hydrogen. Its compound (D20) is known as heavy water. Slight differences between the properties of ordinary water and heavy water allow their separation, notably by electrolysis, in which ordinary water is decomposed and heavy water bccomcs concentrated in the water left in the apparatus.
The nuclcar structure of ordinary hydrogen consists of one proton, the unit of matter. This is the same as a hydrogen ion. An electron as the sole planet in this system completes the structure of hydrogen atom.
The difference between ordinary hydrogen and heavy hydrogen (deuterium) lies in the fact that deuterium has a neutron in the nucleus in addition to the proton. Addition of the neutron adds weight but docs not changc the chemical characteristics. This is in accordance with a general rule covering structures of elements.
Tritium has a nucleus consisting of one proton and two neutrons.
Hydrogen is given off by some natural gas wells, but it escapes into the upper air. It is not found uncombincd on the earth. It is recognized in the stars by its spectrum lines in the light that we rcccive from them.
In combination with oxygen, in the form of water, and with carbon, in the many organic compounds, hydrogen is one of the most abundant elements on the earth.
Hydrogen combines with other elements and forms different kinds of compounds, some of which ionize in solution, others which arc joined with covalcnt bonds, yielding organic types of compounds.
Although formerly it used to be classcd with the alkali metals of Group I in the periodic table, hydrogen acts as a negative part of the molcculc when it is in combination with those metals. It forms hydrides which arc in general colourlcss crystals. Similar compounds arc formed with calcium and barium of Group II. The hydrides decompose in water,
releasing hydrogen. This property has been used as a convenient way to store hydrogen.
Hydrogen is usually obtained by action of sulphuric acid (H2SO4) on zinc. The metal replaces the hydrogen, which bubbles ofT a gas. Electrolysis of water also liberates hydrogen at the cathode, while oxygen comes off at the anode.
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