Oxygen

By

Agnes Grobelny

It is the most abundant element on Earth. The chemical symbol is O. It is essential to all planets’ life forms. As the gas O2 it is in the lower atmosphere in the air that is breathed. In the upper atmosphere as ozone (O3) it shields the ultraviolet rays. Ozone is a pale blue gas that is explosive and poisonous even in low concentrations. Oxygen constitutes about two thirds of the human body and nine tenths (by weight) of the water in the rivers, lakes, and oceans. In combination with other chemical elements, it forms hundreds of thousands of compounds, making up by weight almost half of the rock and sand in the Earth’s crust. In its free state, oxygen is a gas without color, odor, or taste. It turns into a pale blue liquid at temperatures below –297F (-183C) and becomes solid at about –360F (-218C). Oxygen is very active chemically and combines readily with nearly all other elements. These elements are then said to be oxidized. Sometimes heat aids or accelerates this chemical combination, as when wood burns. Burning is simply rapid oxidation. The oxygen of the air combines with the hydrogen and carbon in the wood to form water (H20) and the invisible gas carbon dioxide (CO2). Nearly all planets and animals require oxygen for respiration. Even fish respire with the aid of the free oxygen dissolved in water. Body tissues contain carbon and hydrogen compounds that are constantly being oxidized in the same way as burning wood. They combine with the oxygen in the air and return to the air as carbon dioxide and water. This oxidation creates enough heat to keep some animals warm. There are three natural atomic forms, or isotopes, of oxygen. The lightest and by the most abundant of the isotopes is oxygen-16, has eight protons and eight neutrons in its nucleus. From about 1900 until 1961 oxygen –16 was used as standard of atomic weights.

A principal stated in 1811 by the Italian chemist Amadeo Avogadro, that equal volumes of gases at the same temperature and pressure contain the same number of molecules regardless of their chemical nature and physical properties. This number (Avogadro’s number) is 6.023x10^23. It is the number of molecules of any gas present in a volume of 22.21L and is the same for the lightest gas (hydrogen) as for a heavy gas such as carbon dioxide or bromine. Avogadro’s number is one of the fundamental constants of chemistry. It permits calculation of the amount of pure substance (mole), the basis of stoichiometric relationships. It is also makes possible determination of how much heavier a simple molecule of one gas is than of another, as a result the relative molecular weights of gases can be ascertained by comparing the weights of equal volumes.