Oxygen and hydrogen are present in almost all organic compounds which create living organisms. The percentage of carbon in the mass of living matter is 19.4. Carbon is the basic building unit contained in living matter. In spite of their differences in abundance, all elements and the chemical reactions between them obey the same chemical and physical laws regardless of whether they are a part of the living or non-living world. In all living systems we can always find 4 basic elements: carbon, oxygen, nitrogen and hydrogen. For example, the atmosphere is rich in nitrogen and oxygen but contains little carbon and hydrogen, while the earth’s crust, although it contains oxygen and a small amount of hydrogen, has little nitrogen and carbon. In the non-living world, elements are found in different proportions, and some elements common to living organisms are relatively rare on the earth as a whole, as shown in Table 1. The four elements common to all living organisms are oxygen (O), carbon (C), hydrogen (H), and nitrogen (N). Other elements’ chemical symbols derive from their Latin names for example, the symbol for sodium is Na, referring to natrium, the Latin word for sodium. Some elements follow the English term for the element, such as C for carbon and Ca for calcium. Each element is designated by its chemical symbol, which is a single capital letter or, when the first letter is already “taken” by another element, a combination of two letters.
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