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The periodic table is a common reference in chemistry, where all known elements are arranged by increasing atomic number.

An element’s symbol is usually one or two letters and often comes from its English name or from its Latin or German name. For example, sodium is abbreviated as Na, from the Latin word natrium.

Out of the 92 elements that occur naturally, 25 are essential elements for living organisms. Of those 25, four elements—oxygen, carbon, hydrogen, and nitrogen—make up about 96 percent of living matter.

Plants release oxygen during photosynthesis. Carbon forms the backbone of organic molecules such as glucose. Hydrogen is found in water, which fills aquatic environments. Nitrogen is present in the atmosphere and helps build cellular components.

Most of the remaining 4 percent consists of calcium, phosphorus, potassium, and sulfur.

Trace elements, such as iron in red blood cells and iodine in the thyroid gland, are also essential, but organisms need them only in very small amounts to support key functions.