There are more than 20 essential elements in living organisms, and four elements, Carbon, hydrogen, oxygen and nitrogen (C, H, O and N） are the main elements that make up cells, accounting for about 96% of human total weight.
Common essential elements include Carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, potassium, calcium, chlorine, magnesium, iron, manganese, zinc, copper, boron, molybdenum (C, H, O, N, P, S, K, Ca, Mg, Fe, Mn, Zn, Cu, B, Mo, Cl) etc. Elements with content greater than 0.01% are called major elements, such as C, H, O, N, P, S, K, Ca, Mg; elements with content less than 0.01% are called trace elements or minor elements, such as Fe, Mn, Zn, Cu, B, Mo, Cl, etc. Although trace elements are very small in living organisms, they are components of enzymes, hormones and proteins, which are necessary for the life activities of living organisms. Thyroid hormones contain iodine and if the body is lacking in iodine, goiter will occur. Chlorophyll contains magnesium and if a plant is deficient in magnesium, it will appear the yellow leaves or even wither.
The most abundant element in the human body is oxygen, because various organic compounds contain oxygen atoms and two-thirds of the body weight is water(H₂O). Carbon is the second most abundant element since organics are made up of carbon chains or rings. The types of elements are roughly the same in different organisms, but their content varies greatly.
Among these elements, carbon needs to gain four electrons to form a stable structure. Therefore, carbon atoms share electron pairs with other atoms, such as Hydrogen, oxygen, nitrogen, phosphorus, and sulfur (H, O, N, P, and S), to form covalent bonds, and they are also connected to each other to form a long chain or a ring that makes the backbone of organics , resulting in various types of organic compound. Carbon is the basis of life on the earth.