Elements are the smallest units of matter that cannot be broken down further by chemical processes. There are 118 known elements, but not all of these are naturally-occurring, and fewer still are essential for life. Living matter is composed primarily of carbon, nitrogen, hydrogen, and oxygen, with smaller amounts of other elements like calcium, phosphorus, potassium, and sulfur. Other elements are also necessary for life but only in trace amounts.
The periodic table organizes elements based on their physical and chemical properties. The atomic number of an element corresponds to the number of protons found in its nucleus, and each square in the periodic table also provides the full name, chemical symbol, and atomic weight of an element. The number of protons provides information about the size of an element, but it is not the only organizational principle underlying the structure of the periodic table. Elements are organized into columns (groups) and rows (periods) based on other physical and chemical properties, such as reactivity, the location of their outermost electrons, and the ability to make certain types of bonds. Elements in the same group (i.e., column) vary in size but have many chemical properties in common with one another. By contrast, elements in the same period (i.e., row) are more similar in size and have their electrons located in a similar place, but vary greatly in their chemical properties.
All life on Earth contains the elements oxygen, carbon, hydrogen, and nitrogen. More precisely, 96% of the human body is made up of these four elements. The remaining 4% is composed primarily of calcium, phosphorus, potassium, sulfur, sodium, chlorine, and magnesium, in order of relative abundance. Further, some elements are essential for humans but are found in the body in amounts of less than 0.01%; these are called trace elements. Even though they are only present in small amounts, trace elements are still critical for health. Iron, for example, plays an essential role in red blood cells, helping to bind oxygen so that it can be transported throughout the circulatory system. Too little iron can result in iron-deficiency anemia, which is characterized by symptoms that stem from a lack of oxygen, including fatigue, shortness of breath, weakness, and irregular heart rhythms.
Some elements have detrimental health effects even in small doses. Mercury, for example, is one of several heavy metals that can produce a number of symptoms in small doses—depending on the tissue that is affected—and causes death in larger doses. It can accumulate in the tissues of multicellular organisms over time, so repeated exposure is a concern. Novel ways of removing heavy metal contaminants from the environment using biological methods—bioremediation—requires research to understand both the chemistry of the contaminants and the biology of the first organisms that they affect. Heavy metals often enter the food web at the level of primary producers before they affect organisms at higher trophic levels such as humans.
