2.1: The Periodic Table and Organismal Elements
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 only a few of them 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.
Periodic Table Provides Information about the Physical and Chemical Properties of Elements
The periodic table organizes elements based on their physical and chemical properties. Each square in the periodic table displays the full name, chemical symbol, atomic number, and atomic weight of an element. The atomic number of an element corresponds to the number of protons found in its nucleus and provides information about the size of an element. Elements are organized in the periodic table into columns (groups) and rows (periods) mainly based on their atomic numbers. They are also grouped according to 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.
Major Elements and Trace Elements Make up the Human Body
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 comprise less than 0.01% of the body; 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, causing fatigue, shortness of breath, weakness, and irregular heart rhythms.
Some Elements are Harmful to Living Organisms
Some elements have detrimental health effects even in small doses. Mercury, for example, is one of the 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. Heavy metal contaminants often enter the food web at the level of primary producers, i.e., plants, before they affect organisms at higher trophic levels such as humans.