There are 92 elements that occur naturally on Earth. For living things, only 11 of these elements are found in larger than trace quantities.

For vertebrates, such as humans, there are two additional elements that occur in larger than trace amounts these are Iodine and Iron. The periodic table of elements below is color coded to show the elements found in the human body.

Click to enlarge. Of the elements found in the human body, four of them make up the largest percentage of our body weight (96.2%).

Before you start thinking we should float away with all the oxygen, hydrogen, and nitrogen atoms, remember that the oxygen molecules are mainly part of the water in our body (H2O). In fact, over half of the human body is made up of water (50-70%).

The other trace elements (less than 0.01%) are: boron (B), cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), fluorine (F), iodine (I), iron (Fe), manganese (Mn), molybdenum (Mo), selenium (Se), silicon (Si), tin (Sn), vanadium (V), and zinc (Zn).

Because it is difficult to detect low levels of some essential elements, the trace elements were relatively slow to be recognized as essential. Iron was the first.

It was not until the 19th century, however, that trace amounts of iodine were found to eliminate goiter (an enlarged thyroid gland). This is why common table salt is “iodized”: a small amount of iodine is added.

Molybdenum was not known to be an essential element until 1953, and the need for chromium, selenium, vanadium, fluorine, and silicon was demonstrated only in the last 50 years. It seems likely that in the future other elements, possibly including tin, will be found to be essential at very low levels.

In fact, there is some evidence that one bacterium has replaced phosphorus with arsenic, although the finding is controversial. This has opened up the possibility of a “shadow biosphere” on Earth in which life evolved from an as yet undetected common ancestor.

First, the toxicity of an element often depends on its chemical form—for example, only certain compounds of chromium are toxic, whereas others are used in mineral supplements. Second, as shown in Figure 1.27 “Possible Concentrations of an Essential Element in the Diet”, every element has three possible levels of dietary intake: deficient, optimum, and toxic in order of increasing concentration in the diet.

Over some range of higher intake levels, an organism is able to maintain its tissue concentrations of the element at a level that optimizes biological functions. Finally, at some higher intake level, the normal regulatory mechanisms are overloaded, causing toxic symptoms to appear.

Both the width of the plateau and the specific concentration corresponding to the center of the plateau region differ by as much as several orders of magnitude for different elements. In the adult human, for example, the recommended daily dietary intake is 10–18 mg of iron, 2–3 mg of copper, and less than 0.1 mg of chromium and selenium.

The deficient, optimum, and toxic concentrations are different for different elements.

A period on the periodic table is a row of chemical elements. All elements in a row have the same number of electron shells.

Arranged this way, elements in the same group (column) have similar chemical and physical properties, reflecting the periodic law. For example, the halogens lie in the second-to-last group (group 17) and share similar properties, such as high reactivity and the tendency to gain one electron to arrive at a noble-gas electronic configuration.

Modern quantum mechanics explains these periodic trends in properties in terms of electron shells. As atomic number increases, shells fill with electrons in approximately the order shown in the ordering rule diagram.

In the s-block and p-block of the periodic table, elements within the same period generally do not exhibit trends and similarities in properties (vertical trends down groups are more significant). However, in the d-block, trends across periods become significant, and in the f-block elements show a high degree of similarity across periods.

There are currently seven complete periods in the periodic table, comprising the 118 known elements. Any new elements will be placed into an eighth period.

The elements are colour-coded below by their block: red for the s-block, yellow for the p-block, blue for the d-block, and green for the f-block.

They therefore do not follow the octet rule, but rather a duplet rule. Chemically, helium behaves like a noble gas, and thus is taken to be part of the group 18 elements.

Hydrogen readily loses and gains an electron, and so behaves chemically as both a group 1 and a group 17 element.

They include the biologically most essential elements besides hydrogen: carbon, nitrogen, and oxygen.

All but the noble gas argon are essential to basic geology and biology.

These include iron, the heaviest element forged in main-sequence stars and a principal component of the Earth, as well as other important metals such as cobalt, nickel, and copper. Almost all have biological roles.

Completing the fourth period are six p-block elements: gallium, germanium, arsenic, selenium, bromine, and krypton.

Of the three heaviest elements with biological roles, two (molybdenum and iodine) are in this period. tungsten, in period 6, is heavier, along with several of the early lanthanides.

Period 6 is the first period to include the f-block, with the lanthanides (also known as the rare earth elements), and includes the heaviest stable elements. Many of these heavy metals are toxic and some are radioactive, but platinum and gold are largely inert.

All elements of period 7 are radioactive. This period contains the heaviest element which occurs naturally on Earth, plutonium.

Whilst five of these (from americium to einsteinium) are now available in macroscopic quantities, most are extremely rare, having only been prepared in microgram amounts or less. Some of the later elements have only ever been identified in laboratories in quantities of a few atoms at a time.

Although the rarity of many of these elements means that experimental results are not very extensive, periodic and group trends in behaviour appear to be less well defined for period 7 than for other periods. Whilst francium and radium do show typical properties of groups 1 and 2, respectively, the actinides display a much greater variety of behaviour and oxidation states than the lanthanides.

No element of the eighth period has yet been synthesized. A g-block is predicted.

There may therefore be no ninth period.

The smallest, most fundamental material components of the human body are basic chemical elements. In fact, chemicals called nucleotide bases are the foundation of the genetic code with the instructions on how to build and maintain the human body from conception through old age.

Human chemistry includes organic molecules (carbon-based) and biochemicals (those produced by the body). Human chemistry also includes elements.

All of the elements that contribute to chemical reactions, to the transformation of energy, and to electrical activity and muscle contraction—elements that include phosphorus, carbon, sodium, and calcium, to name a few—originated in stars. These elements, in turn, can form both the inorganic and organic chemical compounds important to life, including, for example, water, glucose, and proteins.

The chapter then builds the framework of life from there. Look around you.

Life is chemistry organized into astonishing complexity and intricacy. To make sense of this organization we can look at life’s chemistry as a hierarchy—levels of organization.

A person is between 1–2 meters (m) tall, but there are many length scales and biological levels of detail which are important for understanding anatomy and physiology. For perspective on size difference, consider an atom.

Atoms contain a positive center (nucleus) surrounded by a cloud of electrons that allow interatomic interactions. We can’t really grasp how small atoms are, but think big instead of small.

The smallest length scale that we will cover is the size of individual atoms, but the movement of subatomic particles called electrons, can change atomic charge. Ions are atoms that carry either a positive or negative charge from altered numbers of electrons, and many atoms and molecules exist in the body as ions.

Ions play an essential role in physiological processes, particularly as they move across cell membranes. Appropriate intracellular and extracellular concentrations of sodium, potassium and calcium ions are required for nerve impulses and heart beats, enable cell-to-cell communication and initiate cellular processes.

Transport of ions across membranes may occur by passive diffusion, through ion channels, or through pumps. Pumps often move ions against a concentration gradient.

Anesthetic drugs such as Novocain block sodium channels. Neurotoxins from some snakes and puffer fish work by blocking ion movements that mormally occur in nerve transmission.

Even subatomic particles, which are too small to see with the best microscopes in the world, play an extremely important role in maintaining proper physiology. The substance of the universe—from a grain of sand to a star—is called matter.

An object’s mass and its weight are related concepts, but not quite the same. An object’s mass is the amount of matter contained in the object, and the object’s mass is the same whether that object is on Earth or in the zero-gravity environment of outer space.

Where gravity strongly pulls on an object’s mass its weight is greater than it is where gravity is less strong. An object of a certain mass weighs less on the moon, for example, than it does on Earth because the gravity of the moon is less than that of Earth.

A piece of cheese that weighs a pound on Earth weighs only a few ounces on the moon. All matter in the natural world is composed of one or more of the 92 fundamental substances called elements.

While your body can assemble many of the chemical compounds needed for life from their constituent elements, it cannot make elements. They must come from the environment.

Calcium is essential to the human body. it is absorbed and used for a number of processes, including strengthening bones.

Among these is calcium, which, because it is an element, cannot be broken down further. The elemental calcium in cheese, therefore, is the same as the calcium that forms your bones.

The elements in the human body are shown in the table below, beginning with the most abundant: oxygen (O), carbon (C), hydrogen (H), and nitrogen (N). Each element’s name can be replaced by a one- or two-letter symbol.

All the elements in your body are derived from the foods you eat and the air you breathe. In nature, elements rarely occur alone.

A compound is a substance composed of two or more elements joined by chemical bonds. For example, the compound glucose is an important body fuel.

Moreover, the elements that make up any given compound always occur in the same relative amounts. In glucose, there are always six carbon and six oxygen units for every twelve hydrogen units.

An atom is the smallest quantity of an element that retains the unique properties of that element. In other words, an atom of hydrogen is a unit of hydrogen—the smallest amount of hydrogen that can exist.

The period at the end of this sentence is millions of atoms wide. Atoms are made up of even smaller subatomic particles, three types of which are important: the proton, neutron, and electron.

The number of negatively-charged electrons that “spin” around the nucleus at close to the speed of light equals the number of protons. An electron has about 1/2000th the mass of a proton or neutron.

In the planetary model, helium’s two electrons are shown circling the nucleus in a fixed orbit depicted as a ring. Although this model is helpful in visualizing atomic structure, in reality, electrons do not travel in fixed orbits, but whiz around the nucleus erratically in a so-called electron cloud.

Two Models of Atomic Structure. (a) In the planetary model, the electrons of helium are shown in fixed orbits, depicted as rings, at a precise distance from the nucleus, somewhat like planets orbiting the sun.

An atom’s protons and electrons carry electrical charges. Protons, with their positive charge, are designated p+.

An atom’s neutrons have no charge: they are electrically neutral. Just as a magnet sticks to a steel refrigerator because their opposite charges attract, the positively charged protons attract the negatively charged electrons.

The attraction by the positively charged nucleus helps keep electrons from straying far. The number of protons and electrons within a neutral atom are equal, thus, the atom’s overall charge is balanced.

One proton is the same as another, whether it is found in an atom of carbon, sodium (Na), or iron (Fe). The same is true for neutrons and electrons.

The answer is the unique quantity of protons each contains. Carbon by definition is an element whose atoms contain six protons.

Moreover, all atoms of carbon, whether found in your liver or in a lump of coal, contain six protons. Thus, the atomic number, which is the number of protons in the nucleus of the atom, identifies the element.

In their most common form, many elements also contain the same number of neutrons as protons. The most common form of carbon, for example, has six neutrons as well as six protons, for a total of 12 subatomic particles in its nucleus.