Fundamental Building Blocks

Atoms and Elements


Atoms are tiny particles that cannot be subdivided into smaller substances without losing their chemical properties.

Matter is anything that has mass and occupies space. All matter—both living and nonliving—is composed of tiny particles called atoms. An atom is the smallest unit of matter that retains all the chemical properties of an element. It cannot be subdivided into smaller substances without losing its properties. The structure and function of microorganisms (microbes) are ultimately determined by the atoms that form them. Processes that keep microbes alive and properties that distinguish them from other living things and from one another are determined at the atomic level. For instance, atomic structure defines the chemical processes by which microbes obtain energy, some of which are unique to these groups. Many physical properties of microbes, such as the structure of their deoxyribonucleic acid (DNA), the components that make up their ribosomes, and the lipids that compose the cell membrane, are also determined by atomic structure.

The subatomic particles making up atoms are distributed in either the nucleus (the center of an atom) or in orbitals surrounding the nucleus. These particles are categorized in part by their electric charge. A proton is a positively charged subatomic particle in the nucleus of an atom. A neutron is a subatomic particle that has a neutral charge in the nucleus of an atom. An electron is a negatively charged subatomic particle that moves in the orbitals around the nucleus. An orbital is a region in which an electron has a high probability of being located. Orbitals make up electron subshells which compose electron shells. The shell in which an electron is found describes its energy, electron subshells describe the shape of the region where an electrons are found, and the specific orbital describes the orientation of this region. Subatomic particles also have mass, though the mass of electrons is negligible compared to that of protons and neutrons.

Although all atoms of a given element have the same number of protons, the number of electrons or neutrons can vary. An isotope is one of two or more atoms of an element that have the same number of protons but different numbers of neutrons and thus different atomic masses. An ion is a charged particle formed when an atom gains or loses electrons. There are two types of ions: anions and cations. An anion is a negatively charged ion that has more electrons than protons. A cation is a positively charged ion that has more protons than electrons. Ions form through movements of valence electrons, those in the outermost electron shells of atoms. Atoms become more stable by filling these outermost electron shells, which is accomplished by gaining and losing electrons. This electron activity drives chemical bonding and the chemical reactions that sustain all living things, including microbes.

Atoms and Electron Orbitals

Electron shells, which contain subshells and subsequently electron orbitals, surround the nucleus of an atom. Electrons fill each shell starting closest to the nucleus and moving outward. The first shell holds only two electrons and is completely filled before electrons fill the second shell, which holds eight electrons. The outermost shell with electrons (called the valence shell) may or may not be full. In the Bohr model of an atom (in which the nucleus, orbitals, and electrons are shown), "n" refers to the shells and their relative energy level.


Elements are made up entirely of one unique type of atom and are defined by the subatomic particles that come together in different combinations to form those atoms.

An element is a substance made up entirely of one unique type of atom. An element cannot be broken down into simpler substances by chemical reactions. Each element is defined by the subatomic particles that come together in a specific combination to form its atoms. All atoms of a particular element have the same number of protons. Different elements are formed by the different combinations of subatomic particles.

Each of the 118 elements identified by scientists is represented on the periodic table by its chemical symbol, shown as one or two letters. The periodic table also lists each element's atomic weight—the weighted average of the mass of atoms—and its atomic number—the number of protons in each nucleus.
The periodic table groups all elements. Each element's listing shows atomic number (number of protons in the nucleus), chemical symbol (that element's abbreviation), and atomic weight (the weighted average of the mass of the element's atoms).
Six elements are particularly important to living things, including microbes: carbon (C), hydrogen (H), nitrogen (N), oxygen (O), phosphorus (P), and sulfur (S). Carbon, hydrogen, and oxygen are present in all organic molecules, and nitrogen is a key component of amino acids and nucleic acids, the building blocks of proteins and DNA, respectively. Sulfur is an element present in specific amino acids that provides structure to proteins, and phosphorus is specifically found in lipids. These six elements plus potassium (K), calcium (Ca), magnesium (Mg), and iron (Fe) make up 95 percent of the dry weight of microbial cells. Potassium is critical to enzyme function. Magnesium and iron also play roles in enzyme activity. Additionally, magnesium contributes to the stability of cell membranes and ribosomes. Iron is a component of cytochromes, which transfer electrons within metabolic pathways, including cellular respiration. Calcium is important to the heat resistance of endospores of bacteria.