Types of Chemical Bonds

A chemical bond is an attractive force that joins atoms or ions. It forms when two or more atoms lose, gain, or share valence electrons, electrons in the outermost shell of an atom. The bond creates stability between the atoms. A compound is a substance made of atoms of two or more elements bonded together in a certain ratio. A molecule is a group of atoms held together by one or more covalent bonds. A covalent bond is a chemical bond that forms when valence electrons are shared between atoms. Atoms in a molecule can be from the same element or from different elements. The oxygen in Earth's atmosphere exists as molecules formed by two atoms of oxygen (O₂) and is not a compound. Water, however, is a compound whose molecules consist of two atoms of hydrogen and one atom of oxygen (H₂O). Covalent bonds can be polar or nonpolar. A polar covalent bond is a covalent bond in which the electron density is more localized on one end of the bond. One end is slightly positive, and one end is slightly negative. By contrast, a nonpolar covalent bond is a covalent bond that forms when two atoms share electrons equally. Two compounds important to microbes, water (H₂O) and methane (CH₄), are examples of each type of covalent bond. Water is a compound with polar covalent bonds. The oxygen atom in water has a partial negative charge and strongly attracts hydrogen electrons. The hydrogen atoms carry partial positive charges. In the nonpolar bonds of a methane molecule, however, electrons are shared equally. Nonpolar bonds are thus stronger than polar bonds.

An ionic bond is a chemical bond that forms when valence electrons are transferred between atoms and not shared. Ionic bonds hold together cations and anions, oppositely charged ions, in ionic compounds. Atoms of certain groups of elements commonly form ionic bonds, such as nonmetals, alkali metals, and alkaline earth metals. Sodium chloride (NaCl)—table salt—is an ionic compound. For many microbes, ions play critical roles in energy production due to their charged nature.

Molecules and compounds are also held together by bonds that are weaker or more temporary than covalent or ionic bonds. A hydrogen bond is a weak intermolecular force that results from an attraction between a positively charged hydrogen atom in one molecule and a negatively charged atom in a nearby molecule. The negatively charged atom that bonds with hydrogen is usually fluorine, oxygen, or nitrogen. For example, neighboring water molecules (H₂O) form a hydrogen bond between the positively charged hydrogen atom of one molecule and the negatively charged oxygen atom of the nearby molecule. These bonds give water unique properties that make it critical to the growth and survival of many microbes. They are responsible for water's cohesion (ability to stick to itself) and adhesion (ability to stick to other substances). Hydrogen bonds are also responsible for water's high heat of vaporization (the amount of heat required to transform a quantity of water from liquid to gas). A van der Waals force (also called a London dispersion force) is the weakest intermolecular force, which forms because of attraction between molecules. Van der Waals forces form between atoms and molecules of any element. They arise from the temporary charge created by unequal distribution of electrons, which are constantly in motion. All molecules with covalent chemical bonds have van der Waals forces, including methane, a compound produced by some microbes as a metabolic byproduct.

Cells of living things, including microbes, require a steady supply of energy in order to function. This energy, often in the form of ATP (adenosine triphosphate), is released in chemical reactions that take place in the cells. ATP is a nucleotide consisting of the sugar ribose, the base adenine, and three phosphate groups. ATP contains chemical energy in the bonds between its phosphate groups. When a phosphate ion is released from ATP, energy is released along with it as the electrons involved in bonding move from an excited state to a lower-energy state. The molecule that remains is adenosine diphosphate, or ADP. The formation and breaking of the bonds between these molecules provide the energy that drives the processes of life. The energy is obtained from the chemical bond of molecules serving as nutrient sources. In all chemical reactions, the starting substances are the reactants, and the substances produced by a reaction are the products. (Each reaction can be written as a chemical equation, with reactants on the left and products on the right.) A chemical reaction takes place when two or more atoms form chemical bonds or when chemical bonds between atoms are broken. Some chemical reactions are reversible, which means the products can undergo the reaction in reverse without additional help. An important metabolic chemical reaction is fermentation, in which a carbohydrate breaks down with the release of energy. The carbohydrate glucose (C₆H₁₂O₆), for example, breaks down to produce ethanol (C₂H₅OH), carbon dioxide (CO₂), and ATP. Glucose is the reactant, and ethanol, carbon dioxide, and ATP are the products. Microbes employ a wide range of chemical reactions to generate energy for cells. Bacteria live in almost every known habitat on Earth; in order to survive they are able to carry out hundreds of different types of chemical reactions to produce the energy they need. Some bacteria obtain energy from chemical reactions involving organic compounds. An organic compound is a compound consisting of molecules that contain one or more carbon-hydrogen bonds. Processes in microbes that break down organic compounds for energy include cellular respiration and fermentation.

Other microbes generate energy through chemical reactions involving inorganic compounds. An inorganic compound is a compound consisting of molecules that do not contain a carbon-hydrogen bond. Some bacteria are able to produce their own chemical energy from inorganic compounds, a process referred to as autotrophy. Generating energy through photosynthesis is a specific form of autotrophy involving the absorption of light, called phototrophy. Chemotrophy refers to forms of autotrophy that do not involve the absorption of light. One example of chemotrophy is sulfur oxidation, in which bacteria convert hydrogen sulfide (H₂S) into sulfuric acid (H₂SO₄) to produce energy in the form of ATP.