Covalent Bonds and Ionic 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.
Types of Chemical Bonds
|Type of Chemical Bond||Description||Example|
|Nonpolar covalent bond||Covalent bond that forms when two atoms share electrons equally||Bond between each amino acid that builds the proteins necessary for cellular function|
|Polar covalent bond||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.||Bonds found in the sugar molecules necessary for metabolism and growth|
|Ionic bond||Chemical bond that forms when valence electrons are transferred between atoms and not shared||Bonds that shape the 3D structure of many proteins|
|Hydrogen bond||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||Bonds between nucleotides that hold the DNA double helix together|
|van der Waals force||Weakest intermolecular force, which forms because of attraction between molecules||Force that holds together the two lipid layers of the plasma membrane surrounding a cell|
Chemical Reactions in Cells
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.