Science, Tech, Math › Science Bonds Definition in Chemistry What Is a Chemical Bond? Share Flipboard Email Print In molecular models, single bonds are represented by solid lines, while double bonds are represented by two lines between atoms. ALFRED PASIEKA/SCIENCE PHOTO LIBRARY, Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated July 03, 2019 In chemistry, a bond or chemical bond is a link between atoms in molecules or compounds and between ions and molecules in crystals. A bond represents a lasting attraction between different atoms, molecules or ions. Why Bonds Form Most of bonding behavior can be explained by the attraction between two opposite electrical charge. The electrons of an atom or ion are attracted to their own positively-charged nucleus (containing protons), yet also to the nuclei of nearby atoms. Species that participate in chemical bonds are more stable when the bond is formed, typically because they had an imbalance of charge (greater or fewer number of electrons than protons) or because their valence electrons did not fill or half-fill electron orbitals. Examples of Chemical Bonds The two main types of bonds are covalent bonds and ionic bonds. Covalent bonding is where atoms share electrons more or less equally between each other. In an ionic bond, an electron from one atom spends more time associated with the nucleus and electron orbitals of the other atom (essentially donated). However, pure covalent and ionic bonding is relatively rare. Usually a bond is intermediate between ionic and covalent. In a polar covalent bond, electrons are shared, but the electrons participating in the bond are more attracted to one atom than to the other. Another type of bonding is a metallic bond. In a metallic bond, electrons are donated to an "electron sea" between a group of atoms. Metallic bonding is very strong, but the fluid nature of the electrons allows for a high degree of electrical and thermal conductivity.