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Chemical Bonds
The electrons of the outermost energy level determine the energetic stability of the atom and its tendency to form chemical bonds with other atoms. The innermost electron shell has a maximum capacity of two electrons, but the next two electron shells can each have a maximum of eight electrons. This is known as the octet rule, which states that, with the exception of the innermost shell, atoms are most stable energetically when they have eight electrons in their valence shell, the outermost electron shell.
However, since not all elements have enough electrons to fill their outermost shells, atoms form chemical bonds with other atoms, thereby obtaining the electrons they need to attain a stable electronic configuration. Therefore, a chemical bond is the force that holds atoms together in a molecule or compound.
Ionic Bonds
Atoms can gain or lose electrons to fill their valence shell. In doing so, they become electrically charged and are called ions. A cation is a positively charged atom that loses electrons, while an atom that gains electrons and becomes negatively charged is an anion. The opposite charges of cations and anions exert a moderately strong mutual attraction that keeps the atoms in close proximity forming an ionic bond. An ionic bond is an ongoing, close association between ions of opposite charge. Common table salt owes its existence to ionic bonding.
Covalent Bonds
Another way the octet rule can be satisfied is by the sharing of electrons between atoms to form covalent bonds. These bonds are stronger and much more common than ionic bonds in the molecules of living organisms. Covalent bonds are commonly found in carbon-based organic molecules, such as our DNA and proteins. Covalent bonds are also found in inorganic molecules like water, carbon dioxide, and oxygen.
Hydrogen Bonds
A hydrogen bond is formed when a weakly positive hydrogen atom already bonded to one electronegative atom (for example, the oxygen in the water molecule) is attracted to another electronegative atom from another molecule. The most common example of hydrogen bonding in the natural world occurs between molecules of water. It happens whenever two raindrops merge into a larger bead or a creek spills into a river. Hydrogen bonding occurs because the weakly negative oxygen atom in one water molecule is attracted to the weakly positive hydrogen atoms of two other water molecules.
This text is partially adapted from Openstax, Anatomy and Physiology 2e, Section 2.2: Chemical Bonds. Openstax, Biology for AP® Courses Section 2.1 Atoms, Isotopes, Ions, and Molecules: The Building Blocks.
