If we take a purely mechanistic view of life, we can consider each living cell as an intricate web of chemical reactions. Atoms combine with other atoms to form molecules and then recombine again during other chemical reactions to form different molecules. These chemical reactions are the basis of a cell's structure and metabolism.

In the accompanying animation, we examine chemical bonds, which are the attractive forces between atoms. We focus, in particular, on the role of electrons in chemical bonds. The behavior of electrons explains most aspects of how atoms interact with each other in the living and nonliving world.


Two main properties influence how or whether bonds form between atoms. One, the state of an atom's outermost electron shell, determines whether an atom is reactive. Helium and neon have outer shells that are completely filled with electrons, and, for this reason, these atoms are inert. If an atom has a partially filled outer shell, however, it has a tendency to form bonds with other atoms. The second main property is an atom's electronegativity, which influences the kinds of bonds an atom forms. If two atoms have similar electronegativities, they tend to share electrons equally in nonpolar covalent bonds. However, if electronegativities differ, then the atoms tend to form either polar covalent bonds or ionic bonds.

Bonds between atoms in molecules also influence how these molecules interact with other molecules. For example, if a molecule contains polar covalent bonds, then the molecule itself is likely polar. A polar molecule can form hydrogen bonds with water molecules and therefore dissolve in the watery milieu of a cell. Once dissolved, it can more easily find and interact with other molecules and thereby participate in the cell's metabolism. Hydrogen bonds are also important because they hold together regions of large molecules, such as DNA or proteins, maintaining the shapes and functions of these important biomolecules.

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Textbook Reference: Concept 2.2 Atoms Interact and Form Molecules