Chemical Bonds

Core Concept

Atoms bond to complete their outer electron shells. The two primary ways they do so are covalent bonding, in which atoms share pairs of electrons, and ionic bonding, in which one atom transfers electrons to another and the resulting oppositely charged ions attract each other. Which happens depends on electronegativity — the atom's pull on shared electrons. When two atoms have similar electronegativity, they share electrons more or less equally (nonpolar covalent); when they differ, the more electronegative atom pulls the shared electrons closer (polar covalent); when the difference is large enough, one atom simply takes the electron outright (ionic).

A second, subtler tier of chemistry is just as important to life: weak interactions between molecules — hydrogen bonds and van der Waals forces. These are not covalent bonds, but because they are numerous, they collectively hold water together, fold proteins, and pair the two strands of DNA.

Key Concepts

Covalent Bonds: Nonpolar and Polar

A covalent bond is the sharing of a pair of valence electrons between two atoms. In a nonpolar covalent bond, electrons are shared equally because the two atoms have the same or nearly the same electronegativity — for example, the two oxygen atoms in O₂ or a carbon–hydrogen bond. In a polar covalent bond, electrons are shared unequally: the more electronegative atom holds the electrons closer, gaining a partial negative charge (δ−), while the other atom gains a partial positive charge (δ+). The O–H bonds in water are the classic example, with oxygen pulling electron density away from hydrogen.

Ionic Bonds, Cations, and Anions

When the electronegativity difference is very large, one atom completely strips an electron from the other. The atom that loses an electron becomes a positively charged ion, or cation; the atom that gains it becomes a negatively charged ion, or anion. The opposite charges attract, forming an ionic bond (or, in a crystal lattice, a network of such attractions). In sodium chloride, sodium (Na) loses one electron to become Na⁺, and chlorine (Cl) gains it to become Cl⁻.

Hydrogen Bonds

A hydrogen bond is a weak attraction between a hydrogen atom that is covalently bonded to a highly electronegative atom (oxygen, nitrogen, or fluorine) and another electronegative atom nearby. Because the hydrogen carries a partial positive charge and the other atom a partial negative charge, they attract. Hydrogen bonds are individually weak but collectively strong: they give water its cohesion, stabilize the alpha helix and beta sheet of proteins, and hold the two strands of DNA together between complementary bases.

Van der Waals Interactions

Van der Waals interactions are even weaker, transient attractions that arise from momentary, fluctuating asymmetries in electron distribution. When two molecules are extremely close, these fleeting dipoles can produce a weak stickiness. Individually they are negligible, but over large surface areas they add up — for example, they help a gecko's foot pads cling to a ceiling and contribute to the close packing of molecules inside a cell.

Strong Bonds vs. Weak Interactions

Covalent and ionic bonds are strong — they hold atoms together within a molecule or crystal and require significant energy to break. Hydrogen bonds and van der Waals interactions are weak and form or break readily at physiological temperatures, which is precisely why they are useful: cells can rapidly assemble and disassemble structures (a protein folding, an enzyme binding its substrate) without destroying the underlying molecules.

How It Works

Consider what happens when sodium meets chlorine. Sodium (1 valence electron) and chlorine (7 valence electrons) both "want" a full outer shell. Sodium's one electron is held loosely, so it transfers it to chlorine. Now both have complete shells: Na⁺ has the stable electron configuration of neon, and Cl⁻ has that of argon. The resulting opposite charges attract, and the ions pack into a crystal of NaCl. This is a cause-and-effect chain — an electronegativity imbalance drives electron transfer, which produces charged ions, which produces an electrostatic attraction. By contrast, when two identical or similar atoms meet (O with O, or C with H), neither can take an electron outright, so they share — a covalent bond — and the evenness of that sharing decides polarity.

How it works

Consider what happens when sodium meets chlorine. Sodium (1 valence electron) and chlorine (7 valence electrons) both "want" a full outer shell. Sodium's one electron is held loosely, so it transfers it to chlorine. Now both have complete shells: Na⁺ has the stable electron configuration of neon, and Cl⁻ has that of argon. The resulting opposite charges attract, and the ions pack into a crystal of NaCl. This is a cause-and-effect chain — an electronegativity imbalance drives electron transfer, which produces charged ions, which produces an electrostatic attraction. By contrast, when two identical or similar atoms meet (O with O, or C with H), neither can take an electron outright, so they share — a covalent bond — and the evenness of that sharing decides polarity.

Common confusions

• "A hydrogen bond is just a covalent bond involving hydrogen." No. A hydrogen bond is a weak intermolecular attraction; the hydrogen is already covalently bonded to O, N, or F within its own molecule.
• "Ionic compounds are individual molecules like NaCl." Ionic compounds form lattices of many ions, not discrete two-atom molecules; the "bond" is the overall electrostatic attraction across the crystal.
• "Polar covalent and ionic are the same thing." They form a spectrum. In a polar covalent bond electrons are shared unequally but still shared; in an ionic bond the electron is essentially transferred.
• "Weak bonds are unimportant because they are weak." Their very weakness (reversibility) plus their number makes them essential — that's why protein folding and enzyme function work at body temperature.
• "Van der Waals forces are permanent." They are transient, arising from momentary fluctuations in electron distribution.

Quick review

• Electronegativity differences predict bond type: nonpolar covalent < polar covalent < ionic.
• Covalent bonds share electrons; ionic bonds transfer electrons and create cations and anions.
• Hydrogen bonds are weak attractions involving H on O/N/F; they are not covalent.
• Van der Waals interactions are even weaker and transient but add up over large surfaces.
• Strong intramolecular bonds hold molecules together; weak intermolecular forces govern shape and interaction.
• Life depends on weak, reversible interactions: water cohesion, protein folding, and DNA pairing.