07 Ionic and Covalent Bonding

Two Models of Chemical Bonding

Chemical bonds are attractive interactions that hold atoms or ions together. Two useful models describe bonding: ionic bonding, based mainly on attraction between oppositely charged ions, and covalent bonding, based on shared electron density. Many real bonds fall between these models rather than being perfectly ionic or perfectly covalent.

Ionic Bonding and Ionic Solids

In ionic bonding, a metal commonly transfers one or more electrons to a nonmetal. The metal becomes a positively charged cation, and the nonmetal becomes a negatively charged anion. Their opposite charges attract. For example, sodium and chlorine can form $\mathrm{Na^+}$ and $\mathrm{Cl^-}$, respectively.

Ionic solids are usually crystalline lattices rather than collections of separate molecules. Their formulas show the simplest whole-number ratio of ions that produces an electrically neutral solid. In $\mathrm{Al_2O_3}$, for example, two $\mathrm{Al^{3+}}$ ions contribute a total charge of $+6$, balanced by three $\mathrm{O^{2-}}$ ions contributing $-6$. The ions occur in a $2:3$ ratio.

The strong attractions throughout an ionic lattice generally make ionic compounds high-melting solids. In a solid, ions are fixed in place and cannot move to carry electric charge. When an ionic compound is molten or dissolved in water, its mobile ions can conduct electricity. These are common patterns, not exception-free rules.

Takeaway: Ionic compounds consist of oppositely charged ions arranged in a neutral ratio; their properties reflect the attractions and mobility of those ions.

Covalent Bonds, Polarity, and Properties

In covalent bonding, atoms share electron density. Covalent bonds commonly form between nonmetals and often produce discrete molecules such as $\mathrm{H_2O}$ and $\mathrm{CO_2}$.

When two identical atoms share electrons, they share them equally. The bond in $\mathrm{H_2}$ is therefore nonpolar. In $\mathrm{HCl}$, chlorine attracts the shared electrons more strongly than hydrogen does. The bond is polar covalent: chlorine is partially negative, $\delta^-$, and hydrogen is partially positive, $\delta^+$.

A covalent bond has a characteristic bond length: the internuclear distance at which the bonded atoms have their lowest potential energy. Breaking a bond requires energy, while forming a bond releases energy. In general, multiple bonds between the same pair of atoms are shorter and stronger than single bonds, although exact lengths and strengths depend on the atoms and their chemical environment.

Molecular covalent substances often have lower melting and boiling points than ionic solids. This broad trend does not mean that the covalent bonds inside each molecule are weak: melting a molecular substance usually separates molecules from one another rather than breaking their internal covalent bonds.

Takeaway: Covalent bonds involve shared electrons; the equality of that sharing affects bond polarity, while bond strength and the attractions between molecules help explain physical properties.

Electronegativity and the Bonding Continuum

Electronegativity describes how strongly an atom attracts shared electrons in a bond. It differs from electron affinity, which concerns the energy change when an isolated gaseous atom gains an electron.

To estimate bond polarity, calculate the absolute difference between the electronegativities of the two bonded atoms:

A small $\Delta \mathrm{EN}$ indicates more equal sharing; a larger value indicates more unequal sharing and greater ionic character. For example, $\mathrm{H-H}$ has $\Delta \mathrm{EN}=0$ and is nonpolar covalent. $\mathrm{H-Cl}$ has a nonzero difference and is polar covalent, while $\mathrm{Na-Cl}$ has a large difference and is commonly modeled as ionic.

Introductory charts sometimes use numerical cutoffs to classify bonds, but these boundaries are approximate, not universal dividing lines. Use electronegativity as a guide alongside the elements involved: metal–nonmetal combinations are often ionic, while nonmetal–nonmetal combinations are often covalent. These are predictions, not absolute rules.

Some compounds contain both types of bonding. In potassium nitrate, $\mathrm{KNO_3}$, attraction between $\mathrm{K^+}$ and $\mathrm{NO_3^-}$ is ionic, while the $\mathrm{N-O}$ bonds within the nitrate ion are covalent.

Takeaway: Electronegativity difference helps estimate how electrons are shared, but ionic and covalent bonding are best understood as ends of a continuum.