Chapter 5: Molecules, Compounds, and Chemical Equations

5.2 Covalent Bonding

Learning Outcomes

  • Describe the formation of covalent bonds
  • Define electronegativity and assess the polarity of covalent bonds

Ionic bonding results from the electrostatic attraction of oppositely charged ions that are typically produced by the transfer of electrons between metallic and nonmetallic atoms. A different type of bonding results from the mutual attraction of atoms for a “shared” pair of electrons. Such bonds are called covalent bonds. Covalent bonds are formed between two atoms when both have similar tendencies to attract electrons to themselves (i.e., when both atoms have identical or fairly similar ionization energies and electron affinities). For example, two hydrogen atoms bond covalently to form an H2 molecule; each hydrogen atom in the H2 molecule has two electrons stabilizing it, giving each atom the same number of valence electrons as the noble gas He.

Compounds that contain covalent bonds exhibit different physical properties than ionic compounds. Because the attraction between molecules, which are electrically neutral, is weaker than that between electrically charged ions, covalent compounds generally have much lower melting and boiling points than ionic compounds. In fact, many covalent compounds are liquids or gases at room temperature, and, in their solid states, they are typically much softer than ionic solids. Furthermore, whereas ionic compounds are good conductors of electricity when dissolved in water, most covalent compounds are insoluble in water; since they are electrically neutral, they are poor conductors of electricity in any state.

Formation of Covalent Bonds

Nonmetal atoms frequently form covalent bonds with other nonmetal atoms. For example, the hydrogen molecule, [latex]\ce{H2}[/latex], contains a covalent bond between its two hydrogen atoms. Figure 5.2.1 illustrates why this bond is formed. Starting on the far right, we have two separate hydrogen atoms with a particular potential energy, indicated by the red line. Along the x-axis is the distance between the two atoms. As the two atoms approach each other (moving left along the x-axis), their valence orbitals (1s) begin to overlap. The single electrons on each hydrogen atom then interact with both atomic nuclei, occupying the space around both atoms. The strong attraction of each shared electron to both nuclei stabilizes the system, and the potential energy decreases as the bond distance decreases. If the atoms continue to approach each other, the positive charges in the two nuclei begin to repel each other, and the potential energy increases. The bond length is determined by the distance at which the lowest potential energy is achieved.

A graph is shown with the x-axis labeled, “Internuclear distance ( p m )” while the y-axis is labeled, “Energy ( J ).” One value, “0,” is labeled midway up the y-axis and two values: “0” at the far left and “0.74” to the left, are labeled on the x-axis. The point “0.74” is labeled, “H bond H distance.” A line is graphed that begins near the top of the y-axis and to the far left on the x-axis and drops steeply to a point labeled, “negative 7.24 times 10 superscript negative 19 J” on the y-axis and 0.74 on the x-axis. This low point on the graph corresponds to a drawing of two spheres that overlap considerably. The line then rises to zero on the y-axis and levels out. The point where it almost reaches zero corresponds to two spheres that overlap slightly. The line at zero on the y-axis corresponds to two spheres that are far from one another.
Figure 5.2.1. The potential energy of two separate hydrogen atoms (right) decreases as they approach each other, and the single electrons on each atom are shared to form a covalent bond. The bond length is the internuclear distance at which the lowest potential energy is achieved.

It is essential to remember that energy must be added to break chemical bonds (an endothermic process), whereas forming chemical bonds releases energy (an exothermic process). In the case of [latex]\ce{H2}[/latex], the covalent bond is very strong; a large amount of energy, 436 kJ, must be added to break the bonds in one mole of hydrogen molecules and cause the atoms to separate:

[latex]\ce{H2}(g)\longrightarrow \ce{2H}(g)\qquad\Delta H=436\text{kJ}[/latex]

Conversely, the same amount of energy is released when one mole of [latex]\ce{H2}[/latex] molecules forms from two moles of H atoms:

[latex]\ce{2H}(g)\longrightarrow\ce{H2}(g)\qquad\Delta H=-436\text{kJ}[/latex]

Key Concepts and Summary

Covalent bonds form when electrons are shared between atoms and are attracted by the nuclei of both atoms. In pure covalent bonds, the electrons are shared equally.

Try It

  1. Why is it incorrect to speak of a molecule of solid [latex]\ce{NaCl}[/latex]?
  2. What information can you use to predict whether a bond between two atoms is covalent or ionic?
  3. Predict which of the following compounds are ionic and which are covalent, based on the location of their constituent atoms in the periodic table:
    1. [latex]\ce{Cl2CO}[/latex]
    2. [latex]\ce{MnO}[/latex]
    3. [latex]\ce{NCl3}[/latex] 
    4. [latex]\ce{CoBr2}[/latex] 
    5. [latex]\ce{K2S}[/latex]
    6. [latex]\ce{CO}[/latex]
    7. [latex]\ce{CaF2}[/latex] 
    8. [latex]\ce{HI}[/latex]
    9. [latex]\ce{CaO}[/latex]
    10. [latex]\ce{IBr}[/latex]
    11. [latex]\ce{CO2}[/latex]
Selected Answers
  1. [latex]\ce{NaCl}[/latex] consists of discrete ions arranged in a crystal lattice, not covalently bonded molecules.
  2. Options (b), (d), (e), (g), and (i) are ionic; Options (a), (c), (f), (h), (j), and (k) are covalent.


bond length: distance between the nuclei of two bonded atoms at which the lowest potential energy is achieved

covalent bond: bond formed when electrons are shared between atoms

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