Chapter 6: Chemical Bonding and Molecular Geometry

Learning Outcomes

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.

Pure vs. Polar Covalent Bonds

If the atoms that form a covalent bond are identical, as in [latex]\ce{H2}[/latex], [latex]\ce{Cl2}[/latex], and other diatomic molecules, then the electrons in the bond must be shared equally. We refer to this as a pure covalent bond. Electrons shared in pure covalent bonds have an equal probability of being near each nucleus.

In the case of [latex]\ce{Cl2}[/latex], each atom starts off with seven valence electrons, and each [latex]\ce{Cl}[/latex] shares one electron with the other, forming one covalent bond:

[latex]\ce{Cl}+\ce{Cl}\rightarrow \ce{Cl2}[/latex]

The total number of electrons around each individual atom consists of six nonbonding electrons and two shared (i.e., bonding) electrons for eight total electrons, matching the number of valence electrons in the noble gas argon. Since the bonding atoms are identical, [latex]\ce{Cl2}[/latex] also features a pure covalent bond.

Two diagrams are shown and labeled “a” and “b.” Diagram a shows a small sphere labeled, “H” and a larger sphere labeled, “C l” that overlap slightly. Both spheres have a small dot in the center. Diagram b shows an H bonded to a C l with a single bond. A dipole and a positive sign are written above the H and a dipole and negative sign are written above the C l. An arrow points toward the C l with a plus sign on the end furthest from the arrow’s head near the H.
Figure 6.2.1 (a) The distribution of electron density in the HCl molecule is uneven. The electron density is greater around the chlorine nucleus. The small, black dots in the center of the green spheres indicate the location of the hydrogen and chlorine nuclei in the molecule. (b) Symbols [latex]\delta+[/latex] and [latex]\delta-[/latex] indicate the polarity of the [latex]\ce{H-Cl}[/latex] bond.

When the atoms linked by a covalent bond are different, the bonding electrons are shared, but no longer equally. Instead, the bonding electrons are more attracted to one atom than the other, giving rise to a shift of electron density toward that atom. This unequal distribution of electrons is known as a polar covalent bond, characterized by a partial positive charge on one atom and a partial negative charge on the other. The atom that attracts the electrons more strongly acquires the partial negative charge and vice versa. For example, the electrons in the [latex]\ce{H-Cl}[/latex] bond of a hydrogen chloride molecule spend more time near the chlorine atom than near the hydrogen atom. Thus, in an [latex]\ce{HCl}[/latex] molecule, the chlorine atom carries a partial negative charge and the hydrogen atom has a partial positive charge. Figure 6.2.1 shows the distribution of electrons in the [latex]\ce{H-Cl}[/latex] bond. Note that the shaded area around [latex]\ce{Cl}[/latex] is much larger than it is around [latex]\ce{H}[/latex]. Compare this to the even distribution of electrons in a [latex]\ce{H2}[/latex] nonpolar bond.

We sometimes designate the positive and negative atoms in a polar covalent bond using a lowercase Greek letter “delta,” [latex]\delta[/latex], with a plus sign or minus sign to indicate whether the atom has a partial positive charge ([latex]\delta{+}[/latex]) or a partial negative charge ([latex]\delta{-}[/latex]). This symbolism is shown for the [latex]\ce{H-Cl}[/latex] molecule in Figure 6.2.1.


Whether a bond is nonpolar or polar covalent is determined by a property of the bonding atoms called electronegativity. Electronegativity is a measure of the tendency of an atom to attract electrons (or electron density) towards itself. It determines how the shared electrons are distributed between the two atoms in a bond. The more strongly an atom attracts the electrons in its bonds, the larger its electronegativity. Electrons in a polar covalent bond are shifted toward the more electronegative atom; thus, the more electronegative atom is the one with the partial negative charge. The greater the difference in electronegativity, the more polarized the electron distribution and the larger the partial charges of the atoms.

Figure 6.2.2 shows the electronegativity values of the elements as proposed by one of the most famous chemists of the twentieth century: Linus Pauling (Figure 6.2.3). In general, electronegativity increases from left to right across a period in the periodic table and decreases down a group. Thus, the nonmetals, which lie in the upper right, tend to have the highest electronegativities, with fluorine the most electronegative element of all (EN = 4.0). Metals tend to be less electronegative elements, and the group 1 metals have the lowest electronegativities. Note that noble gases are excluded from this figure because these atoms usually do not share electrons with others atoms since they have a full valence shell. (While noble gas compounds such as [latex]\ce{XeO2}[/latex] do exist, they can only be formed under extreme conditions, and thus they do not fit neatly into the general model of electronegativity.)

Part of the periodic table is shown. A downward-facing arrow is drawn to the left of the table and labeled, “Decreasing electronegativity,” while a right-facing arrow is drawn above the table and labeled “Increasing electronegativity.” The electronegativity for almost all the elements is given.
Figure 6.2.2 The electronegativity values derived by Pauling follow predictable periodic trends with the higher electronegativities toward the upper right of the periodic table.

Electronegativity versus Electron Affinity

We must be careful not to confuse electronegativity and electron affinity. The electron affinity of an element is a measurable physical quantity, namely, the energy released or absorbed when an isolated gas-phase atom acquires an electron, measured in kJ/mol. Electronegativity, on the other hand, describes how tightly an atom attracts electrons in a bond. It is a dimensionless quantity that is calculated, not measured. Pauling derived the first electronegativity values by comparing the amounts of energy required to break different types of bonds. He chose an arbitrary relative scale ranging from 0 to 4.

Portrait of a Chemist: Linus Pauling

A photograph of Linus Pauling is shown.
Figure 6.2.3. Linus Pauling (1901–1994) made many important contributions to the field of chemistry. He was also a prominent activist, publicizing issues related to health and nuclear weapons.

Linus Pauling, shown in Figure 6.2.3, is the only person to have received two unshared (individual) Nobel Prizes: one for chemistry in 1954 for his work on the nature of chemical bonds and one for peace in 1962 for his opposition to weapons of mass destruction. He developed many of the theories and concepts that are foundational to our current understanding of chemistry, including electronegativity and resonance structures.

Pauling also contributed to many other fields besides chemistry. His research on sickle cell anemia revealed the cause of the disease—the presence of a genetically inherited abnormal protein in the blood—and paved the way for the field of molecular genetics. His work was also pivotal in curbing the testing of nuclear weapons; he proved that radioactive fallout from nuclear testing posed a public health risk.

Electronegativity and Bond Type

The absolute value of the difference in electronegativity (ΔEN) of two bonded atoms provides a rough measure of the polarity to be expected in the bond and, thus, the bond type. When the difference is very small or zero, the bond is covalent and nonpolar. When it is large, the bond is polar covalent or ionic. The absolute values of the electronegativity differences between the atoms in the bonds [latex]\ce{H-H}[/latex], [latex]\ce{H-Cl}[/latex], and [latex]\ce{Na-Cl}[/latex] are 0 (nonpolar), 0.9 (polar covalent), and 2.1 (ionic), respectively. The degree to which electrons are shared between atoms varies from completely equal (pure covalent bonding) to not at all (ionic bonding). Figure 6.2.4 shows the relationship between electronegativity difference and bond type.

Two flow charts and table are shown. The first flow chart is labeled, “Electronegativity difference between bonding atoms.” Below this label are three rounded text bubbles, connected by a downward-facing arrow, labeled, “Zero,” “Intermediate,” and “Large,” respectively. The second flow chart is labeled, “Bond type.” Below this label are three rounded text bubbles, connected by a downward-facing arrow, labeled, “Pure covalent,” “Polar covalent,” and “Ionic,” respectively. A double ended arrow is written vertically to the right of the flow charts and labeled, “Covalent character decreases; ionic character increases.” The table is made up of two columns and four rows. The header line is labeled “Bond type” and “Electronegativity difference.” The left column contains the phrases “Pure covalent,” “Polar covalent,” and “Ionic,” while the right column contains the values “less than 0.4,” “between 0.4 and 1.8,” and “greater than 1.8.”
Figure 6.2.4 As the electronegativity difference increases between two atoms, the bond becomes more ionic.

A rough approximation of the electronegativity differences associated with covalent, polar covalent, and ionic bonds is shown Table 6.2.1. This table is just a general guide, however, with many exceptions. For example, the [latex]\ce{H}[/latex] and [latex]\ce{F}[/latex] atoms in [latex]\ce{HF}[/latex] have an electronegativity difference of 1.9, and the [latex]\ce{N}[/latex] and [latex]\ce{H}[/latex] atoms in [latex]\ce{NH3}[/latex] a difference of 0.9, yet both of these compounds form bonds that are considered polar covalent. Likewise, the [latex]\ce{Na}[/latex] and [latex]\ce{Cl}[/latex] atoms in [latex]\ce{NaCl}[/latex] have an electronegativity difference of 2.1, and the Mn and I atoms in [latex]\ce{MnI2}[/latex] have a difference of 1.0, yet both of these substances form ionic compounds.

The best guide to the covalent or ionic character of a bond is to consider the types of atoms involved and their relative positions in the periodic table. Bonds between two nonmetals are generally covalent; bonding between a metal and a nonmetal is often ionic.

Some compounds contain both covalent and ionic bonds. The atoms in polyatomic ions, such as [latex]\ce{OH-}[/latex], [latex]\ce{NO3-},[/latex] and [latex]\ce{NH4+},[/latex] are held together by polar covalent bonds. However, these polyatomic ions form ionic compounds by combining with ions of opposite charge. For example, potassium nitrate, [latex]\ce{KNO3}[/latex], contains the [latex]\ce{K+}[/latex] cation and the polyatomic [latex]\ce{NO3-}[/latex] anion. Thus, bonding in potassium nitrate is ionic, resulting from the electrostatic attraction between the ions [latex]\ce{K+}[/latex] and [latex]\ce{NO3-},[/latex] as well as covalent between the nitrogen and oxygen atoms in [latex]\ce{NO3-}[/latex].

Example 6.2.1: Electronegativity and Bond Polarity

Bond polarities play an important role in determining the structure of proteins. Using the electronegativity values in Figure 7.6, arrange the following covalent bonds—all commonly found in amino acids—in order of increasing polarity. Then designate the positive and negative atoms using the symbols [latex]\delta{+}[/latex] and [latex]\delta{-}[/latex]:

[latex]\ce{C-H, C-N, C-O, N-H, O-H, S-H}[/latex]

Show Solution

The polarity of these bonds increases as the absolute value of the electronegativity difference increases. The atom with the [latex]\delta{-}[/latex] designation is the more electronegative of the two. Table 1 shows these bonds in order of increasing polarity.

Table 6.2.1 Bond Polarity and Electronegativity Difference
Bond [latex]\Delta[/latex]EN Polarity
[latex]\ce{C-H}[/latex] 0.4 [latex]\stackrel{\delta -}{\ce{C}}-\stackrel{\delta \text{+}}{\ce{H}}[/latex]
[latex]\ce{S-H}[/latex] 0.4 [latex]\stackrel{\delta -}{\ce{S}}-\stackrel{\delta \text{+}}{\ce{H}}[/latex]
[latex]\ce{C-N}[/latex] 0.5 [latex]\stackrel{\delta \text{+}}{\ce{C}}-\stackrel{\delta -}{\ce{N}}[/latex]
[latex]\ce{N-H}[/latex] 0.9 [latex]\stackrel{\delta -}{\ce{N}}-\stackrel{\delta \text{+}}{\ce{H}}[/latex]
[latex]\ce{C-O}[/latex] 1.0 [latex]\stackrel{\delta \text{+}}{\ce{C}}-\stackrel{\delta -}{\ce{O}}[/latex]
[latex]\ce{O-H}[/latex] 1.4 [latex]\stackrel{\delta -}{\ce{O}}-\stackrel{\delta \text{+}}{\ce{H}}[/latex]

Check Your Learning

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. In polar covalent bonds, the electrons are shared unequally, as one atom exerts a stronger force of attraction on the electrons than the other. The ability of an atom to attract a pair of electrons in a chemical bond is called its electronegativity. The difference in electronegativity between two atoms determines how polar a bond will be. In a diatomic molecule with two identical atoms, there is no difference in electronegativity, so the bond is nonpolar or pure covalent. When the electronegativity difference is very large, as is the case between metals and nonmetals, the bonding is characterized as ionic.

Try It

  1. Explain the difference between a nonpolar covalent bond, a polar covalent bond, and an ionic bond.
  2. From its position in the periodic table, determine which atom in each pair is more electronegative:
    1. [latex]\ce{N}[/latex] or [latex]\ce{P}[/latex]
    2. [latex]\ce{N}[/latex] or [latex]\ce{Ge}[/latex]
    3. [latex]\ce{S}[/latex] or [latex]\ce{F}[/latex]
  3. From their positions in the periodic table, arrange the atoms in each of the following series in order of increasing electronegativity:
    1. [latex]\ce{As}[/latex], [latex]\ce{H}[/latex], [latex]\ce{N}[/latex], [latex]\ce{P}[/latex], [latex]\ce{Sb}[/latex]
    2. [latex]\ce{Cl}[/latex], [latex]\ce{H}[/latex], [latex]\ce{P}[/latex], [latex]\ce{S}[/latex], [latex]\ce{Si}[/latex]
    3. [latex]\ce{Br}[/latex], [latex]\ce{Cl}[/latex], [latex]\ce{Ge}[/latex], [latex]\ce{H}[/latex], [latex]\ce{Sr}[/latex]
  4. Which of the following molecules or ions contain polar bonds?
    1. [latex]\ce{O3}[/latex]
    2. [latex]\ce{S8}[/latex]
    3. [latex]\ce{O2^{2-}}[/latex]
Selected Answers


covalent bond: bond formed when electrons are shared between atoms

electronegativity: tendency of an atom to attract electrons in a bond to itself

polar covalent bond: covalent bond between atoms of different electronegativities; a covalent bond with a positive end and a negative end

pure covalent bond: (also, nonpolar covalent bond) covalent bond between atoms of identical electronegativities



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