# 5.8: Characteristics of Covalent compounds

Learning Objectives

• Recognize bond characteristics of covalent compounds: bond length and bond energy

## Bond Length and Bond Energy

Bond length is the separation distance between two nuclei forming a covalent bond. The bond length is usually measured in meters or picometers (1 pm = 1x10-12m). For example, the covalent bond in the hydrogen molecule (H2) has a certain length (about 7.4 × 10−11 m). Other covalent bonds also have known bond lengths, which are dependent on both the identities of the atoms in the bond and whether the bonds are single, double, or triple bonds. Table $$\PageIndex{1}$$ lists the approximate bond lengths for some single covalent bonds. The exact bond length may vary depending on the identity of the molecule but will be close to the value given in the table.

Bond energy is defined as the energy required to break a particular bond in a molecule in the gas phase, expressed in kilojoules per mole of bonds (kJ/mol). This value depends on the identity of the bonded atoms and the nature of the molecule (for example, the energy required to break the O-H bond in H2O is different than in H2O2). Except for diatomic molecules, the bond energies listed in Table $$\PageIndex{1}$$ are average values for all bonds of a given type in a range of molecules. Figure $$\PageIndex{1}$$: representation of bond length and bond energy between two atoms A and B

Table $$\PageIndex{1}$$: Approximate Bond Lengths and Bond Energies of Some Single Bonds
Bond Length (× $$10^{−12}\, m$$) Bond Energy (kJ/mol)
H–H 74 436
H–C 110 414
H–N 100 386
H–O 97 459
H–I 161 295
C–C 154 350
C–N 147 305
C–O 143 358
N–N 145 167
O–O 145 142

Table $$\PageIndex{2}$$ compares the lengths and bond energies of single covalent bonds with those of double and triple bonds between the same atoms. Without exception, as the number of covalent bonds between two atoms increases, the bond length decreases and the bond energy increases.

By comparing both tables, we can conclude that the more energy needed to break a bond, the higher its stability, and the shorter the bond length between the atoms

Table $$\PageIndex{2}$$: Comparison of Bond Lengths and Bond Energies for Single and Multiple Bonds
Bond Length (× $$10^{−12}\, m$$) Bond Energy (kJ/mol)
C–C 154 350
C=C 134 614
C≡C 120 839
C–N 147 305
C=N 128 615
C≡N 116 891
C–O 143 358
C=O 120 745
C≡O 113 1072
N–N 145 167
N=N 123 918
N≡N 110 941
O–O 145 142
O=O 121 495

## Physical properties of covalent compounds

Covalent compounds form discrete molecules that can exist independently from each other, in contrast to the crystal structure of ionic compounds in which formula units cannot exist individually but only as a part of a lattice of ions. Therefore, the physical properties of covalent molecules depend heavily on the nature of their interaction with other molecules (intermolecular forces). Depending on the nature of these intermolecular interactions, covalent compounds may exist as a solid, a liquid, or a gas at room temperature and normal atmospheric pressure. As a general rule, we can say that covalent compounds have lower melting and boiling points compared to ionic compounds with similar molecular mass.

Since there are no ions in their structure, covalent compounds do not exhibit any electrical conductivity, either in pure form or when dissolved in water. There are however some exceptions that we will discuss in CHE 104. These exceptions involve ionizable covalent compounds such as acids (these molecules dissociate in water forming ions, and therefore, solutions of ionizable covalent compounds can conduct the electricity).

## Key Takeaways

• Covalent bonds between different atoms have different bond lengths and bond energies.
• The stronger the covalent bond, the higher its bond energy and the shorter its bond distance
•  As a general rule, covalent compounds have lower melting and boiling points compared to ionic compounds with similar molecular mass.
• The physical properties of covalent molecules depend heavily on the nature of their interaction with other molecules