Covalent Bonds (College Board AP® Chemistry): Revision Note

Non-polar Covalent bonds

• A covalent bond is formed when two nonmetals share a pair of electrons

• Atoms form covalent bonds to gain a full outer shell of electrons

• Compounds that are formed by elements joined by covalent bonds are molecules

• Covalent bonds come in different presentations:

  • A single covalent bond occurs when one pair of electrons is shared, and each atom contributes with one valence electron

  • A double covalent bond occurs when two pairs of electrons are shared, and each atom contributes with two valence electrons

  • A triple covalent bond occurs when three pairs of electrons are shared, and each atom contributes with three valence electrons

• The nature of the covalent bond depends in the difference of electronegativity between the atoms that share the pairs of electrons

  • Using the Pauling’s scale of electronegativity, if the difference is smaller than 0.4, the covalent bond is nonpolar

• Nonpolar covalent bonds are formed when two elements with similar or equal electronegativity share pairs of electrons

  • E.g. Chlorine gas is a diatomic element. Therefore, it exists in nature as a molecule of Cl₂ instead of an individual atom Cl

  • The two chlorine atoms form a single covalent bond sharing a pair of electrons as shown in the image below

  • Since both atoms are the same, their difference in electronegativity is 0

  • Therefore, the nature of the covalent bond is nonpolar 

The Formation of a Chlorine Molecule, Cl₂

Diagram showing how a covalent bond forms between two chlorine atoms

Polar covalent bonds

• A polar covalent bond is formed when the nonmetals that share pairs of electrons have a greater electronegativity difference

  • Using the Pauling’s scale of electronegativity, if the difference is between than 0.4 - 2.0, the covalent bond is polar

• Electronegativity is ability of an atom to attract the pair electrons of a covalent bond

• One of the atoms will have greater Coulombic attraction to the electrons, pulling them towards itself

• When this occurs, a dipole moment (δ) is formed because there was a separation of positive and negative charges

• A dipole moment is created when particles of equal magnitude but opposite sign charges are separated

  • E.g. This occurs in the molecule of hydrofluoric acid (HF) that is shown in the image below

  • The chlorine atom and the hydrogen atom form a single covalent bond sharing a pair of electrons as shown in the image below

  • Their difference in electronegativity is calculated as following

 $\mathrm{\Delta }\mathrm{electronegativity}=\mathrm{Highest}\mathrm{electronegativity}-\mathrm{Lowest}\mathrm{electronegativity}$

$\mathrm{\Delta }\mathrm{electronegativity}=4.0-2.1$

$\mathrm{\Delta }\mathrm{electronegativity}=1.9$ 

• Since the difference is between 0.4 and 2.0, the nature of the covalent bond is polar

• Fluorine will acquire a negative dipole δ^-

• Hydrogen will acquire a positive dipole δ⁺

• If the difference in electronegativity is greater, the bond dipoles are greater too

  • This occurs because one of the atoms is pulling the electrons with stronger coulombic attraction, therefore the distance between the charges is shorter

The H-F molecule

Polar covalent bond in hydrofluoric acid (HF)

Ionic character

• Ionic bonds are formed when electrons are transferred from one atom to another

• In polar covalent bonds, there is a partial transfer of electrons towards the most electronegative atom

• Therefore, polar covalent bonds have some ionic character

• The ionic character of a covalent bond increases if the difference of electronegativity between the atoms is greater