Covalent Bonds (College Board AP Chemistry)

• 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

• 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

• 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 negative 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 attrraction, therefore the distance between the charges is closer

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 increase if the difference of electronegativity between the atoms is greater