Covalent Bonds



 

Learning Objective

  • Differentiate between covalent and ionic bonds

Key Points

    • Covalent bonds involve two atoms, typically nonmetals, that share electron density to form strong bonding interactions.
    • Covalent bonds include single, double, and triple bonds and are composed of sigma and pi bonding interactions where 2, 4, or 6 electrons are shared respectively.
    • Covalent compounds typically have lower melting and boiling points than ionic compounds.

Terms

  • electronegativityThe tendency of an atom or molecule to attract electrons and thus form bonds.
  • single bondA type of covalent bond where only two electrons are shared between atoms.

Example

    • A fluorine atom has seven valence electrons. If it shares one electron with a carbon atom (which has four valence electrons), the fluorine will have a full octet (its seven electrons plus the one it is sharing with carbon). Carbon will then have five valence electrons (its four and the one its sharing with fluorine). Covalently sharing two electrons is also known as a “single bond.” Carbon will have to form four single bonds with four different fluorine atoms to fill its octet. The result is CF4 or carbon tetrafluoride.

Forming Covalent Bonds

Covalent bonds are a class of chemical bonds where valence electrons are shared between two atoms, typically two nonmetals. The formation of a covalent bond allows the nonmetals to obey the octet rule and thus become more stable. For example:

  • A fluorine atom has seven valence electrons. If it shares one electron with a carbon atom (which has four valence electrons), the fluorine will have a full octet (its seven electrons plus the one it is sharing with carbon).
  • Carbon will then have five valence electrons (its four and the one its sharing with fluorine). Covalently sharing two electrons is also known as a “single bond.” Carbon will have to form four single bonds with four different fluorine atoms to fill its octet. The result is CF4 or carbon tetrafluoride.

Covalent bonding requires a specific orientation between atoms in order to achieve the overlap between bonding orbitals. Covalent bonding interactions include sigma-bonding (σ) and pi-bonding (π). Sigma bonds are the strongest type of covalent interaction and are formed via the overlap of atomic orbitals along the orbital axis. The overlapped orbitals allow the shared electrons to move freely between atoms. Pi bonds are a weaker type of covalent interactions and result from the overlap of two lobes of the interacting atomic orbitals above and below the orbital axis.

Covalent bonds can be single, double, and triple bonds.

  • Single bonds occur when two electrons are shared and are composed of one sigma bond between the two atoms.
  • Double bonds occur when four electrons are shared between the two atoms and consist of one sigma bond and one pi bond.
  • Triple bonds occur when six electrons are shared between the two atoms and consist of one sigma bond and two pi bonds (see later concept for more info about pi and sigma bonds).

Ionic Compounds v. Molecular Compounds

Unlike an ionic bond, a covalent bond is stronger between two atoms with similar electronegativity. For atoms with equal electronegativity, the bond between them will be a non-polar covalent interaction. In non-polar covalent bonds, the electrons are equally shared between the two atoms. For atoms with differing electronegativity, the bond will be a polar covalent interaction, where the electrons will not be shared equally.

Ionic solids are generally characterized by high melting and boiling points along with brittle, crystalline structures. Covalent compounds, on the other hand, have lower melting and boiling points. Unlike ionic compounds, they are often not soluble in water and do not conduct electricity when solubilized.