45 The Covalent Bond
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Comparison between Covalent and Ionic Compounds
Covalent and ionic compounds have distinct physical properties.
LEARNING OBJECTIVES
Identify element pairs which are likely to form ionic or covalent bonds
KEY TAKEAWAYS
Key Points
- Ionic compounds are formed from strong electrostatic interactions between ions, which result in higher melting points and electrical conductivity compared to covalent compounds.
- Covalent compounds have bonds where electrons are shared between atoms. Due to the sharing of electrons, they exhibit characteristic physical properties that include lower melting points and electrical conductivity compared to ionic compounds.
Key Terms
- valence electrons: Electrons in the outermost principal energy (valence) level of an atom that can participate in the formation of chemical bonds with other atoms.
- octet rule: Atoms lose, gain, or share electrons in order to have a full valence level of eight electrons. Hydrogen and helium are exceptions because they can hold a maximum of two valence electrons.
- electronegativity: The tendency of an atom or molecule to attract electrons and form bonds.
Two Classes of Compounds
Compounds are defined as substances containing two or more different chemical elements. They have distinct chemical structures characterized by a fixed ratio of atoms held together by chemical bonds. Here, we discuss two classes of compounds based on the bond type that holds the atoms together: ionic and covalent.
Covalent Compounds
Covalent bonds are characterized by the sharing of electrons between two or more atoms. These bonds mostly occur between nonmetals or between two of the same (or similar) elements.Two atoms with similar electronegativity will not exchange an electron from their outermost shell; the atoms instead share electrons so that their valence electron shell is filled.
Examples of compounds that contain only covalent bonds are methane ([latex]\text{CH}_4[/latex]), carbon monoxide ([latex]\text{CO}[/latex]), and iodine monobromide ([latex]\text{IBr}[/latex]).
Ionic Compounds
Ionic bonding occurs when there is a large difference in electronegativity between two atoms. This large difference leads to the loss of an electron from the less electronegative atom and the gain of that electron by the more electronegative atom, resulting in two ions. These oppositely charged ions feel an attraction to each other, and this electrostatic attraction constitutes an ionic bond.
Ionic bonding occurs between a nonmetal, which acts as an electron acceptor, and a metal, which acts as an electron donor. Metals have few valence electrons, whereas nonmetals have closer to eight valence electrons; to easily satisfy the octet rule, the nonmetal will accept an electron donated by the metal. More than one electron can be donated and received in an ionic bond.
Some examples of compounds with ionic bonding include [latex]\text{NaCl}[/latex], [latex]\text{KI}[/latex], [latex]\text{MgCl}_2[/late