1.8: Ionic and Covalent Bonds

There are many types of chemical bonds and forces that hold atoms or ions together in compounds. The two most basic types of bonds are characterized as either ionic or covalent. In ionic bonding, electrons are transferred from one atom to another. Ionic bonds require at least atom that easily loses electrons to form a cation and at least one atom that easily gains electrons to form an anion. The cation and anion are attracted to one another and form a bond. In contrast, atoms that are more similar to one another can share electrons in covalent bonds which will be covered later in the course.

Introduction

Ionic bonding is the complete transfer of valence electron(s) between atoms. In ionic bonds, a metal can lose electrons to become a positively charged cation, whereas a nonmetal accepts those electrons to become a negatively charged anion. Ionic bonds require an electron donor, often a metal, and an electron acceptor, a nonmetal.

Ionic bonding is observed because metals have few electrons in their outer-most shell. You can think of an atom like an onion with many layers. If the outermost layer of the onion is torn and incomplete, that is similar to an incomplete octet. If you remove that outermost, incomplete layer of the onion, there is a complete layer underneath that is now the new outermost layer. By losing their valence electrons, metals can satisfy the octet rule. This sodium atom donates the lone electron in its valence orbital in order to achieve octet configuration. This creates a positively charged cation due to the loss of electron.

On the other hand, nonmetals generally have close to 8 electrons in their valence shells and can more easily accept electrons to achieve an octet. This chlorine atom receives one electron to achieve its octet, resulting in a negatively charged anion.

Because sodium ions have a 1+ charge and chloride ions have a 1- charge, the ratio of sodium ions to chloride ions in sodium chloride is 1:1. Its formula is NaCl. In the example below you will also see MgCl₂ in which the ratio of cations to anions is 1:2 because magnesium ions have a 2+ charge whereas chloride ions only have a 1- charge. 

In ionic bonding, more than one electron can be donated or received to satisfy the octet rule. The charges on the anion and cation correspond to the number of electrons donated or received. In ionic bonds, the net charge of the compound must be zero. This is important for understanding what happens in nature and for predicting the formulas of ionic compounds! The ratio of cations and anions in an ionic compound always results in an overall charge of zero.

Example \(\PageIndex{1}\): Chloride Salts

In this example, the sodium atom is donating its 1 valence electron to the chlorine atom. This creates a sodium cation and a chlorine anion. Notice that the net charge of the resulting compound is 0.

In this example, the magnesium atom is donating both of its valence electrons to chlorine atoms. Each chlorine atom can only accept 1 electron before it can achieve its noble gas configuration; therefore, 2 atoms of chlorine are required to accept the 2 electrons donated by the magnesium. Notice that the net charge of the compound is 0.

References

1. Petrucci, Ralph H. General Chemistry: Principles and Modern Applications. Upper Saddle River, NJ: Pearson Education, 2007.
2. Brown, Theodore L., Eugene H. Lemay, and Bruce E. Bursten. Chemistry: The Central Science. 6th ed. Englewood Cliffs, NJ: Prentice Hall, 1994.

Problems

1. Do these compounds contain ionic or covalent bonds?

CH₄    Fe₂O₃    KNO₃    H₂O    BeCl₂

 

2. In the following reaction, indicate whether the reactants and products are ionic or covalently bonded.

 

Solutions

1. From left to right: Covalent, Ionic, Ionic, Covalent, Ionic. In general, a compound containing both metals and nonmetals has at least one ionic bond and compounds containing only nonmetals have covalent bonds. 
2. All products and reactants are ionic.