Ionic Bonds



 

Learning Objective

  • Summarize the characteristic features of ionic bonds

Key Points

    • Ionic bonds are formed through the exchange of valence electrons between atoms, typically a metal and a nonmetal.
    • The loss or gain of valence electrons allows ions to obey the octet rule and become more stable.
    • Ionic compounds are typically neutral. Therefore, ions combine in ways that neutralize their charges.

Terms

  • valence electronsThe electrons of an atom that can participate in the formation of chemical bonds with other atoms. They are the furthest electrons from the nucleus.
  • octet ruleAn atom is most stable when there are eight electrons in its valence shell.

Forming an Ion

Ionic bonds are a class of chemical bonds that result from the exchange of one or more valence electrons from one atom, typically a metal, to another, typically a nonmetal. This electron exchange results in an electrostatic attraction between the two atoms called an ionic bond. An atom that loses one or more valence electrons to become a positively charged ion is known as a cation, while an atom that gains electrons and becomes negatively charged is known as an anion.

This exchange of valence electrons allows ions to achieve electron configurations that mimic those of the noble gases, satisfying the octet rule. The octet rule states that an atom is most stable when there are eight electrons in its valence shell. Atoms with less than eight electrons tend to satisfy the duet rule, having two electrons in their valence shell. By satisfying the duet rule or the octet rule, ions are more stable.

A cation is indicated by a positive superscript charge (+ something) to the right of the atom. An anion is indicated by a negative superscript charge (- something) to the right of the atom. For example, if a sodium atom loses one electron, it will have one more proton than electron, giving it an overall +1 charge. The chemical symbol for the sodium ion is Na+1 or just Na+. Similarly, if a chlorine atom gains an extra electron, it becomes the chloride ion, Cl. Both ions form because the ion is more stable than the atom due to the octet rule.

Forming an Ionic Bond

Once the oppositely charged ions form, they are attracted by their positive and negative charges and form an ionic compound. Ionic bonds are also formed when there is a large electronegativity difference between two atoms. This difference causes an unequal sharing of electrons such that one atom completely loses one or more electrons and the other atom gains one or more electrons, such as in the creation of an ionic bond between a metal atom (sodium) and a nonmetal (fluorine).

Formation of sodium fluorideThe transfer of electrons and subsequent attraction of oppositely charged ions.

Determining the Formula of an Ionic Compound

To determine the chemical formulas of ionic compounds, the following two conditions must be satisfied:

  1. Each ion must obey the octet rule for maximum stability.
  2. Ions will combine in a way that the overall ionic compound will be neutral. In other words, the charges of the ions must balance out.

Magnesium and fluorine combine to form an ionic compound. What is the formula for the compound?

Mg most commonly forms a 2+ ion. This is because Mg has two valence electrons and it would like to get rid of those two ions to obey the octet rule. Fluorine has seven valence electrons and usually forms the F ion because it gains one electron to satisfy the octet rule. When Mg2+ and F combine to form an ionic compound, their charges must cancel out. Therefore, one Mg2+ needs two F ions to neutralize the charge. The 2+ of the Mg is balanced by having two -1 charged ions. Therefore, the formula of the compound is MgF2. The subscript two indicates that there are two fluorines that are ionically bonded to magnesium.

On the macroscopic scale, ionic compounds form crystalline lattice structures that are characterized by high melting and boiling points and good electrical conductivity when melted or solubilized.

Example

Magnesium and fluorine combine to form an ionic compound. What is the formula for the compound?

Mg most commonly forms a 2+ ion. This is because Mg has two valence electrons and it would like to get rid of those two ions to obey the octet rule. Fluorine has seven valence electrons and as such, usually forms the F ion because it gains one electron to satisfy the octet rule. When Mg2+ and F combine to form an ionic compound, their charges must cancel out. Therefore, one Mg2+ needs two F ions to balance. The 2+ of the Mg is balanced by having two -1 charged ions. Therefore, the formula of the compound is MgF2. The subscript two indicates that there are two fluorines that are ionically bonded to magnesium.