IONIC BOND: CHARACTERISTICS, HOW IT IS FORMED AND EXAMPLES - CHEMISTRY - 2025

The ionic bond is a type of chemical bond in which there is an electrostatic attraction between oppositely charged ions. That is, a positively charged ion forms a bond with a negatively charged ion, transferring electrons from one atom to another.

This type of chemical bond occurs when valence electrons from one atom are permanently transferred to another atom. The atom that loses electrons becomes a cation (positively charged), and the one that gains electrons becomes an anion (negatively charged).

Ionic bond example: sodium fluoride. Sodium loses one valence electron and gives it up to fluorine. Wdcf

Ionic bond concept

The ionic bond is one by which electrically charged particles, called ions, interact to give rise to ionic solids and liquids. This bond is the product of electrostatic interactions between hundreds of millions of ions, and is not limited to just a couple of them; that is, it goes beyond the attraction between a positive charge towards a negative charge.

Consider for example the ionic compound sodium chloride, NaCl, better known as table salt. In NaCl, the ionic bond predominates, so it is composed of Na ⁺ and Cl ^- ions. Na ⁺ is the positive ion or cation, while Cl ^- (chloride) is the negative ion or anion.

The Na + and Cl- ions in sodium chloride are held together by ionic bonding. Source: Eyal Bairey via Wikipedia.

Both Na ⁺ and Cl ^- are attracted to being of opposite electrical charges. The distances between these ions allow others to come closer together, so that NaCl pairs and pairs appear. The Na ⁺ cations will repel each other because they are of equal charges, and the same happens with each other with the Cl ^- anions.

There comes a time when millions of Na ⁺ and Cl ^- ions manage to unify, unite, to create a structure that is as stable as possible; one governed by ionic bonding (top image). Na ⁺ cations are smaller than Cl ^- anions because of the increasing effective nuclear force of their nucleus on external electrons.

Ionic bond of NaCl. Rhannosh / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

The ionic bond is characterized by establishing ordered structures where the distance between the ions (Na ⁺ and Cl ^- in the case of NaCl) is small compared to that of other solids. So we speak of an ionic crystalline structure.

How is an ionic bond formed?

The ionic bonding only takes place if a distribution of electrons occurs so that the charges of the ions arise. This type of bond can never occur between neutral particles. There must necessarily be cations and anions. But where do they come from?

Ionic bond illustration. a) Sodium has a net negative charge. b) Sodium gives up an electron to chlorine. Sodium remains with a net positive charge and chlorine with a net negative charge, generating the ionic bond. This type of bond between millions of Na and Cl atoms gives rise to the physical salt. OpenStax College / CC BY (https://creativecommons.org/licenses/by/3.0)

There are many pathways by which ions originate, but essentially many are based on an oxidation-reduction reaction. Most inorganic ionic compounds consist of a metallic element bonded with a non-metallic element (those in the p block of the periodic table).

The metal must oxidize, lose electrons, to become a cation. On the other hand, the nonmetallic element is reduced, gains these electrons, and becomes an anion. The following image illustrates this point for the formation of NaCl from sodium and chlorine atoms:

Formation of an ionic bond. Source: Shafei at Arabic Wikipedia / Public domain

The Na atom donates one of its valence electrons to Cl. When this distribution of electrons occurs, the Na ⁺ and Cl ^- ions are formed, which begin to attract each other immediately and electrostatically.

It is therefore said that Na ⁺ and Cl ^- do not share any pair of electrons, unlike what might be expected for a hypothetical Na-Cl covalent bond.

Ionic bond properties

The ionic bond is non-directional, that is, its force is not present in a single direction, but rather spreads through space as a function of the distances that separate the ions. This fact is important, as it means that the ions are strongly bound, which explains several of the physical properties of ionic solids.

Melting point

The ionic bond is responsible for the salt to melt at a temperature of 801 ºC. This temperature is considerably high compared to the melting points of various metals.

This is because NaCl must absorb enough heat for its ions to begin to flow freely out of its crystals; that is, the attractions between Na ⁺ and Cl ^- must be overcome.

Boiling point

The melting and boiling points of ionic compounds are especially high due to their strong electrostatic interactions: their ionic bonding. However, as this bond involves many ions, this behavior is usually attributed rather to intermolecular forces, and not properly to ionic bonding.

In the case of salt, once the NaCl has melted, a liquid composed of the same initial ions is obtained; only now they move more freely. The ionic bond is still present. The Na ⁺ and Cl ^- ions meet at the surface of the liquid to create a high surface tension, which prevents ions from escaping into the gas phase.

Therefore, the molten salt must increase its temperature even more to boil. The boiling point of NaCl is 1465 ° C. At this temperature, the heat exceeds the attractions between Na ⁺ and Cl ^- in the liquid, so NaCl vapors begin to form with a pressure equal to atmospheric.

Electronegativity

It was previously said that the ionic bond is formed between a metallic element and a non-metallic element. In short: between a metal and a non-metal. This is ordinarily so as regards inorganic ionic compounds; especially, the binary type, such as NaCl.

For a partition of electrons (Na ⁺ Cl ^-) to occur and not a sharing (Na-Cl), there must be a large difference in electronegativity between the two atoms. Otherwise, there would be no ionic bond between the two of them. Possibly the Na and Cl get closer together, interact, but immediately Cl, due to its higher electronegativity, "takes" an electron from Na.

However, this scenario applies only to binary compounds, MX, such as NaCl. For other salts or ionic compounds, their formation processes are more complicated and cannot be approached from a purely atomic or molecular perspective.

Types

There are no different types of ionic bonds, since the electrostatic phenomenon is purely physical, varying only the way in which the ions interact, or the number of atoms they possess; that is, if they are monatomic or polyatomic ions. Likewise, each element or compound originates a characteristic ion that defines the nature of the compound.

In the examples section we will delve into this point, and it will be seen that the ionic bond is the same in essence in all compounds. When this is not fulfilled, it is said that the ionic bond has a certain covalent character, which is the case of many transition metal salts, where the anions coordinate with the cations; for example, FeCl ₃ (Fe ³⁺ -Cl ^-).

Examples of ionic bonds

Several ionic compounds will be listed below, and their ions and proportions will be highlighted:

- Magnesium chloride

MgCl ₂, (Mg ²⁺ Cl ^-), in a 1: 2 ratio (Mg ²⁺: 2 Cl ^-)

- Potassium fluoride

KF, (K ⁺ F ^-), in a 1: 1 ratio (K ⁺: F ^-)

- Sodium sulfide

Na ₂ S, (Na ⁺ S ^2-), in a 2: 1 ratio (2Na ⁺: S ^2-)

- Litho hydroxide

LiOH, (Li ⁺ OH ^-), in a 1: 1 ratio (Li ⁺: OH ^-)

- Calcium fluoride

CaF ₂, (Ca ²⁺ F ^-), in a 1: 2 ratio (Ca ²⁺: 2F ^-)

- Sodium carbonate

Na ₂ CO ₃, (Na ⁺ CO ₃ ^2-), in a 2: 1 ratio (2Na ⁺: CO ₃ ^2-)

- Calcium carbonate

CaCO ₃, (Ca ²⁺ CO ₃ ^2-), in a 1: 1 ratio (Ca ²⁺: CO ₃ ^2-)

- Potassium permanganate

KMnO ₄, (K ⁺ MnO ₄ ^-), in a 1: 1 ratio (K ⁺: MnO ₄ ^-)

- Copper sulphate

CuSO ₄, (Cu ²⁺ SO ₄ ^2-), in a 1: 1 ratio (Cu ²⁺: SO ₄ ^2-)

- Barium hydroxide

Ba (OH) ₂, (Ba ²⁺ OH ^-), in a 1: 2 ratio (Ba ²⁺: OH ^-)

- Aluminum bromide

AlBr ₃, (Al ³⁺ Br ^-), in a 1: 3 ratio (Al ³⁺: 3Br ^-)

- Iron (III) oxide

Fe ₂ O ₃, (Fe ³⁺ O ^2-), in a 2: 3 ratio (2Fe ³⁺: 3O ^2-)

- Strontium oxide

SrO, (Sr ²⁺ O ^2-), in a 1: 1 ratio (Sr ²⁺: O ^2-)

- Silver chloride

AgCl, (Ag ⁺ Cl ^-), in a 1: 1 ratio (Ag ⁺: Cl ^-)

- Others

-CH ₃ COONa, (CH ₃ COO ^- Na ⁺), in a 1: 1 ratio (CH ₃ COO ^-: Na ⁺)

- NH ₄ I, (NH ₄ ⁺ I ^-), in a 1: 1 ratio (NH ₄ ⁺: I ^-)

Each of these compounds has an ionic bond where millions of ions, corresponding to their chemical formulas, are electrostatically attracted and form a solid. The greater the magnitude of its ionic charges, the stronger the electrostatic attractions and repulsions.

Therefore, an ionic bond tends to be stronger the greater the charges on the ions that make up the compound.

Solved exercises

Here are some exercises that put into practice the basic knowledge of ionic bonding.

- Exercise 1

Which of the following compounds is ionic? The options are: HF, H ₂ O, NaH, H ₂ S, NH _3, and MgO.

An ionic compound must by definition have an ionic bond. The greater the electronegativity difference between its constituent elements, the greater the ionic character of said bond.

Therefore, the options that do not have a metallic element are ruled out in principle: HF, H ₂ O, H ₂ S and NH ₃. All these compounds are made up of only non-metallic elements. The NH ₄ ⁺ cation is an exception to this rule, as it does not have any metals.

The remaining options are NaH and MgO, which have the metals Na and Mg, respectively, attached to non-metallic elements. NaH (Na ⁺ H ^-) and MgO (Mg ²⁺ O ^2-) are ionic compounds.

- Exercise 2

Consider the following hypothetical compound: Ag (NH ₄) ₂ CO ₃ I. What are its ions and in what proportion are they found in the solid?

Decomposing the compound into its ions we have: Ag ⁺, NH ₄ ⁺, CO ₃ ^2- and I ^-. These are joined electrostatically following the ratio 1: 2: 1: 1 (Ag ⁺: 2NH ₄ ⁺: CO ₃ ^2-: I ^-). This means that the amount of NH ₄ ⁺ cations is double that of Ag ⁺, CO ₃ ^2- and I ^- ions.

- Exercise 3

KBr is composed of K ⁺ and Br ^- ions, with a magnitude of charge. Then, CaS possesses the ions Ca ²⁺ and S ^2-, with charges of double magnitude, so it could be thought that the ionic bond in CaS is stronger than in KBr; and also stronger than Na ₂ SO ₄, since the latter is composed of Na ⁺ and SO ₄ ^2- ions.

Both CaS and CuO may have an equally strong ionic bond, since they both contain ions with double magnitude charges. Next, we have AlPO ₄, with Al ³⁺ and PO ₄ ^3- ions. These ions have triple magnitude charges, so the ionic bond in AlPO ₄ should be stronger than in all the previous options.

And finally, we have the winner Pb ₃ P ₄, because if we assume that it is made up of ions, these become Pb ⁴⁺ and P ^3-. Their charges have the highest magnitudes; and therefore, Pb ₃ P ₄ is the compound that probably has the strongest ionic bond.

References

1. Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
2. Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
3. Wikipedia. (2020). Ionic bonding. Recovered from: en.wikipedia.org
4. Helmenstine, Anne Marie, Ph.D. (February 11, 2020). Ionic vs Covalent Bonds - Understand the Difference. Recovered from: thoughtco.com
5. The Editors of Encyclopaedia Britannica. (January 31, 2020). Ionic bond. Encyclopædia Britannica. Recovered from: britannica.com
6. Chemicool Dictionary. (2017). Definition of Ionic Bonding. Recovered from: chemicool.com