Lesson Explainer: Ionic Bonding Chemistry • Second Year of Secondary School

In this explainer, we will learn how to describe ionic bonding in terms of electrostatic attraction and use it to explain the properties of ionic structures.

Most people are familiar with at least a few different types of ionic compounds because ionic compounds are a mainstay of modern living. People regularly use common table salt to make food more palatable, and they use chalk to write on blackboards. Manufacturers use magnesia as a principal fireproofing ingredient for construction materials, and hospital staff use potassium chloride to treat hypokalemia. Ionic compounds are clearly very common and useful, and this is a good reason to understand their chemical structures and properties.

Ionic compounds are formed of positively and negatively charged ions. The ions group together, and they make a giant three-dimensional lattice structure. Ionic lattice structures have many interesting properties, and we are going to list and explain these properties in the following paragraphs.

Common table salt is mostly made up of the sodium chloride compound. The sodium chloride lattice contains both positively charged sodium ions () and negatively charged chloride ions (). The sodium ions have the charge state and the chloride ions have the charge state. There is strong electrostatic attraction between these oppositely charged ions, causing the sodium chloride lattice to be relatively stable.

The following figure shows how the sodium and chloride ions are arranged in a three-dimensional space. The image on the left-hand side shows a ball-and-stick model, and the image on the right-hand side shows the corresponding space-filling model. Ball-and-stick models are intentionally designed with too much space between ions. They also tend to be made so that the diameter of at least one ion type is unrealistically large or small. The ball-and-stick models are designed to help people understand how ions are bonded in an ionic lattice rather than to accurately show bond lengths and ion diameters. Space-filling models provide a more realistic perspective of ion diameters and bond lengths, but they tend to look cluttered. Each representation model has its limitations, and this explains why chemists use more than one model to teach and understand the structure of an ionic lattice. The two representation methods are clearly very different, but they do both indicate that sodium chloride has the or empirical formula. The two representation methods show that sodium chloride has one sodium ion () for every chloride ion ().

Example 1: Understanding the Structure of Sodium Chloride Compounds

A representation of the structure of sodium chloride is shown below.

1. What is the formula unit for sodium chloride?
2. Which of the following is a disadvantage of using this type of representation for ionic structures?
   1. It shows the bonded ions in different colors.
   2. The size of the ions and distances between them are not correct.
   3. It shows the 3D structure of the ions in the lattice.
   4. It shows the arrangement of the ions in the lattice.
   5. The ratio of positively charged to negatively charged ions can be determined.

Answer

Part 1

The formula unit is the lowest whole number ratio of ions represented in an ionic compound. It is used to indicate the relative number of positively charged and negatively charged ions in a giant ionic lattice structure. The figure shows one part of the sodium chloride lattice. The figure can be used to determine that the sodium chloride lattice contains one sodium ion for every chloride ion. The figure suggests that option E must be the correct answer for this question because option E states that the formula unit for sodium chloride is . Option E indirectly states that the sodium chloride lattice has one sodium ion for every chloride ion.

Part 2

Ball-and-stick models are intentionally designed with too much space between ions. They also tend to be made so that the diameter of at least one ion type is unrealistically large or small. The ball-and-stick models are designed to help people understand how ions are bonded in an ionic lattice rather than to accurately show the sizes of bonds or ions. It can sometimes be undesirable to use ball-and-stick models for teaching purposes because they do not accurately show the sizes of chemical bonds or charged ions. These statements are supported by option B. We can conclude therefore that option B is the correct answer for this question.

Three-dimensional models are usually difficult to draw or construct and it is sometimes easier to use a simpler two-dimensional diagram to describe the structure of an ionic lattice. The two-dimensional diagrams show how positively and negatively charged ions are organized in space. They show that ions of one charge are always surrounded by ions of the opposite charge. They also show that there is very little space in between the positively and negatively charged ions. The following figure shows how a relatively simple two-dimensional diagram can be used to describe the structure of a giant ionic lattice.

Example 2: Understanding How to Represent the Crystal Structure of an Ionic Compound

Which of the following diagrams best represents the crystal structure of an ionic compound?

Answer

Ionic compounds contain positively and negatively charged ions. Each positively charged ion is surrounded by negatively charged ions, and each negatively charged ion is surrounded by positively charged ions. The oppositely charged ions are grouped together through attractive electrostatic interactions, and they end up forming a giant three-dimensional lattice. There is very little space in between the oppositely charged ions, and taken together, this means the correct answer is D.

Sodium chloride can be prepared by reacting chlorine gas with a heated sample of liquid sodium. The reaction is extremely exothermic, and a lot of heat and light is generated as the sodium and chlorine atoms react and make the sodium chloride compound. The chemical reaction happens as sodium atoms transfer single valence electrons to chlorine atoms. The following equation shows how sodium metal can be reacted with chlorine gas to make the sodium chloride compound:

Chemists usually use the octet rule to explain why sodium and chlorine atoms react with each other. The octet rule states that atoms transfer or share electrons to attain the same electron configuration as the nearest noble gas.

Chlorine atoms have seven outer-shell (valence) electrons, and they need to gain a single electron to have the same electron configuration as an argon atom. Sodium atoms have a single outer-shell electron. They need to lose this electron to have eight valence electrons and the same electron configuration as a neon atom. Sodium and chlorine atoms can both attain the same electron configuration as a noble gas atom if single outer-shell electrons move from sodium atoms to chlorine atoms. The following figure shows how sodium and chlorine atoms attain the same electron configuration as a noble gas atom when single valence electrons move from sodium atoms to chlorine atoms.

The sodium and chlorine atoms generate oppositely charged ions that have attractive electrostatic interactions. The oppositely charged ions interact with each other, and they end up forming a giant ionic lattice structure. Each sodium cation ends up being surrounded by six chloride ions, and each chloride anion ends up being surrounded by six sodium ions.

Lewis structure diagrams are simple schematic illustrations that show how outer-shell electrons are shared or transferred during chemical reactions. Lewis structure diagrams can be used to show how the sodium chloride compound is formed when electrons move from sodium atoms to chlorine atoms. The following Lewis structure diagram shows how sodium and chloride ions are made when single outer-shell electrons move from sodium atoms to chlorine atoms.

Atoms will sometimes lose or gain more than a single outer-shell electron when they form ionic compounds. Calcium atoms lose two electrons when they form calcium carbonate (), and magnesium atoms lose two electrons when they form magnesia (). Oxygen atoms gain two electrons when they form sodium oxide (), and they also gain two electrons when they form most other oxides like beryllium oxide () or barium oxide ().

The tendency of an atom to gain or lose electrons can be determined from its group number and its position in the periodic table. Atoms tend to gain electrons if they have a high group number and they are on the right-hand side of the periodic table. Atoms tend to lose electrons if they have a low group number and they are on the left-hand side of the periodic table. The following table shows how the atoms of a periodic table column (group) tend to lose or gain a set number of electrons during chemical reactions. Group 1 atoms tend to form state ions as they lose a single valence electron, and group 16 atoms tend to form state ions as they gain two valence electrons.

The chemical composition of any one ionic compound can simply and effectively be described with its formula unit. A formula unit is the lowest whole number ratio of ions represented in a compound. The formula unit of an ionic compound describes the relative abundance of positively and negatively charged ions. It indicates how many positively charged ions there are for each negatively charged ion. The formula unit for any ionic compound is always set so that the charge of the cations exactly balances the charge of the anions. The formula unit will always contain the same number of cations and anions if they have equal but opposite charges.

Sodium chloride has the formula unit because sodium ions have the charge state and chloride ions have the equal but opposite charge state. Magnesium oxide has the formula unit because magnesium ions have the charge state and oxygen ions have the equal but opposite charge state. The sodium chloride and magnesium oxide compounds contain the same number of cations and anions because their cations and anions have equal but opposite charges.

The formula unit of an ionic compound always contains an unequal number of cations and anions if the ions do not have equal but opposite charges. There has to be an imbalance of ions so that the whole ionic system has no overall electric charge. Sodium oxide has the formula unit because its oxygen ions have the charge state and its sodium ions have the charge state. The sodium oxide compound can only have no overall charge if it contains two sodium ions for every oxygen ion.

Ionic compounds always have a ratio of oppositely charged ions that gives them zero overall electric charge. The following table shows how the formula unit of an ionic compound is always balanced such that positive and negative charges cancel each other out.

Example 4: Understanding How to Draw Lewis Structure Diagrams for Compounds of Sodium Oxide

Which electron dot diagram shows the correct structure for sodium oxide?

Answer

Atoms tend to obey the octet rule when they react and form ionic compounds. Oxygen is a group 16 element, and its atoms gain two electrons when they form ionic compounds. Sodium is a group 1 element, and its atoms lose one electron when they form ionic compounds. Options A and E correctly show that the sodium and oxygen atoms obey the octet rule when they react and form sodium () and oxygen () ions. Sodium oxide must have the formula unit because oxygen ions have the charge state and sodium ions have the charge state. The ionic lattice can only have zero electric charge if there are two sodium ions for every oxygen ion. This implies that option E is wrong, and option A must be correct.

The following table shows some of the common physical and chemical properties of giant ionic lattice structures.

Ionic compounds usually have high melting points because it takes a lot of energy to break the strong electrostatic bonds between positively and negatively charged ions. The electrostatic bonds are stronger when the ions have the or charge state instead of the charge state. Magnesia () is used as a fireproofing ingredient for construction materials because it contains divalent and ions. There are very strong electrostatic interactions between the divalent ions, and it takes a lot of heat energy to break them apart.

Example 5: Understanding How Ion Charge Affects the Melting Points of Ionic Compounds

The bar chart below shows the melting points of three ionic compounds. Which of the following assignments is correct?

1. X:
   Y:
   Z:
2. X:
   Y:
   Z:
3. X:
   Y:
   Z:
4. X:
   Y:
   Z:
5. X:
   Y:
   Z:

Answer

Heat is needed to break electrostatic bonds between negatively and positively charged ions in ionic compounds. The bonds are stronger when the ions have the or charge state instead of the charge state. The sodium fluoride () lattice contains and charge state ions. The magnesium fluoride () lattice contains and charge state ions. The magnesium oxide () contains and charge state ions.

It takes a lot of energy to break the ionic bonds in the lattice and less energy to break the ionic bonds in the lattice. It takes even less energy to break the ionic bonds in the lattice. We can use these statements to determine that magnesium oxide must have the highest melting point and sodium fluoride must have the lowest melting point. Magnesium fluoride must have a melting point that is in between these two extremes. This line of reasoning can be used to determine that option C is the correct answer for this question.

Ionic compounds are almost always hard, and they can withstand large forces that are applied over a large area due to the forces of attraction between their oppositely charged ions. Sodium chloride and magnesium oxide have hardness values that are similar to some minerals like gypsum and calcite.

Ionic compounds are hard, but they are also brittle. Ionic lattices are made up of neatly aligned ions and, this highly symmetric structure can be disturbed if force is applied over a small surface area. The stress can push some ions out of place and make ions of the same charge strongly repel each other. The electrostatic repulsion forces destabilize the ionic lattice and the lattice usually ends up cracking and shattering. The following figure shows how an ionic lattice can be destabilized when it is struck with force (stress) that is applied over a small surface area.

Ionic compounds are almost always highly soluble in water because water molecules are polar, and they have strong electrostatic interactions with the positively and negatively charged ions that make up an ionic lattice. The water molecules break down giant ionic lattice structures as they interact with the charged ions and progressively pull them apart from each other. The separated ions end up being dissolved in the water. They can freely move throughout the water, and they can even conduct an electric current.

Ionic compounds do not conduct electricity when they are solid, but they can conduct electricity when they are melted or dissolved in water. Materials can only conduct electricity if they contain some type of mobile charge-carrying particle. Positively and negatively charged particles are not mobile when they are trapped in a giant three-dimensional lattice, but they can move freely when the lattice is melted or dissolved in water.