In this lesson, you'll learn about ionic compounds and how they form. Additionally, you'll learn the properties of ionic compounds, such as their high melting and boiling points, their ability to conduct electricity, and the fact that they form crystals.
Most of the rocks and minerals that surround us are made of ions held together through ionic bonding, the electrical attraction between oppositely charged ions. Look closely at a crystal of salt. What does it look like? Look even closer. You know that table salt is composed of Na+ and Cl-, which combine with ionic bonds to make NaCl. But, do you see NaCl? No. You see Na and Cl all in a neat lattice structure.
Remember the Lewis dot structure for both Na and Cl. They are both originally neutral with their one and seven valence electrons respectively. The sodium, which is a metal, will easily lose that electron to become a positively charged cation. Chlorine, a non-metal, will happily gain one electron to become negatively charged. The sodium transfers its electron to chlorine, which makes both of them happy.
Sodium donates its electron to chlorine, forming an ionic compound
This is an example of an ionic compound. An ionic compound is a compound held together by ionic bonds. Examples of ionic compounds include pyrite, FeS2. Remember that an ionic bond is formed through the transfer of electrons. These compounds are usually formed between metals and non-metals. The ratio of cations to anions is always in a way that there is no net charge. What happens is the positively charged metal cation forms a bond with a negatively charged non-metal anion. This bond is electrically neutral and strong, but its strength varies depending on something called the lattice energy.
The strength of ionic compounds is measured with something called lattice energy. It is the energy released when one mole of an ionic compound is formed. This means when the individual ions of the compound come together to form the crystal lattice, they need less energy to stay together, so they release it, and the energy released is called the lattice energy. The bond force between ions of opposite charge is strongest when the ions are small. This is because the valence electrons are closest to their nucleus, and the nucleus has a strong force or pull over them. It exerts this same force on neighboring atoms. The bond is also stronger as the charge on the ions get larger. So, the force of the bond between a +1 cation and a -1 anion isn't as strong as the force between a +3 cation and a -2 anion.
Some real examples of these differences:
|| Charge on each ion
|| Lattice energy (kJ/mol)
|| sodium chloride
|| 1, -1
|| sodium bromide
|| 1, -1
|| calcium fluoride
|| +2, -1
|| magnesium oxide
|| +2, -2
The lattice energy is negative to show that energy is released when the compound comes together. So, the more negative the number, the more energy was released and the stronger the bond. In comparing the sodium bromide with the sodium chloride, both have the same charge on their ions, but the bromine is bigger than the chlorine, so it has a lower energy. The magnesium oxide is both smaller and has a larger charge than the calcium fluoride.
The more energy released when the compound comes together, the stronger the bond
Properties of Ionic Compounds
1. Ionic compounds have unique properties. They all form crystals. A crystal is made up of an orderly and symmetrical pattern of atoms called a crystal lattice. The crystal lattice shape is an arrangement that takes the least amount of energy to maintain. Within the crystal, there are forces between oppositely charged ions and between the nuclei and electrons of adjacent ions.
2. They have high melting and boiling points. This is due to the strength of the ionic bond. The bonds are so strong that it takes a lot of energy to move them from one state to another, whether it is from solid to liquid or liquid to gaseous.
3. They are hard and brittle. The ionic bonds in these compounds are strong and don't allow the molecules to move around much, which makes them easily breakable.
4. They conduct electricity well when dissolved in water. This is because the ionic compound is made up of ions, which are all either positively or negatively charged. When the ions are separated in water, they allow the electrons to flow.
Ionic compounds conduct electricity well when dissolved in water
5. As a solid, ionic compounds are good insulators.
Most rocks and minerals on Earth are ionic compounds held together by ionic bonds. These strong bonds are formed between a positively charged metal cation and a negatively charged non-metal anion.
The properties of ionic compounds include forming crystal lattice structures, having high melting and boiling points, being hard and brittle and conducting electricity well when dissolved in water, but being insulators when in the solid form.
The lattice energy is the energy of the bonds when in its crystal shape. This energy increases when the atoms making up the compound are small and when the charges on each atom are large.
After watching this video, you'll be able to:
- Describe what an ionic compound is
- Define lattice energy
- List the properties of ionic compunds