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Ionic Compounds: Formation, Lattice Energy and Properties

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  • 0:05 Ionic Compound
  • 1:44 Lattice Energy
  • 3:42 Properties
  • 4:47 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

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.

Ionic Compounds

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
Sodium Transfers Electron to Chlorine

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.

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:

Compound Name Charge on each ion Lattice energy (kJ/mol)
NaCl sodium chloride 1, -1 -787.5
NaBr sodium bromide 1, -1 -751.4
CaF2 calcium fluoride +2, -1 -2634.7
MgO magnesium oxide +2, -2 -3760

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.

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