MICSP - Structure of Ionic Solids (Lesson)
Structure of Ionic Solids
Ionic Solids: Salts
What properties make salt a salt? Table salt is the flavor enhancing, wonderful compound most people are familiar with known technically as sodium chloride. What is it about sodium and chlorine that makes them join together and become something very different from the elements sodium and chlorine?
Salts are the result of ionic bonds. Ionic bond is the word given to the electrostatic attraction between cations and anions. As each cation is attracted to any negative anion, as well each anion is experiencing the same electrostatic attraction for any positive particle. These attractions give rise to the structure of salts as a crystalline lattice of ions.
Properties
This crystalline arrangement accounts for many properties of salts. Salts can conduct electricity, but not while the ions are locked in the crystal lattice. When in molten form or when dissolved in water, the ions become free to move. Electricity is simply the flow of charge, so when the charged ions are free from the crystal, salts are conductors.
Salts are hard and brittle. The orderly arrangement allows for tight packing that results in strong attractions between the charges explaining hardness. The brittle nature of salts is seen when a stress dislodges ions from the lattice. As particles with the same charge come into closer proximity than expected, the repulsive force breaks or cleaves the salt crystal along the stress line. The brittle yet hard nature of salt is illustrated here where an applied force causes a shift in the crystal and the resulting repulsive forces between ions cause fracture.
The packing of ions in the lattice depends on both the size and charge of the ions involved. As metal atoms form cations by losing an entire energy level, the cation has a small radius when compared to the neutral atom. The anion is slightly larger than the atom from which it forms as the number of electrons exceeds the number of protons, expanding the radius from atomic size. This increase in size comes from the increased amount of electron repulsion due to the extra electrons. The tightness of ion packing in the crystal then is a factor of ion size.
Energy
The formation of a salt releases energy to the environment and produces a greater stability for the ions that make it up. Ions, as free charged particles, attract and repel everything else with a charge. When cations and anions arrange into the crystal lattice structure, the potential for charge interactions is reduced for those ions, so we say the crystal lattice arrangement lowers chemical potential energy.
Born-Haber Diagram
A Born-Haber diagram traces the energy flow through the formation of a salt, starting with neutral, but reactive atoms all the way through changes that end with the new chemical product of the salt compound.
Please watch the following video to learn more about how to create an energy profile for NaCl.
You Try It!
Hydrated Salts
For some salts, the size and spacing of the lattice structure allow water molecules to fit in between the ions inside crystal. The source of this water is often crystals formed from evaporation of water from aqueous solutions. Like everything in chemistry, the reason that many ionic compounds tend to associate with a specific number of water molecules is energy. Nature always seeks to minimize energy. Hydrated salts use the number of associated water molecules as part of the hydrate name. Magnesium sulfate heptahydrate indicates seven waters are found for every one MgSO4 formula unit. Copper (II) sulfate has five waters of hydration, hence the name copper (II) sulfate pentahydrate.
Many salts are more stable when they organize into regular crystalline structures, structures where each ionic compound is situated in a regular, predictable way. Many such crystals can't be formed unless a few water molecules form bridges between molecules by hydrogen bonding. Take a look at how the water is incorporated as part of the crystal structure.
The sulfate ion is in the center. The water is shown around the orange cation and the blue dotted lines show the connection of the water and the anion.
Please Note: When using hydrated salts, we have to be careful to incorporate the waters of hydration when calculating the formula weight. For example, the formula weight of copper (II) sulfate is 159.6 g/mol, but the FW of CuSO4· 5H2O is 249.7 g/mol. So when doing calculations with hydrated salts, you should include the water in the molar mass.
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