ionic+solids

Ionic compounds solidify such that their cations and anions occupy //lattice points// of a crystal. The **crystal lattice** may take on many geometric patterns, such as cubic, hexagonal, or monoclinic.



Sodium chloride is an example of a simple cubic lattice, one that you can actually see if you look closely at table salt:



The **lattice strength** of a crystalline solid will affect its physical properties (e.g. melting point, solubility, index of refraction).

Lattice strength is dependent on variables such as:
 * variations of particle size
 * charge differences of ions
 * strength of intermolecular forces



For example, compare the two salts LiF and MgO.
 * First, the two cations (Li + and Mg 2+ ) are similar in size (radii of 90 and 86 pm, respectively), as are the two anions F - and O 2- (119 and 126 pm, respectively). Therefore, particle size variation is controlled.
 * The LiF salt has a charge ratio of 1:1 (+1 to -1). The MgO salt has a charge ratio of 2:2 (+2 to -2). Therefore, MgO has a larger charge difference.
 * This larger charge difference in MgO gives it a stronger lattice strength than LiF, as is evidenced by the difference in melting point. MgO melts at 2850 o C, while LiF melts at a cooler 845 o C.

Now compare the melting points of lithium fluoride (LiF) and lithium iodide (LiI).
 * Both compounds have the same charge ratio of 1:1 (+1 to -1).
 * The flouride ion is much smaller (119 pm) than the iodide ion (206 pm). This creates a large variation in particle size at the lattice points of the crystal for lithium iodide.
 * Therefore, lithium iodide has a weaker lattice strength and thus melts at a much lower temperature (469 o C) than lithium fluoride (845 o C).

A very nice summary of lattice energies for salts is given here.


 * Minerals** are crystalline structures of ionic compounds. For example, calcite (calcium carbonate) has the ability to create double images due to its index of refraction (birefringence).



Check out webmineral.com to explore some crystalline structures of ionic compounds.



The Cave of Crystals, discovered in Mexico in 2000, contains the largest crystals ever found. Here is a brief video on this amazing cavern.

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