Covalent, Metallic or Ionic Bonds | Chemical Bonds in Molecular Atoms | Salt Crystals | How Atoms and Molecules Move in Liquid? | Lewis Dot Structure and Molecular Shape | Electronegativity, Ionization, and Bond Formation | Van Der Waals Forces
|CCSTD HS Chemistry 2.c.|
A Salt Crystal is the solid form of an ionic compound.
Most of us think of table salt, or Sodium Chloride (NaCl), when we hear the word ‘salt’, but in chemistry there are many salts. Additionally, we think of glassware or prisms when we hear the word ‘crystal’. However, when we speak of Salt Crystals in chemistry, we are referring to the solid form of an ionic compound.
The repeating geometric pattern of a salt crystal is called the Crystal Lattice, and the smallest unit of the lattice is known as a Unit Cell.
In comparing patterned wallpaper to a crystal lattice; just like the wallpaper has a repeating pattern, the Crystal Lattice has a repeating Unit Cell.
The three assumptions of the ionic Crystal Lattice model are;
There are seven types of Unit Cells, and these cells give rise to fourteen Crystal Lattices.
A listing of the fourteen Crystal Lattice types can be seen at:
The electrostatic forces that hold ions together in salt crystals are very strong. These ionic bonds are quite difficult to break, which translates to great amounts of energy contained in the bond. The strength of the ionic bond leads to relatively high melting points and extremely high boiling points for ionic Salt Crystals.
At the same time, drop this incredibly tough compound into a glass of
water (H2O), and it will easily dissolve. What gives?
In the event that NaCl dissolves in water, the partial positive charges of the water will attract the salt anions (Cl-), and the partial negative charge will attract the cations (Na+), resulting in a breaking apart of the ionic solid.
Salt solubility, or how well the salt dissolves, in water, is an important characteristic of any ionic compound. We will discuss salt solubility in Lesson 7.
Other Crystal Salts
Sodium Chloride is not the only crystal salt. There are other ions that come together to form crystal salts. Some of those salts have multiple charges on their cations, such as in the case of Calcium Fluorite (CaF2) , which is composed of one Ca++ cation and two F- anions. Others have multiple charges on their anions, such as Sodium Phosphate (Na3PO4), where there are three Sodium cations (Na+) for each Phosphate anion (PO4-3).
The salts that arise from ionic bonding have a number of characteristics in common, such as;
Types of Salts
There are two types of salts: acid salts and normal salts. In order to understand the difference between these two types of salts, we will first need to briefly discuss acids and bases.
An acid can be thought of as any compound that is willing to give up a Hydrogen cation (H+). This cation is often called a Proton since the Hydrogen cation has no electrons, but it does have one proton in its nucleus. A base is any compound that is willing to accept a Proton (the H+ kind).
With these definitions in mind, let’s first consider the neutralization of the strong acid, Hydrochloric Acid (HCl or H+ and Cl-) by a strong base, Sodium Hydroxide (NaOH or Na+ and OH-).
You can see that water and the salt crystal, Sodium Chloride, are products of the neutralization reaction. This reaction produces water and a normal salt, or a salt that contains no Hydrogen in its molecular structure.
Another reaction example is Phosphoric Acid (H3PO4) being neutralized with Sodium Hydroxide. Phosphoric Acid has three cations (H+) and one anion (PO4-3). Let’s say we react a given amount of H3PO4 with the same amount of basic Sodium Hydroxide (NaOH). We will get;
If we add more NaOH, we get;
With this sequence of reactions, we end up with a total of three
molecules of water; two acid salts, and one
When a salt has no Protons (H+) or Hydroxyl ions (OH-),
it is called a normal salt. Can you guess which product of reaction (3) is
the normal salt?
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