What is Salt in Chemistry? - Definition & Formula

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  • 0:00 What Do Chemists Mean By Salt?
  • 1:10 How is a Salt Bonded Together?
  • 3:25 What Are the…
  • 4:50 Finding a Salt's…
  • 8:25 Lesson Summary
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Lesson Transcript
Instructor: Nicola McDougal

Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.

What exactly do we mean by 'salt' in chemistry? Learn more about the definition, chemical, and physical properties and how we can easily figure out the chemical formula of many salts. Test what you have learned with a quiz.

What Do Chemists Mean by 'Salt'?

We've all heard of salt, right? Those white crystals we put on our fries are the kind of salt we are most familiar with, but what do we really mean in chemistry when we refer to something as a salt?

In chemistry, a salt is an ionic compound which is made up of two groups of oppositely charged ions. The ion with a positive charge is called a cation, and the one with a negative charge is called an anion. How many of each type of ion the salt has is important because the compound must have an overall electrical charge of zero - that is, an equal balance between positive charge and negative charge.

We'll cover more of this later.

Salts can be easily identified since they usually consist of positive ions from a metal with negative ions from a non metal.

The salt we put on our fries is actually sodium chloride and is made up of a Na1+ (that's our metal) and a Cl1- (our non-metal). Often you will see this written as Na+ and Cl- (the 1 is dropped), or simply NaCl.

Sodium chloride salt
sodium chloride compound

How is a Salt Bonded Together?

Before we can understand the glue that bonds ions together, we need to learn why certain atoms become ions at all. Ions are formed one of two ways.

First, an atom can lose an electron to become a cation. Remember that electrons are negatively charged, so if an atom loses negative charge, it becomes positive.

An atom can also gain an electron to become an anion. This happens because of the atom's electron distribution and the magic number eight. Chemists often refer to the octet rule, which, put simply, just means that an atom will achieve stability when it can get eight valence, or outer, electrons. The atom will be similar to its closest noble gas in the periodic table. One way this can be achieved is by losing or gaining electrons to form an ion.

We have already learned that metals form cations by losing electrons. For example, sodium is a group 1 metal, and like all group 1 metals, it has one valence electron. This electron is not held very tightly by the atom and is easily lost, which forms the Na1+ cation.

Chlorine is a nonmetal and is found in group 17 (or 7A) on the periodic table. It has seven valence electrons, and it needs just one more electron to achieve the magic eight for stability! Getting that extra electron will form the Cl1- anion. Notice that both ions (the Na1+ and the Cl1-) have the same number charge, just opposite signs.

So, what happens when a metal sodium atom meets a gaseous chlorine atom? A vigorous reaction occurs as both atoms form ions. The sodium transfers its extra electron to the chlorine - that gives the chlorine its magic eight valence electrons and gives the sodium a positive charge. These ions are immediately attracted to each other, and the salt sodium chloride is formed. The ions are glued together by ionic bonds, which are the electrostatic attraction between opposite charged ions, which are the electrostatic attraction between opposite charged ions.

During the formation of an ionic bond an electron (here shown as a red circle) is transferred from the sodium to the chlorine atom forming the Na+ cation and Cl- anion. The attraction between the two ions instantaneously bonds them together forming NaCl.
diagram of ionic bonding in sodium chloride

What Are the Properties of a Salt?

The physical and chemical properties of materials are closely linked to how they are bonded together. We now know that ions in a salt are strongly attracted to each other, forming strong ionic bonds. It takes a lot of energy to break apart an ionic bond, and the stronger the attraction, the stronger the bond. This attraction between ions means that a compound with ionic bonds will have a strong, ordered structure.

Salts often form a crystal structure or crystal lattice, a highly ordered formation of molecules. This is why we get crystals of salt on our fries.

Crystals of salt
photograph of salt crystals

This ordered structure and strong ionic bonding leads many salts to have some special properties. First, they tend to be crystalline solids with crystal structures. The solids also tend to be hard and brittle due to strong ionic bonding throughout the crystal. Salts also have high boiling and melting points because it takes a lot of energy to break those bonds and change the salt's matter state. Finally, salts are electrolytes, meaning they dissolve in water to create free moving ions, which are able to conduct electricity.

Keep in mind, though, while molten salts also conduct electricity, solid salts do not; ions must be free to conduct electricity.

Finding a Salt's Chemical Formula

So far, we have only talked about sodium chloride, abbreviated NaCl, but all combinations of metals and nonmetals form salts. Examples include magnesium iodide, abbreviated MgI2, and aluminum oxide, Al2O3. Remember, we always write the cation first, followed by the anion. The number following the atom tells us how many of that atom type are contained in that compound; where there is no number, there is just one atom.

For aluminum oxide, there are two aluminum atoms and three oxygen atoms, but why that number? Why not just one of each, or one aluminum and two oxygens? Because the overall salt is always electrically neutral; in other words, the positive charge must equal the negative charge, so they cancel each other out. With different charges for each atom, some salts will demand different numbers of atoms for each element.

The periodic table is the chemist's best tool for figuring out charges of ions and predicting the chemical formula of the salts they form.

Ion charges shown in the periodic table

The periodic table organizes elements into vertical groups. Main group metals - those in groups 1 and 2, plus aluminum, form just one positive ion. Nonmetals will only ever form one negative ion. Here's a summary of the different charges groups of the periodic table will have.

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