Aluminum Chloride: Formula, Molar Mass & Decomposition

Instructor: Matthew Bergstresser
Aluminum chloride has a common use in daily hygiene. In this lesson, we will learn aluminum chloride's formula, molar mass, and see how we can create a decomposition reaction.


Many people use store-bought antiperspirants daily. Some people are more prone to perspiring and need prescription antiperspirants. Most of these medications are solutions of aluminum chloride. When rubbed on the skin, the aluminum clogs the pores preventing perspiration.

Let's learn more about aluminum chloride including its chemical formula, molar mass, and how it decomposes.

Chemical Formula

Aluminum chloride is an ionic salt, which means it is the combination of a metal ion (aluminum) and a non-metal ion (chloride). Atoms are turned into ions when the positive charge of the nucleus doesn't equal the number of electrons in that atom's electron cloud.

The chemical formula of aluminum chloride is AlCl3. Let's see why three chloride ions are needed for one aluminum ion.

A general rule in chemistry is that an atom will gain or lose electrons to have a total of eight electrons in the outermost shell of its electron cloud. Aluminum has three electrons in its outer shell, and chlorine has seven in its outer shell.

AlCl 3 in a Lewis dot diagram. The dots represent outer-shell electrons for each element.

Aluminum wants to get rid of its three electrons. This will make the next inner electron shell its outer shell, which is already full with eight electrons. Chlorine, having seven in its outer shell, has one empty space available.

So if aluminum and chlorine come close to each other, they can help each other out.

Chlorine is the greedy one in terms of electrons. Since one aluminum atom can give up three electrons, three chlorine atoms pull off those three electrons, which stabilizes the aluminum. It also stabilizes the chlorine atoms because they get their one empty spaced filled.

Three chlorine atoms take one electron each from aluminum atom

This electron transfer gives us four charged particles that stick together because opposites charges attract.

All charges add up to equal 0

As you can see, aluminum chloride has no net charge. It is neutral just like all ionic compounds. Instead of writing symbols with dots around them, we can write

Al+3 + 3Cl-1 → AlCl3

which gives us aluminum chloride's formula, AlCl3. From this formula, we can determine the molar mass of this compound.

Molar Mass

Molar mass is the mass of one mole of a substance. The mass of one mole of aluminum chloride is the sum of one mole of aluminum and three moles of chlorine.

The periodic table has the molar mass of each element, so we just need to look there and then we can organize all of the information in a chart.

The values in red are the masses of one mole of that element

Element Quantity Molar Mass of Element (g/mole) Total Molar Mass in AlCl3 (g/mole)
Aluminum 1 26.982 26.982
Chlorine 3 35.45 106.35

Adding the values in column number 4 we get

26.982 g/mole + 106.35 g/mole ≈ 133.33 g/mole

So we have determined the formula for aluminum chloride, and determined its molar mass. Now, let's break it apart in a decomposition reaction!


Decomposition reactions are when a compound is broken down chemically into its elements. A 2.3 voltage source will provide enough electrical current through liquid aluminum chloride to separate the aluminum atom from the three chlorine atoms according to the equation:

AlCl3 → Al + Cl2

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