formula unit

noun

, Chemistry.

1. (of an ionic compound that does not form molecules, as most salts) the chemical formula with the least number of elements out of the set of empirical formulas having the same proportion of ions as elements:

   NaCl is the formula unit for the ionic compound sodium chloride.

More About Formula Unit

What is a formula unit?

A formula unit is the chemical formula of an ionic compound that lists the ions in the lowest ratio that equals a neutral electrical charge.

In chemistry, we use chemical formulas to describe the elements or molecules that make up a substance. For example, two hydrogen atoms bond with one oxygen atom to form a molecule of water, giving it a chemical formula of water is H₂O. We refer to this formula as a molecular formula because water is made of many H₂O molecules.

However, ionic compounds aren’t made of distinct molecules with specific properties. Instead they form complex structures of humongous numbers of charged ions that are attracted to each other. For this reason, we instead use a formula that describes the ratio of ions in the compound that would have an equal balance of positive and negative charges. This formula is called the formula unit.

For example, lithium nitride is made of lithium ions and nitride ions. Lithium ions have a +1 charge and nitride ions have a -3 charge. In order for these charges to become neutral, we need 3 lithium ions for every 1 nitride ion because (1 x 3) + (-3) = 0. Therefore, the formula unit of lithium nitride is Li₃N.

Why is formula unit important?

The first records of the term formula unit come from around 1869. It is used to specifically describe the composition of ionic compounds.

While molecular formulas and formula units both tell us what specific compound we are dealing with, there is a key difference. A molecular formula, like that for water, tells you exactly how many atoms the molecule is made up of. The formula unit tells you what ions make up the compound rather than atoms or molecules.

Did you know … ?

It is possible for the ratio in the formula unit to be 1:1. Table salt, for example, has a formula unit of NaCl because the charge of a sodium ion (Na) is +1 and the charge of a chloride ion (Cl) is -1.

What are real-life examples of formula unit?

Chemistry students will often learn what a formula unit is when they are introduced to ionic compounds.

The back of my head is about to cave in with all this ionic compound molecular formula unit molar mass stuff flowing around.

— endea (@_maryjaneplain) October 24, 2012

We have our first ionic compound entry from Caleb. No molecular geometry here, but a nice representation of the formula unit for Calcium Chloride. pic.twitter.com/eUTWyax29w

— Bridget B. Adkins (@BridgetBAdkins) January 16, 2018

 

What other words are related to formula unit?

• ion
• compound
• salt
• chemistry
• empirical formula
• molecular formula
• structural formula

Quiz yourself!

True or False?

A formula unit lists the ions in the lowest ratio that equals a neutral electrical charge.