Formula unit, not molecule
Formula unit mass of ionic compounds.
No molecules in salt
A crystal of an ionic compound is a huge, regular array of ions. It is not made of separate molecules.
So how do we write the mass of the smallest repeating group of ions, such as Na₂O?
What this lesson covers
The idea
Ionic compounds form 3-D crystals, not molecules; the simplest whole number ratio of ions is a formula unit, and its mass, the sum of the atomic masses of its atoms, is the formula unit mass.
No molecules in salt
A crystal of an ionic compound is a huge, regular array of ions. It is not made of separate molecules.
So how do we write the mass of the smallest repeating group of ions, such as Na₂O?
- As the sum of the atomic masses in its formula
- We cannot give it any mass
- As the mass of one molecule
From a crystal to a formula unit
Press Next step. This flat picture only shows the idea of a crystal.
Formula unit mass
Because an ionic compound is not made of molecules, we talk about a formula unit. The formula unit mass is found just like the molecular mass: add the atomic masses of all the atoms in the formula unit.
Ionic compounds form 3-D crystals, not molecules. The simplest whole number ratio of ions is a formula unit, and its mass, the sum of the atomic masses of its atoms, is the formula unit mass.
Notes
Ionic compounds form 3-D crystals, not molecules. A formula unit is the simplest whole number ratio of ions; its mass is the formula unit mass.
Check yourself
Atomic masses: Na = 23 u and O = 16 u. What is the formula unit mass of sodium oxide, Na₂O, in u?
Answer: 62 u
(23 u × 2) + (16 u × 1) = 62 u.
Atomic masses: Ca = 40 u, N = 14 u, O = 16 u. What is the formula unit mass of calcium nitrate, Ca(NO₃)₂, in u?
Answer: 164 u
40 + (14 + 16 × 3) × 2 = 40 + 62 × 2 = 164 u.
Why do we say formula unit and not molecule for an ionic compound?
What is a formula unit?
- Its ions form a 3-D crystal, not molecules — correct. Yes!
- It has only one atom. An ionic compound has many ions arranged in a crystal.
- It has no mass. A formula unit has a mass, called the formula unit mass.
- A single molecule of an ionic compound. Ionic compounds do not form molecules.
- The simplest whole number ratio of ions in an ionic compound — correct. Yes!
- The charge on one ion. A formula unit is a collection of ions in the simplest whole number ratio, not a charge.