What Is Relative Atomic Mass (Ar)?

Relative atomic mass (Ar) is the average mass of the atoms of an element compared to 1/12 of the mass of a carbon-12 atom.

Because elements exist as a mixture of isotopes, the atomic mass listed in the periodic table is an average value based on the abundance of those isotopes.

Relative atomic mass has no units because it is a comparison.

Example:

Relative atomic mass of oxygen = 16

This means an oxygen atom is 16 times heavier than 1/12 of a carbon-12 atom.

Why Relative Atomic Mass Is an Average

Many elements exist as mixtures of isotopes with different masses.

For example, chlorine has two main isotopes:

• Chlorine-35
• Chlorine-37

Since these isotopes exist in different proportions, the relative atomic mass is calculated as a weighted average.

For chlorine, the relative atomic mass is approximately 35.5.

What Is Relative Molecular Mass (Mr)?

Relative molecular mass (Mr) is the sum of the relative atomic masses of all the atoms in a molecule.

It represents the total mass of a molecule compared with 1/12 of a carbon-12 atom.

Relative molecular mass also has no units.

How to Calculate Relative Molecular Mass

To calculate Mr:

1. Identify the chemical formula

2. Find the relative atomic masses of each element

3. Multiply by the number of atoms

4. Add the total

Example 1 – Water (H₂O)

Relative atomic masses:

H = 1
O = 16

Calculation:

(2 × 1) + (1 × 16)

= 2 + 16
= 18

Relative molecular mass of water = 18

Example 2 – Carbon Dioxide (CO₂)

Relative atomic masses:

C = 12
O = 16

Calculation:

12 + (2 × 16)

= 12 + 32
= 44

Relative molecular mass of carbon dioxide = 44

Example 3 – Methane (CH₄)

Relative atomic masses:

C = 12
H = 1

Calculation:

12 + (4 × 1)

= 12 + 4
= 16

Relative molecular mass of methane = 16

Relative Formula Mass

For ionic compounds, we often use the term relative formula mass instead of molecular mass.

This is because ionic compounds do not exist as molecules.

Example:

Sodium chloride (NaCl)

Relative atomic masses:

Na = 23
Cl = 35.5

Relative formula mass:

23 + 35.5 = 58.5

Why Ar and Mr Are Important

Relative atomic mass and relative molecular mass are essential for:

• Stoichiometry calculations
• Determining molecular masses
• Understanding chemical equations
• Calculating moles of substances

They are widely used in many chemistry calculations.

Exam Tip (5070)

Students are commonly asked to:

• Define relative atomic mass (Ar)
• Define relative molecular mass (Mr)
• Calculate Mr from a chemical formula

Example exam question:

Calculate the relative molecular mass of NH₃.

Answer:

N = 14
H = 1

14 + (3 × 1) = 17

Practice Question

Calculate the relative molecular mass of H₂SO₄.

Relative atomic masses:

H = 1
S = 32
O = 16

Answer

(2 × 1) + 32 + (4 × 16)

= 2 + 32 + 64
= 98

Relative molecular mass of H₂SO₄ = 98

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