Relative Atomic & Molecular Mass (¹²C Scale)

Understanding Relative Atomic Mass (Ar)

The relative atomic mass of an element, symbolized as $A_r$, is the weighted average mass of the atoms in a naturally occurring sample of the element, compared to $\frac{1}{12}$ of the mass of a carbon-12 atom.

For example:

• The $A_r$ of chlorine is approximately 35.5 because it consists of two isotopes: $^{35}\text{Cl}$ and $^{37}\text{Cl}$.

Calculating Relative Atomic Mass

To calculate $A_r$, use the formula:

$$A_r = \frac{\sum (\text{isotope mass} \times \text{abundance})}{\text{total abundance}}$$

Where:

• Isotope mass is the mass number of each isotope.
• Abundance is the percentage of each isotope in the sample.

Understanding Relative Molecular Mass (Mr)

The relative molecular mass of a compound, symbolized as $M_r$, is the sum of the relative atomic masses of the atoms in its molecular formula.

For example:

• The $M_r$ of water ($\text{H}_2\text{O}$) is calculated as: $2 \times A_r(\text{H}) + A_r(\text{O}) = 2 \times 1 + 16 = 18$.

Steps to Calculate Relative Molecular Mass

1. Identify the molecular formula of the compound.
2. Find the $A_r$ of each element in the compound.
3. Multiply the $A_r$ by the number of atoms of each element in the formula.
4. Add the results to get the $M_r$.