Edexcel iGCSE Chemistry -1.17 Relative Atomic Mass- Study Notes- New Syllabus
Edexcel iGCSE Chemistry -1.17 Relative Atomic Mass- Study Notes- New syllabus
Edexcel iGCSE Chemistry -1.17 Relative Atomic Mass- Study Notes -Edexcel iGCSE Chemistry – per latest Syllabus.
Key Concepts:
1.17 be able to calculate the relative atomic mass of an element (Ar) from isotopic abundances
1.17 Calculating Relative Atomic Mass (Ar) from Isotopic Abundances
Most elements exist naturally as a mixture of isotopes.
The relative atomic mass (Ar) is the weighted mean mass of the isotopes of an element, taking into account their percentage abundances.
Weighted Mean Formula
\( A_r = \dfrac{(m_1 \times a_1) + (m_2 \times a_2) + (m_3 \times a_3) + \dots}{100} \)
Where:
• \( m \) = isotopic mass
• \( a \) = percentage abundance
The answer is usually given to 1 or 2 decimal places.
Step-by-Step Method
1. Multiply each isotope mass by its percentage abundance.
2. Add the results together.
3. Divide by 100.
Why Ar Is Not a Whole Number
Because it is an average of isotopes with different masses.
For example, chlorine has isotopes of mass 35 and 37, so its Ar is about 35.5.
Common Mistakes
• Forgetting to divide by 100.
• Adding the masses without weighting them.
• Confusing mass number with relative atomic mass.
Example 1 (Conceptual):
Why is the relative atomic mass of chlorine 35.5 and not 35 or 37?
▶️ Answer/Explanation
Chlorine exists as two isotopes, 35 and 37.
Because they occur in different abundances, the relative atomic mass is a weighted average between the two values.
Therefore, it is approximately 35.5.
Example 2 (Numerical):
An element has two isotopes:
Isotope A: mass 24, abundance 75% Isotope B: mass 25, abundance 25%
Calculate the relative atomic mass.
▶️ Answer/Explanation
\( A_r = \dfrac{(24 \times 75) + (25 \times 25)}{100} \)
\( = \dfrac{1800 + 625}{100} \)
\( = 24.25 \)
Example 3 (Hard ):
An element has three isotopes:
Isotope X: mass 58, abundance 60% Isotope Y: mass 60, abundance 30% Isotope Z: mass 61, abundance 10%
Calculate the relative atomic mass and explain why it is not equal to any of the isotopic masses.
▶️ Answer/Explanation
\( A_r = \dfrac{(58 \times 60) + (60 \times 30) + (61 \times 10)}{100} \)
\( = \dfrac{3480 + 1800 + 610}{100} \)
\( = \dfrac{5890}{100} = 58.9 \)
The relative atomic mass is a weighted average of all isotopes.
Because the isotopes exist in different abundances, the value lies between 58 and 61 and does not exactly match any single isotope.