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Relative atomic mass = sum of (isotope mass × abundance) ÷ 100, so chlorine's is 35.5. Free GCSE AQA notes with worked examples and 9 questions answered.
Understanding Relative Atomic Mass is part of A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes in GCSE AQA Higher Chemistry.
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Relative atomic mass = sum of (isotope mass × abundance) ÷ 100, so chlorine's is 35.5. Free GCSE AQA notes with worked examples and 9 questions answered.
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Use this lesson to stay inside A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes while connecting the detail back to the wider Topic 1: Atomic structure and the periodic table topic.
Read the core explanations from Understanding Relative Atomic Mass before testing yourself from memory.
Look at a periodic table. Notice how chlorine has a relative atomic mass of 35.5? That's not a mistake - it's telling us something important about atoms.
Most elements exist as mixtures of isotopes in nature. Chlorine atoms come in two main varieties: some with mass 35, others with mass 37. The 35.5 value is the weighted average of these isotopes based on how common each one is.
Think of isotopes like different models of the same car. A Ford Focus and Ford Focus ST are both Ford Focus cars (same element) but have different engines (different mass numbers). Chlorine-35 and chlorine-37 are both chlorine atoms but have different numbers of neutrons.
**Trap**: Many students think isotopes are different elements. They're not - they're different versions of the same element with identical chemical behaviour.
Relative atomic mass isn't just the average of isotope masses - it's a weighted average. This means more common isotopes have a bigger influence on the final value.
Imagine a class where 75% of students are 16 years old and 25% are 17 years old. The average age isn't 16.5 (simple average). It's closer to 16 because most students are 16. The calculation: (16 × 0.75) + (17 × 0.25) = 16.25 years.
Now let's tackle the calculations that appear in exams. The key is to multiply each isotope mass by its abundance, add them up, then divide by 100.
Worked example: Calculating Relative Atomic Mass
Chlorine has two isotopes: Cl-35 (75% abundance) and Cl-37 (25% abundance). Calculate the relative atomic mass of chlorine. [3 marks]
Answer: 35.5
Notice how the answer (35.5) is much closer to 35 than 37. This makes sense because Cl-35 is three times more abundant than Cl-37, so it dominates the average.
Relative atomic mass questions are mark-winners if you know the method. Here's how to guarantee full marks and avoid the traps that catch other students.
**Common Exam Mistake**: Students often forget to divide by 100 at the end. They calculate (35 × 75) + (37 × 25) = 3550 and think that's the answer. Always remember the final division step.
Exam precision note: relative atomic mass is a weighted average, so you must use both isotope mass and isotope abundance. After multiplying each isotope mass by its abundance, divide the total by 100.
Relative atomic mass is measured on a scale where carbon-12 has a value of exactly 12. This is why the definition refers to one twelfth of the mass of a carbon-12 atom, rather than simply taking an ordinary average of isotope masses.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Relative atomic mass = sum of (isotope mass × abundance) ÷ 100. For chlorine, with Cl-35 at 75% and Cl-37 at 25%: (35 × 75) + (37 × 25) = 2625 + 925 = 3550, and 3550 ÷ 100 = 35.5. Students often forget the final division by 100.
The formula for relative atomic mass is Ar = sum of (isotope mass × abundance) ÷ 100. Multiply each isotope's mass by its percentage abundance, add the results, then divide the total by 100.
Relative atomic mass is the weighted mean mass of an atom of an element compared with one twelfth of the mass of a carbon-12 atom. It is a weighted average of the masses of an element's isotopes, so more common isotopes have a bigger influence on the value.
Chlorine's relative atomic mass is 35.5 because chlorine exists as a mixture of isotopes: chlorine-35 (75%) and chlorine-37 (25%). The value is a weighted average, closer to 35 than 37 because Cl-35 is three times more abundant.
Relative atomic masses are often not whole numbers because most elements exist in nature as mixtures of isotopes with different masses. The relative atomic mass is a weighted average of these isotopes based on how common each one is, such as 35.5 for chlorine.
A weighted average means more common isotopes have a bigger influence on the relative atomic mass. Chlorine's value of 35.5 is much closer to 35 than 37 because chlorine-35 is three times more abundant than chlorine-37.
Isotopes are not different elements; they are different versions of the same element with the same number of protons but different numbers of neutrons. They have identical chemical properties but different masses.
The most common mistake is not dividing by 100 at the end. Calculating (35 × 75) + (37 × 25) gives 3550, which must then be divided by 100 to give the relative atomic mass of 35.5.
Relative atomic mass is measured on a scale where carbon-12 has a value of exactly 12. This is why it is defined as the weighted mean mass of an atom compared with one twelfth of the mass of a carbon-12 atom.