S4.6.3 For the adult
Isotopes
Atoms have a whole number of particles. So why is the mass of chlorine in the periodic table 35.5?
What this chapter teaches
Isotopes are atoms of one element with different numbers of neutrons. They react the same way because they have the same electrons. Relative atomic mass is a weighted mean of the isotope masses.
Three questions to ask
There is no one right wording. Listen for the idea.
- What is the same in chlorine 35 and chlorine 37, and what is different?
- Why can an element have a relative atomic mass that is not a whole number?
- A sample is 50% of an isotope of mass 10 and 50% of mass 12. What is the mean?
After the chapter, the learner can
- Explain what isotopes are and why they react the same way
- Calculate relative atomic mass from isotope amounts
What they copy into their notebook
Isotopes: same protons, different neutrons, so the same reactions. Relative atomic mass is a weighted mean: add mass × percentage for each isotope, then divide by 100. Chlorine: 35.5.