Lesson overview
Isotopes: Can atoms of one element have different masses?
Two bottles of clear water, and one of them is heavy water. Predict which test will tell them apart, weigh 10 cm³ of each and test both with anhydrous copper(II) sulfate, then look inside their hydrogen atoms.
Suggested sequence
- Lesson 1 · Two waters. Isotopes, symbols for isotopes and ions, and why isotopes react in the same way. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Why is chlorine 35.5? Supplement: abundance and relative atomic mass. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: Two waters
- Define isotopes as different atoms of the same element that have the same number of protons but different numbers of neutrons
- Interpret and use symbols for atoms and ions that show the mass number, the proton number and the charge
- Explain why isotopes of the same element have the same chemical properties
Part 2: Why is chlorine 35.5?
- Calculate the relative atomic mass of an element from the masses and abundances of its isotopes
Misconceptions to listen for
- “Atoms with the same mass number are isotopes.” — The mass number counts protons and neutrons together. With 6 and 7 protons they are different elements, even though both mass numbers are 14.
- “Atoms with the same number of neutrons are isotopes.” — The neutrons are what differ between isotopes. Atoms with different numbers of protons are different elements.
- “Bromine-81 has 81 neutrons.” — 81 is the mass number, protons and neutrons together. Take away the 35 protons: 46 neutrons.
- “The ion has 14 electrons, because the charge is 2−.” — A negative charge means extra electrons, which carry the negative charge. 16 protons and 18 electrons give 16 − 18 = −2.
- “The ion has 16 electrons, the same as its protons.” — That is true for an atom. An ion has a charge, so its electrons and protons are different.
- “The particle with 11 protons and mass number 24 is magnesium-24.” — Only the proton number decides the element. 11 protons is sodium, whatever the mass number.
- “The ion with 10 electrons has mass number 22.” — Electrons are not counted in the mass number. It is still 12 + 12 = 24; the missing electrons give the charge.
- “Hydrogen-2 has 2 protons.” — Then it would be helium. The 2 is the mass number: 1 proton and 1 neutron.
- “Hydrogen-3 has 3 electrons.” — Electrons match protons in an atom, and every hydrogen atom has 1 proton. The extra mass is in neutrons.
- “Chlorine-37 reacts differently because it has two more neutrons.” — Neutrons stay in the nucleus and have no charge, so they take no part in reactions. The electrons decide how an atom reacts.
Scope
Isotopes (2.3 Core 1–2, Supplement 3–4): isotopes as atoms of the same element with the same number of protons and different numbers of neutrons, symbols for atoms and ions such as ¹²₆C and ³⁵₁₇Cl⁻, why isotopes have the same chemical properties (the same number of electrons and the same electronic configuration), and how to calculate relative atomic mass from isotope masses and abundances. Relative atomic mass measured against carbon-12, relative molecular mass, radioactivity and mass spectrometry are left to other modules or are not in the course.