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Science lesson plan · 10th grade (Year 11 in England) · 45 minutes

Relative atomic mass from isotope abundances

Atoms, elements and the periodic table

CCSS NGSS.HS-PS1-1CCSS NGSS.SEP 5England: GCSE Combined Science: Atomic structure (relative atomic mass)

Objective and success criteria

Students will: Relative atomic mass from isotope abundances.

  • I can answer questions on today's skill on my own, without help.
  • I can explain each step and say why it works.
  • I can check my answer and spot a mistake.

Before and after

Needs first: Atomic number, mass number and isotopes

Leads to: the next skill in this area.

Materials: Periodic tables, Atom model kits, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).

Warm-up · 5 min

  • Quick retrieval from earlier learning (Atomic number, mass number and isotopes). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. An atom of fluorine is written ¹⁹₉F (mass number 19, atomic number 9). How many neutrons does it have? → 10Prompt: Atomic number = number of protons (= electrons in a neutral atom).
  2. Oxygen has atomic number 8. How many neutrons are in the isotope oxygen-16? → 8Prompt: The number after the name is the mass number.
  3. Which number is the same for all isotopes of an element? (the mass number / the atomic number / the number of neutrons / the relative mass) → the atomic numberPrompt: Isotopes differ only in their neutrons.

Hook · 5 min

  • Show the periodic table and ask what patterns students can see before anything is explained.
  • Share the objective: Relative atomic mass from isotope abundances. Read the success criteria together.

I do: model it · 10 min

  • Work through the example out loud, saying why you do each step, not just what you do.
  • Example: Natural boron contains 20 % of mass 10 and 80 % of mass 11. Calculate its relative atomic mass to 1 decimal place.
  • Step 1: 10 × 20 + 11 × 80 = 1080
  • Step 2: 1080 ÷ 100 = 10.8
  • Step 3: So the answer is: 10.8
  • Answer: 10.8
  • Pause after each step and ask a student to say what comes next.

We do: guided practice · 10 min

  • Do these together. Students try each step on whiteboards before you reveal it.
  • Use the hints as prompts rather than giving the answer.
  1. Natural magnesium contains 79 % of mass 24 and 10 % of mass 25 and 11 % of mass 26. Calculate its relative atomic mass to 1 decimal place. → 24.3Prompt: Multiply each mass by its percentage abundance.
  2. An element has two isotopes: mass 50 (25 %) and mass 52 (75 %). What is its relative atomic mass? Give 1 decimal place. → 51.5Prompt: The answer must lie between the two isotope masses.

You do: independent practice · 10 min

  • Students work on the student sheet. Circulate and check the first two answers of every student early.
  • Students who finish: write a question of their own for a partner, with the answer.

The 8 questions are on the student sheet below, with answers in the key.

Exit ticket · 5 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. An element has two isotopes: mass 73 (20 %) and mass 75 (80 %). What is its relative atomic mass? Give 1 decimal place. → 74.6Prompt: The answer must lie between the two isotope masses.
  2. Do isotopes of the same element react differently in chemical reactions? (No: isotopes never react / Yes: heavier isotopes are always more reactive / No: they have the same electron arrangement, so their chemistry is the same / Yes: they have different numbers of protons) → No: they have the same electron arrangement, so their chemistry is the samePrompt: Chemistry depends on electrons.

Watch out for

  • Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
  • Remembering facts without the reason: ask "why?" and "what is the evidence?"

Support and stretch

Support

  • Review first: Atomic number, mass number and isotopes.
  • Use concrete materials: periodic tables.
  • Give the first step of each question, or a partly worked example to finish.
  • Pair with a confident partner for the guided practice.

Stretch

  • Explain a common mistake in writing and how to fix it.
  • Write a word problem that uses today's skill, then solve a partner's.

Homework and more practice

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-10.

Free lesson plan from eurekami.com/lesson-plan-builder

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Relative atomic mass from isotope abundances

Name

Date

Score/ 8

  1. Question 1

    Carbon-12 and carbon-14 are isotopes. How many neutrons does carbon-14 have? (Carbon has 6 protons.)

    • A. 12
    • B. 8
    • C. 14
    • D. 6

  2. Question 2

    An element has two isotopes: mass 28 (20 %) and mass 30 (80 %). What is its relative atomic mass? Give 1 decimal place.

    Answer

  3. Question 3

    Natural gallium contains 60 % of mass 69 and 40 % of mass 71. Calculate its relative atomic mass to 1 decimal place.

    Answer

  4. Question 4

    Do isotopes of the same element react differently in chemical reactions?

    • A. No: isotopes never react
    • B. No: they have the same electron arrangement, so their chemistry is the same
    • C. Yes: they have different numbers of protons
    • D. Yes: heavier isotopes are always more reactive

  5. Question 5

    An element has two isotopes: mass 40 (20 %) and mass 42 (80 %). What is its relative atomic mass? Give 1 decimal place.

    Answer

  6. Question 6

    Natural lithium contains 7.5 % of mass 6 and 92.5 % of mass 7. Calculate its relative atomic mass to 1 decimal place.

    Answer

  7. Question 7

    Why is the relative atomic mass of chlorine 35.5 and not a whole number?

    • A. Chlorine atoms have half a neutron
    • B. Electrons add 0.5
    • C. It is measured badly
    • D. It is an average of its isotopes, weighted by abundance

  8. Question 8

    An element has two isotopes: mass 49 (40 %) and mass 51 (60 %). What is its relative atomic mass? Give 1 decimal place.

    Answer

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Answer key: Relative atomic mass from isotope abundances

  1. 1. B. 8

  2. 2. 29.6

  3. 3. 69.8

  4. 4. B. No: they have the same electron arrangement, so their chemistry is the same

  5. 5. 41.6

  6. 6. 6.9

  7. 7. D. It is an average of its isotopes, weighted by abundance

  8. 8. 50.2