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Science lesson plan · 10th grade (Year 11 in England) · 60 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 · 6 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 carbon is written ¹²₆C (mass number 12, atomic number 6). How many neutrons does it have? → 6Prompt: Atomic number = number of protons (= electrons in a neutral atom).
  2. Oxygen has atomic number 8. How many neutrons are in the isotope oxygen-17? → 9Prompt: The number after the name is the mass number.
  3. Which number is the same for all isotopes of an element? (the relative mass / the number of neutrons / the atomic number / the mass number) → the atomic numberPrompt: Isotopes differ only in their neutrons.

Hook · 6 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 · 12 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 · 12 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 copper contains 69 % of mass 63 and 31 % of mass 65. Calculate its relative atomic mass to 1 decimal place. → 63.6Prompt: Multiply each mass by its percentage abundance.
  2. An element has two isotopes: mass 75 (60 %) and mass 77 (40 %). What is its relative atomic mass? Give 1 decimal place. → 75.8Prompt: The answer must lie between the two isotope masses.

You do: independent practice · 17 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 · 7 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 51 (90 %) and mass 53 (10 %). What is its relative atomic mass? Give 1 decimal place. → 51.2Prompt: The answer must lie between the two isotope masses.
  2. Natural lithium contains 7.5 % of mass 6 and 92.5 % of mass 7. Calculate its relative atomic mass to 1 decimal place. → 6.9Prompt: Multiply each mass by its percentage abundance.

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

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

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

  2. Question 2

    An element has two isotopes: mass 22 (10 %) and mass 24 (90 %). What is its relative atomic mass? Give 1 decimal place.

    Answer

  3. Question 3

    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.

    Answer

  4. Question 4

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

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

  5. Question 5

    An element has two isotopes: mass 72 (20 %) and mass 74 (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

    An element has two isotopes: mass 36 (10 %) and mass 38 (90 %). What is its relative atomic mass? Give 1 decimal place.

    Answer

  8. Question 8

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

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

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Answer key: Relative atomic mass from isotope abundances

  1. 1. C. It is an average of its isotopes, weighted by abundance

  2. 2. 23.8

  3. 3. 24.3

  4. 4. A. 8

  5. 5. 73.6

  6. 6. 6.9

  7. 7. 37.8

  8. 8. D. No: they have the same electron arrangement, so their chemistry is the same