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Science lesson plan · 8th grade (Year 9 in England) · 30 minutes

Atomic number, mass number and isotopes

Atoms, elements and the periodic table

CCSS NGSS.HS-PS1-1England: KS4 Combined Science: Atomic structure and the periodic table

Objective and success criteria

Students will: Atomic number, mass number and isotopes.

  • 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: Protons, neutrons and electrons

Leads to: Electron shells and the periodic table; Relative atomic mass from isotope abundances; Alpha, beta and gamma radiation

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

Warm-up · 4 min

  • Quick retrieval from earlier learning (Protons, neutrons and electrons). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. Which particle has no charge? (proton / neutron / electron) → neutronPrompt: Two of the particles are in the nucleus.
  2. Atomic structure: which statement is true? (Neutrons have a negative charge. / Protons and electrons have the same mass. / The plum pudding model is today's accepted model of the atom. / Most of the mass of an atom is in the nucleus.) → Most of the mass of an atom is in the nucleus.Prompt: Where is each particle, and what is its charge?
  3. Which particle is in the nucleus and its number defines the element? (proton / neutron / electron) → protonPrompt: Two of the particles are in the nucleus.

Hook · 3 min

  • Show the periodic table and ask what patterns students can see before anything is explained.
  • Share the objective: Atomic number, mass number and isotopes. Read the success criteria together.

I do: model it · 7 min

  • Work through the example out loud, saying why you do each step, not just what you do.
  • Example: An atom of fluorine is written ¹⁹₉F (mass number 19, atomic number 9). How many neutrons does it have?
  • Step 1: 19 − 9 = 10
  • Step 2: So the answer is: 10
  • Answer: 10
  • Pause after each step and ask a student to say what comes next.

We do: guided practice · 6 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. An atom of beryllium is written ⁹₄Be (mass number 9, atomic number 4). How many electrons does it have? → 4Prompt: Atomic number = number of protons (= electrons in a neutral atom).
  2. Chlorine has atomic number 17. How many neutrons are in the isotope chlorine-37? → 20Prompt: The number after the name is the mass number.

You do: independent practice · 7 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 6 questions are on the student sheet below, with answers in the key.

Exit ticket · 3 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. Hydrogen has atomic number 1. How many neutrons are in the isotope hydrogen-1? → 0Prompt: The number after the name is the mass number.
  2. An atom of sodium is written ²³₁₁Na (mass number 23, atomic number 11). How many neutrons does it have? → 12Prompt: Atomic number = number of protons (= electrons in a neutral atom).

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: Protons, neutrons and electrons.
  • 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

  • Move on to: Electron shells and the periodic table; Relative atomic mass from isotope abundances; Alpha, beta and gamma radiation.
  • 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-8.

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

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Atomic number, mass number and isotopes

Name

Date

Score/ 6

  1. Question 1

    What are isotopes?

    • A. atoms with different numbers of protons
    • B. atoms of different elements with the same mass
    • C. charged atoms
    • D. atoms of the same element with different numbers of neutrons

  2. Question 2

    Uranium has atomic number 92. How many neutrons are in the isotope uranium-238?

    Answer

  3. Question 3

    An atom of calcium is written ⁴⁰₂₀Ca (mass number 40, atomic number 20). How many electrons does it have?

    Answer

  4. Question 4

    Which number is the same for all isotopes of an element?

    • A. the mass number
    • B. the relative mass
    • C. the atomic number
    • D. the number of neutrons

  5. Question 5

    Chlorine has atomic number 17. How many neutrons are in the isotope chlorine-37?

    Answer

  6. Question 6

    An atom of sulfur is written ³²₁₆S (mass number 32, atomic number 16). How many electrons does it have?

    Answer

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Answer key: Atomic number, mass number and isotopes

  1. 1. D. atoms of the same element with different numbers of neutrons

  2. 2. 146

  3. 3. 20

  4. 4. C. the atomic number

  5. 5. 20

  6. 6. 16