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Science lesson plan · 8th grade (Year 9 in England) · 45 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 · 5 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 is found in shells around the nucleus? (proton / neutron / electron) → electronPrompt: Two of the particles are in the nucleus.
  2. Atomic structure: which statement is true? (The number of protons is called the atomic number. / The plum pudding model is today's accepted model of the atom. / Neutrons have a negative charge. / Electrons are found in the nucleus.) → The number of protons is called the atomic number.Prompt: Where is each particle, and what is its charge?
  3. Which particle has a charge of +1? (proton / neutron / electron) → protonPrompt: Two of the particles are in the nucleus.

Hook · 5 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 · 10 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 · 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. An atom of aluminium (aluminum) is written ²⁷₁₃Al (mass number 27, atomic number 13). How many electrons does it have? → 13Prompt: 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.

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. Which number is the same for all isotopes of an element? (the mass number / the relative mass / the number of neutrons / the atomic number) → the atomic numberPrompt: Isotopes differ only in their neutrons.
  2. Uranium has atomic number 92. How many neutrons are in the isotope uranium-235? → 143Prompt: The number after the name is the mass number.

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/ 8

  1. Question 1

    An atom has 11 protons, 12 neutrons and 11 electrons. What is its mass number?

    • A. 34
    • B. 12
    • C. 23
    • D. 11

  2. Question 2

    Uranium has atomic number 92. How many neutrons are in the isotope uranium-235?

    Answer

  3. Question 3

    An atom of argon is written ⁴⁰₁₈Ar (mass number 40, atomic number 18). How many electrons does it have?

    Answer

  4. Question 4

    What are isotopes?

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

  5. Question 5

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

    Answer

  6. Question 6

    An atom of helium is written ⁴₂He (mass number 4, atomic number 2). How many neutrons does it have?

    Answer

  7. Question 7

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

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

  8. Question 8

    Oxygen has atomic number 8. How many neutrons are in the isotope oxygen-16?

    Answer

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Answer key: Atomic number, mass number and isotopes

  1. 1. C. 23

  2. 2. 143

  3. 3. 18

  4. 4. C. atoms of the same element with different numbers of neutrons

  5. 5. 146

  6. 6. 2

  7. 7. A. the atomic number

  8. 8. 8