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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 has no charge? (proton / neutron / electron) → neutronPrompt: Two of the particles are in the nucleus.
  2. Atomic structure: which statement is true? (Protons and electrons have the same mass. / The plum pudding model is today's accepted model of the atom. / An atom has no overall charge because it has equal numbers of protons and electrons. / Electrons are found in the nucleus.) → An atom has no overall charge because it has equal numbers of protons and electrons.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 · 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 magnesium is written ²⁴₁₂Mg (mass number 24, atomic number 12). How many protons does it have? → 12Prompt: 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 · 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 atomic number / the number of neutrons) → the atomic numberPrompt: Isotopes differ only in their neutrons.
  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.

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

    Carbon has atomic number 6. How many neutrons are in the isotope carbon-13?

    Answer

  2. Question 2

    An atom of lithium is written ⁷₃Li (mass number 7, atomic number 3). How many electrons does it have?

    Answer

  3. Question 3

    A sodium atom loses one electron to become a sodium ion. What is its charge?

    • A. −1
    • B. 0
    • C. +11
    • D. +1

  4. Question 4

    Hydrogen has atomic number 1. How many neutrons are in the isotope hydrogen-1?

    Answer

  5. Question 5

    An atom of fluorine is written ¹⁹₉F (mass number 19, atomic number 9). How many neutrons does it have?

    Answer

  6. Question 6

    What are isotopes?

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

  7. Question 7

    Chlorine has atomic number 17. How many neutrons are in the isotope chlorine-35?

    Answer

  8. Question 8

    An atom of oxygen is written ¹⁶₈O (mass number 16, atomic number 8). How many protons does it have?

    Answer

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Answer key: Atomic number, mass number and isotopes

  1. 1. 7

  2. 2. 3

  3. 3. D. +1

  4. 4. 0

  5. 5. 10

  6. 6. A. atoms of the same element with different numbers of neutrons

  7. 7. 18

  8. 8. 8