Build the Atom: Protons, Neutrons, Electrons and Isotopes

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Science · Chemistry
SP-CHEM-003 · Free to print and use in your classroom · substituteplans.com
Page 2
Build the Atom: Protons, Neutrons, Electrons
and Isotopes
Science · Chemistry · Grades 9-11 · 50 minutes
Time needed
50 minutes (one class period)
Prep
Copy the 2-page worksheet (one per student).
Technology
None. A basic calculator is optional.
Student
independence
High. Students work alone, then check with a partner.
Copies to make
One 2-page worksheet per student
Materials
Pencils, Worksheet, Calculators (optional)
Goal for today: Students will determine numbers of protons, neutrons and electrons from atomic and mass numbers
and calculate an average atomic mass from isotope abundances.
Before the bell
☐ Copy the worksheet, one per student (2 pages).
☐ Write on the board: protons = atomic number; neutrons = mass number − atomic number; electrons =
protons for a neutral atom.
☐ Keep the answer key on your desk. All reference data is printed on the worksheet, so no periodic table is
needed.
Say this to start
Good morning! Today is about the tiny particles inside atoms. Everything you need is on the worksheet, including a
small reference list. You will fill in a table, work with ions and isotopes, and finish with one calculation. Please work
quietly on your own first and check with a partner at the end.

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Science · Chemistry
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Page 3
Run of show
Minutes
What you do
What students do
0–5
Take attendance. Hand out the worksheet. Ask students to
read the Reference box.
Read the reference box and the three rules.
5–12
Read the worked example aloud: Sodium-23 has atomic
number 11. So 11 protons. Neutrons = 23 − 11 = 12. A
neutral atom has 11 electrons.
Follow along and write the example in the
margin.
12–25
Tell students to complete the particle table in Part A and
the ion table in Part B. Remind them that a positive charge
means electrons were lost, and a negative charge means
electrons were gained.
Fill in protons, neutrons and electrons for
each atom or ion.
25–40
Tell students to do questions 1–3 and the average atomic
mass calculations (questions 4 and 5). Calculators are fine.
Answer the questions and show work in the
boxes.
40–47
Read the answers aloud from the key. Students mark and
correct their own work.
Check answers and fix mistakes.
47–50
Collect the worksheets and put them in a stack for the
teacher.
Hand in the worksheet.
If things go wrong
• If students ask what a mass number is, tell them it is the number of protons plus neutrons added together. It
is also in the reference box.
• If students mix up ion charges, tell them: lose electrons, become more positive; gain electrons, become more
negative.
• If a student’s average atomic mass looks too small, check that they multiplied each mass by its percent and
divided by 100.
• If the class gets loud, remind them this is individual practice and partners may talk only at the end.
Early finishers
• Write the symbol, protons, neutrons and electrons for Carbon-14 (atomic number 6) on the back.
• Explain in two sentences how two isotopes of the same element are alike and different.
Support and challenge
• Support: Complete only Part A and questions 1 and 2. Use the worked example as a model.
• Challenge: Chlorine-35 and chlorine-37 are both found in nature. Explain why the average atomic mass of
chlorine is not a whole number.

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Science · Chemistry
SP-CHEM-003 · Free to print and use in your classroom · substituteplans.com
Page 4
Notes for the teacher (you fill this in)
Jot a few lines before you leave. A full Daily Sub Report is in the free Sub Survival Kit at SubstitutePlans.com.
How far the class got / what to pick up next:
Students who were especially helpful:
Anything the teacher should know: