DSE Chemistry: atomic structure and periodic table patterns (tutor lesson plan)

2026-11-03

Tutor lesson-prep guides · Secondary Chemistry

Students can say that proton number equals atomic number, but freeze when asked why sodium beats magnesium or why chlorine is 35.5. This lesson uses electron arrangement as one thread from position to property to calculation.

Objectives — Students can: (1) write the electron arrangement in the first three shells from the proton number, and reverse it to name the period and group; (2) calculate relative atomic mass from isotope abundance (chlorine: 75% of 35 and 25% of 37 gives 35.5) and explain why the answer is not a whole number; (3) use outer-shell electrons and number of shells as two separate rulers to explain the reactivity trend down a group and the radius trend across a period.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Proton number and symbol	What is the proton number of Na?	11
Percentages to decimals	Write 75% as a decimal	0.75
Reading the periodic table	Where is period 3 group 1?	Third row, left-hand column

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0–10 min	Three particles	Open with a question: an atom is mostly empty, so why is almost all the mass in the centre?	Name protons and neutrons in the nucleus, electrons outside
10–22 min	Electron arrangement	Fill shells from the proton number, 2 then 8, modelling Na and Cl	Both arrangements correct, outer shell identified
22–38 min	Arrangement to position	Students read period and group from the arrangement, then reverse it	State “shells = period, outer electrons = group”
38–48 min	Isotopes and abundance	Combine isotopes 35 and 37: 0.75×35 + 0.25×37 = 35.5	Get 35.5 and explain the decimal
48–55 min	Trends	Compare Na, Mg and Cl using outer electrons and shell number	Each student gives one mechanism
55–60 min	Exit check	Mark on the spot and name the act to reteach	All three correct

03 Teaching script (read it out)

Scene 1: Start with the empty space

Do not open with a definition. Open with an image: if the nucleus were a marble in the middle of a hall, the outermost electron would be near the outside wall.
Then give three facts: protons are positive, neutrons neutral, and an electron has about 1/1836 the mass of a proton. So almost all mass sits in the nucleus while almost all size comes from the electron cloud.
Then bring in proton number: the proton count fixes the element. Na is number 11, so it has 11 electrons, and a mass number of 23 means 12 neutrons in the nucleus. Train students to state three numbers in one sentence: protons, neutrons, electrons.

Ask students: once you know the proton number, what lets you write the electron count?

Scene 2: Fill the shells, then read the table

Only three numbers are needed as limits: first shell 2, second 8, third 8. Na with 11 electrons is 2, 8, 1; Cl with 17 is 2, 8, 7.
Immediately reverse it twice: arrangement to position (shells = period, outer electrons = group, so Na is period 3 group 1 and Cl is period 3 group 7), then position back to arrangement. After two rounds students see that the periodic table is an electron-arrangement table.
Handle two traps: ions change the electron count (Na+ becomes 2, 8), and transition metals break the “outer electrons = group” shortcut, so S4 only covers main groups.

Ask students: why does period 3 hold only eight elements?

Scene 3: From arrangement to property, from abundance to decimals

Two rulers: outer-shell electrons decide whether the atom wants to gain or lose, and the number of shells decides how easy that is. So group 1 reactivity rises down the group (more shells, outer electron leaves easily) while group 7 reactivity falls (more shells, weaker pull).
The other half is isotopes: chlorine exists as 35 and 37, roughly 75% and 25% in nature, giving the weighted mean 0.75×35 + 0.25×37 = 35.5. The answer is 35.5 and not 35 or 37 because both kinds of atom are present in the sample.
Have students say it themselves: relative atomic mass is an average, not the mass of one atom.

Ask students: if chlorine had only the 35 isotope, what number would appear in the table?

04 Question types and worked steps

Type	Example	Steps	Answer
Electron arrangement	Write the arrangement for Mg (12) and S (16)	Count electrons, fill 2 then 8, then check the total equals the proton number	Mg 2,8,2; S 2,8,6
Reverse position	An element is 2,8,7. Which period and group?	Shells = period, outer electrons = group; group 7 means group 17	Period 3, group 7 (a halogen)
Abundance calculation	Isotopes 63 (69%) and 65 (31%): find the relative atomic mass	Multiply each by its fraction, add, quote one decimal place	63×0.69 + 65×0.31 = 63.3
Explanation	Why does group 1 reactivity increase down the group?	More shells is only the start; add that the outer electron is further from the nucleus and easier to lose	More shells, more shielding, outer electron lost more easily, reactivity rises

Sanity check (teach this): Check the numbers: Na has 11 protons, 12 neutrons and arrangement [2, 8, 1]; Cl has 17 protons and arrangement [2, 8, 7]; chlorine relative atomic mass is 35.5 from 0.75×35 + 0.25×37. Every figure here is computed in python, so never quote abundances from memory in class. If a student works backwards from 35.5, accept a ratio of about 3:1 rather than an exact 75:25.

05 Independent practice (3 questions)

Question	Answer	What to watch
Write the arrangement of K (19) and name its period and group.	2,8,8,1; period 4, group 1.	Fourth shell holds one electron and the third is not filled with nine
Isotopes 10 (20%) and 11 (80%): find the relative atomic mass.	10×0.20 + 11×0.80 = 10.8.	Weighted mean used, not a midpoint or a whole number
Using outer electrons and shell number, explain why Cl is more reactive than I.	Cl has three shells, so its outer shell is closer and pulls electrons more strongly; I has more shells and a weaker pull.	Both shell number and pull are mentioned, not just “Cl is smaller”

06 Common mistakes and what to say

Mistake	Student writes	Say this
Mass number read as proton number	“The proton number of sodium is 23.”	“23 is the mass number. How many of those are protons? That count is the proton number.”
Skipping a shell	“Potassium is 2,8,9.”	“Is the third shell full yet? Check the limit before you touch the fourth.”
Plain average for abundance	“35 and 37 average to 36.”	“Only if the mix were 50:50. The question gives 75% and 25%, so weight it.”
Half an explanation for trends	“Chlorine is reactive because it is near the top.”	“Near the top is a position, not a cause. Say what happens to the shells and the outer electrons.”

07 Homework (paper, section and question numbers)

Redo the practice on this page — Questions 1 to 3: arrangements must include period and group, abundance answers must show the working, and question 3 is collected next lesson.
Preview the next chapter: chemical bonding — Find the ionic and covalent bonding chapter on the subject index, read the first objective and write down one question.

08 Exit check (last 5 minutes)

Question	Answer	Decision
What is the electron arrangement of sodium?	2, 8, 1	Answering 2,8,2 or 2,9 means the proton number or the shell limit is not secure
Why is the relative atomic mass of chlorine 35.5?	It is the weighted mean of the 35 and 37 isotopes, not the mass of one atom	Answering “because it has a decimal” misses where the average comes from
Why does reactivity change down a group?	More shells put the outer electron further out: easier to lose in group 1, harder to attract in group 7	Naming shells without a mechanism means act three needs reteaching

09 Tutor FAQ

How many elements must students memorise?

Only the first twenty for arrangement and position; everything else can be derived from the rules in the exam.

What if a student asks about the third shell holding 18?

At S4 only the main-group octet is required; the 18-electron transition series belongs to the elective, and stating that boundary keeps the class calm.

Should students draw the periodic table themselves?

Yes. Drawing the first three periods with outer-electron counts makes the group column obvious.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.