S4 Chemistry: Periodic Trends — Tutor Lesson Plan (45 min, script, four common errors)

2026-11-02

Tutor lesson-prep guides · Primary maths

Students memorise “reactivity increases down Group I” and then apply it to Group VII. This lesson ties every trend to one cause: distance from the nucleus and the number of shells.

Objectives — the student can explain the trend in atomic radius across a period and down a group, predict the trend in first ionisation energy with a reason, state and justify the reactivity trend in Groups I and VII, and use the position of an element to predict whether it forms an ionic or covalent compound.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Atomic structure	How many protons, neutrons and electrons in a sodium-23 atom?	11 protons, 12 neutrons, 11 electrons
Electronic configuration	Write the electronic configuration of chlorine.	2, 8, 7
Ions	Why does magnesium form a 2+ ion?	It loses two outer electrons to reach a stable noble gas configuration

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Hook	Sodium and potassium with water, video or demonstration	Predict which is faster, and why, before naming the trend
5-15 min	Scene 1	Same period, left to right: nuclear charge rises, shells stay the same, radius falls	Plot radius for Na to Cl on the same axes
15-24 min	Scene 2	Same group, top to bottom: one more shell each time, radius rises, outer electron is easier to remove	Explain with distance and shielding, not “bigger atom”
24-32 min	Scene 3	Reactivity: Group I loses an electron more easily down the group, Group VII gains an electron less easily down the group	Say what each group is trying to do
32-39 min	Scene 4	Predicting bonding: metal plus non-metal gives ionic, non-metal plus non-metal gives covalent	Test against three unfamiliar pairs
39-45 min	Exit check	Four predictions in section 09	Mark at once and name the error

03 Teaching script (read it out)

Scene 1: One cause, two directions

Across a period the number of shells stays the same but the nuclear charge increases, so the outer electrons are pulled in and the radius decreases.
Down a group each element has one more shell, so the outer electron is further from the nucleus and shielded by more inner shells.

Draw both cases as a nucleus with shells, side by side, and have the student mark the outer electron each time.

Scene 2: Ionisation energy is a consequence, not a separate fact

First ionisation energy rises across a period because the outer electron is held more strongly, and falls down a group because the outer electron is further away and shielded.
Ask for the reason every time; the number alone earns nothing.

Question pattern: “Why is the first ionisation energy of magnesium higher than that of sodium?” Answer must mention nuclear charge, same shell, stronger attraction.

Scene 3: Group reactivity trends run in opposite directions

Group I metals lose their single outer electron, which becomes easier down the group, so reactivity increases downwards.
Group VII non-metals gain an electron, which becomes harder as the shell gets further from the nucleus and more shielded, so reactivity decreases downwards.

Make the student say the verb — lose or gain — before comparing anything.

04 Question types and worked steps

Type	Example	Steps	Answer
Compare two radii	Which is larger: Na or Cl? Give a reason.	Same period, Cl has the higher nuclear charge, so the outer shell is pulled in	Na is larger
Ionisation energy reasoning	Why is the first ionisation energy of potassium lower than that of sodium?	Outer electron is in a shell further from the nucleus and more shielded	Lower, because the outer electron is easier to remove
Predict the bond type	Predict the bond in calcium chloride and in carbon tetrachloride.	Metal plus non-metal gives ionic; non-metal plus non-metal gives covalent	Ionic, then covalent

Sanity check (teach this): Every trend answer must name the two competing factors: nuclear charge pulling in, and distance plus shielding pushing out. If a student gives only “because it is bigger”, the reasoning is not yet there, even when the final trend is correct.

05 Independent practice (3 questions)

Question	Answer	What to watch
Arrange Mg, Al and Si in order of decreasing atomic radius.	Mg > Al > Si	Same period, radius falls as nuclear charge rises
Explain why fluorine is more reactive than iodine.	Fluorine gains an electron more easily: smaller atom, less shielding, stronger attraction	The verb must be gain, not lose
Predict whether potassium bromide is ionic or covalent and give a reason.	Ionic, metal plus non-metal, so electrons are transferred	Reason must mention transfer, not just “different elements”

06 Common mistakes and what to say

Mistake	Student writes	Say this
Applying the Group I trend to Group VII	The trends run in opposite directions	Ask what the atom must do: lose or gain an electron
Saying radius increases across a period “because there are more electrons”	More electrons are added to the same shell, while nuclear charge rises faster in effect	Draw the shells; extra electrons in the same shell do not expand it
Giving a trend without a reason	The mark is in the reason, not the direction	Require nuclear charge, distance and shielding in the answer
Treating ionisation energy as a fixed property of the element	It changes with each successive electron removed	Ask why the second ionisation energy is always higher than the first

07 Homework (paper, section and question numbers)

HKDSE Chemistry Paper 1 multiple-choice practice paper — Questions on the periodic table, atomic radius and ionisation energy
Secondary chemistry chapter index — Revise electronic configuration before the next lesson

08 Exit check (last 5 minutes)

Question	Answer	Decision
Which is larger, K or Br, and why?	K, it is further left in the same period, lower effective nuclear charge	All correct: move to bonding and structure
Why is caesium more reactive than lithium?	The outer electron is further from the nucleus and more shielded, so it is lost more easily	One wrong: redo the distance and shielding model
Predict the bond type in sodium oxide.	Ionic, metal plus non-metal	Two or more wrong: repeat scene 4

09 Tutor FAQ

Do students need the numerical ionisation energies?

No. They need the trend and the reason. Numbers help as evidence, but a memorised table is not the objective.

How do I handle the exception at Group II to Group III?

Mention it only if a student asks, and keep the main trend clean. At S4 the general model matters more than the exception.

Is “effective nuclear charge” too advanced?

Use “pull of the nucleus on the outer electrons” instead; the idea is the same and the phrase is safer for S4.

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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.