DSE Chemistry | Periodic Table Tutor Prep Guide

DSE Chemistry | Periodic Table Tutor Prep Guide | Period and Group Trends

2026-11-01

Tutor lesson-prep guides · Secondary Chemistry

Students can recite that elements in a group behave alike, but the moment they compare radii or electronegativity they fall back on memorising. They read the table as symbols, not as two axes: shell count across a period and outer electrons down a group.

Objectives — Students can: (1) locate an element by atomic number and give its electron arrangement; (2) use shell count and outer electrons to predict radius, ionisation energy and electronegativity; (3) decide metal or non-metal from position and write the reaction with chlorine; (4) explain trends with two sentences instead of memory.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Atomic structure	protons, neutrons, electrons	Na has 11 protons
Electron arrangement	2,8,8 filling order	Cl is 2,8,7
Ion formation	metals lose electrons	Mg loses two electrons to form Mg2+

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Locate	write arrangement first	Na is period 3 group I
8-20 min	Across a period	left to right	radius 186 pm to 99 pm
20-32 min	Down a group	one more shell	radius increases
32-42 min	Metal reactions	count outer electrons	Mg to Cl2 is 1:2
42-52 min	Electronegativity	rises across a period	0.9 to 3.0
52-60 min	Exit check	collect and mark	independent

03 Teaching script (read it out)

Scene 1: First move: write the electron arrangement before any trend

Na with atomic number 11 is 2,8,1: three shells means period 3, one outer electron means group I. Cl with atomic number 17 is 2,8,7: also period 3, seven outer electrons, group VII. From Na to Ar there are 8 elements because 3s and 3p together hold eight electrons. A student who miscounts shells will reverse every trend that follows. Ask: why must K have a larger radius than Na? (K has one more shell.)

Task: five atomic numbers, thirty seconds to write period, group and arrangement.

Scene 2: Two axes, taught separately

Across period 3 the radius falls because protons increase while shell count stays the same: Na 186 pm against Cl 99 pm, a factor of 1.88. Ionisation energy rises from 496 to 1251 kJ/mol, a factor of 2.52, and electronegativity goes from 0.9 to 3.0. Down a group everything reverses: an extra shell, larger radius, lower ionisation energy, higher metallic character. Put the two axes in two lines of board work so students never swap them. Reminder: compare radius by shells and nuclear charge, never by atomic mass. Compute: how many times larger is the Na radius than the Cl radius? (about 1.88.)

Scene 3: Metals and non-metals: outer electrons decide the equation

One or two outer electrons means a metal that tends to lose them; six or seven means a non-metal that gains them. Na with Cl2 is 2:1, giving 2Na + Cl2 -> 2NaCl. Mg with Cl2 is 1:2, giving Mg + Cl2 -> MgCl2, because Mg must lose two electrons and needs two chlorine atoms. Ask how many outer electrons the metal has and the coefficients follow without memorising. Reactivity increases down group I, which is why K reacts more violently with water than Na. Ask: why is MgO 1:1 while MgCl2 is 1:2? (O needs two electrons, Cl needs one.)

04 Question types and worked steps

Type	Example	Steps	Answer
Locate	atomic number to period and group	arrangement, shells, outer electrons	Cl is period 3 group VII
Compare radius	two atoms or ions	shells first, then nuclear charge	Na 186 pm above Cl 99 pm
Compare ionisation energy	same period or group	smaller radius, higher energy	496 to 1251 kJ/mol
Write an equation	metal plus non-metal	coefficients from outer electrons	Mg + Cl2 -> MgCl2

Sanity check (teach this): A radius in pm sits between 30 and 300; an answer of 0.186 means nm was used as pm. Radius falls left to right, so saying Na is smaller than Cl is the wrong direction. Ionisation energy is always positive because energy goes in. Atoms must balance: Mg + Cl2 has two chlorine atoms and so does MgCl2. Use either roman numerals or 1 to 18 for groups, never a mix.

05 Independent practice (3 questions)

Question	Answer	What to watch
Period and group of Cl	period 3, group VII	start from 2,8,7
Ratio of Na to Cl radius	1.88	186 divided by 99
Coefficients for Mg and Cl2	1 to 1, Mg + Cl2 -> MgCl2	count outer electrons

06 Common mistakes and what to say

Mistake	Student writes	Say this
Swapping period and group trends	says radius rises left to right	Across a period the shell count is fixed and the pull is stronger, so the radius must fall.
Judging radius by atomic mass	says Cl is larger than Na	Radius depends on shells and nuclear charge, not mass.
Unbalanced equation	writes Mg + Cl2 -> MgCl	Mg loses two electrons, so two chlorine atoms are needed.
Wrong units	writes a radius of 0.186 pm	A picometre is 1e-12 m, so the value should read 186.

07 Homework (paper, section and question numbers)

Secondary Chemistry chapter index — questions 2 and 5 are the same type
All prep guides — other chapters in this subject
DSE Chemistry practice papers — periodic table set, questions 1 to 6

08 Exit check (last 5 minutes)

Question	Answer	Decision
Atomic number and period of Ar	18, period 3	three correct allows bonding next
Electronegativity across a period	increases	one wrong means reteach
Equation for Mg and Cl2	Mg + Cl2 -> MgCl2	two or more wrong means restart act one

09 Tutor FAQ

Why are there only eight elements in period 3?

The 3s and 3p subshells hold eight electrons between them, and 3d belongs to period 4.

Do radius and ionisation energy agree?

Yes, a smaller radius holds outer electrons more tightly, so ionisation energy is higher.

Which elements are most electronegative?

Those at the top right, F and O, because they combine small radius with high nuclear charge.

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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.