S3 Science — Atoms, Elements and the Periodic Table: Electron Arrangement and Ions (Lesson Prep Guide)

2026-11-15

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Every student can recite that an atom has protons, neutrons and electrons, then freezes when asked for the electron arrangement of sodium, because nobody ever showed them that the atomic number is a number you can count with. This lesson does three things: count electrons from the atomic number, read the group from the outer shell, and turn the group back into an ion.

Objectives — Students can: (1) write the electron arrangement of any of the first twenty elements using the 2, 8, 8 rule; (2) state which group an element sits in from its outer electrons and work backwards from a group to the outer shell; (3) explain whether a metal loses or a non-metal gains electrons when forming an ion, and write the ion charge and the average atomic mass of an isotope mixture.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Element symbols	What elements do Na, Cl and Mg stand for?	Sodium, chlorine and magnesium
Shell capacity	What is the maximum number of electrons in the first shell?	Two
Three particles	Which three particles make up an atom?	Protons, neutrons and electrons

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	A stuck moment	Write electron arrangement of sodium and ask where to start	Start from atomic number 11
5-15 min	Atomic number is electron count	Protons equal electrons in a neutral atom; count 2, 8, 1	Count hydrogen, oxygen and magnesium themselves
15-30 min	Arrangement and group	2,8,1 matches group 1 and 2,8,7 matches group 17	Match four elements to their groups
30-39 min	Ions and isotopes	Magnesium loses 2 to give Mg2+ and oxygen gains 2 to give O2-, then average atomic mass	Write two ion symbols
39-42 min	Independent practice	Three questions on arrangement, neutrons and ion charge; tour asking where they started	Write the atomic number first every time
42-45 min	Exit check	Collect answers and ask how many outer electrons group 2 has	Two outer electrons

03 Teaching script (read it out)

Scene 1: The atomic number is the only number to count

When you meet an element, find its atomic number first, not its name. In a neutral atom the protons equal the electrons, so the atomic number is the total number of electrons. Sodium has atomic number 11, so eleven electrons must be placed: fill the first shell with 2, the second with 8, and the last one goes into the third shell, written 2,8,1. Require every student to write the atomic number beside the working, because that habit decides whether the rest of the question is right.

Students say which number they started from

Scene 2: The outer shell count is the group number

The periodic table is a map of arrangements, not a list to memorise. Group 1 has one outer electron, group 2 has two, group 17 has seven and group 18 has eight, with helium in period 1 as the exception at two. So 2,8,2 is magnesium in group 2, and 2,8,7 is chlorine in group 17. Then reverse it: ask what the last number of the arrangement must be for an element in group 16.

Students work backwards from a group to the outer shell count

Scene 3: Metals lose electrons, non-metals gain them

Elements with one to three outer electrons tend to lose them and become positive ions, while those with six or seven tend to gain and become negative. Magnesium 2,8,2 loses two, leaving 2,8, so twelve protons against ten electrons gives Mg2+. Oxygen 2,6 gains two, giving 2,8, so ten electrons against eight protons gives O2-. Isotopes need their own sentence: the same proton count means the same element with a different neutron count, so chlorine-35 and chlorine-37 are both chlorine and average to 35.5.

Students say whether the charge is positive or negative and why

04 Question types and worked steps

Type	Example	Steps	Answer
Electron arrangement	Write the electron arrangement of chlorine, atomic number 17.	1) Note the atomic number 17. 2) Put 2 in the first shell and 8 in the second, leaving 7. 3) Write 2,8,7	2,8,7
Neutron count	How many neutrons are in the nucleus of sodium-23?	1) The mass number 23 is protons plus neutrons. 2) Atomic number 11 means eleven protons. 3) Subtract	Twelve neutrons
Average atomic mass	Chlorine is three quarters chlorine-35 and one quarter chlorine-37. What is the average atomic mass?	1) Three times 35 plus 37 gives 142. 2) Divide by four	35.5
Ion formation	What ions do magnesium (12) and oxygen (8) form?	1) Write the arrangements 2,8,2 and 2,6. 2) Magnesium loses two, oxygen gains two. 3) Compare protons and electrons	Mg2+ and O2-

Sanity check (teach this): Three checks: first, the mass number, since protons plus neutrons must equal it and the difference must be a whole number; second, the arrangement, because shells fill left to right and the last shell never exceeds eight outside the first; third, the charge, which equals protons minus electrons and must be written with its sign. Numeric anchors: 11 equals 2 plus 8 plus 1, and 23 minus 11 is 12; three times 35 plus 37 gives 142, and 142 divided by 4 is 35.5; the sodium ion has eleven protons against ten electrons, a difference of one, hence a charge of plus one.

05 Independent practice (3 questions)

Question	Answer	What to watch
Write the electron arrangement of magnesium, atomic number 12.	2,8,2	Check nobody squeezes all twelve electrons into the second shell
How many neutrons are in potassium-39, atomic number 19?	20	Check they subtracted 19 rather than 20
What charge does sulfur, atomic number 16, carry as an ion?	2 minus, having gained two electrons	Check gaining electrons was not written as a positive charge

06 Common mistakes and what to say

Mistake	Student writes	Say this
More than eight in the outer shell	Magnesium written as 2,10	Outside the first shell, cap the outer shell at eight and open a new one
Mass number treated as atomic number	Neutrons in sodium given as 23	The atomic number is the proton count; protons plus neutrons give the mass number
Isotopes treated as different elements	Chlorine-35 and chlorine-37 called two elements	The same proton count means the same element, only the neutrons differ
Ion sign reversed	Magnesium ion written as Mg2-	Metals lose electrons so the charge is positive: Mg2+

07 Homework (paper, section and question numbers)

States of matter and the particle model, question 3 — Same habit of explaining the visible world at particle level
Separating mixtures, question 2 — Elements, compounds and mixtures must be kept apart
Combustion and oxidation, question 4 — Oxidation questions track where the atoms went

08 Exit check (last 5 minutes)

Question	Answer	Decision
What is the electron arrangement of oxygen, atomic number 8?	2,6	Answering 2,8 means eight was used as if it were ten
An element has arrangement 2,8,1. Which group is it in?	Group 1	Answering group 3 means shells were counted instead of outer electrons
What is the symbol of the chloride ion?	Cl minus	Answering Cl plus means gaining an electron was read as losing one

09 Tutor FAQ

Must the whole periodic table be memorised?

At junior level only the first twenty symbols and atomic numbers, and even those follow from the arrangement rules. The rules matter, not the table.

Why is the arrangement 2, 8, 8?

A shell holds up to 2n squared electrons, so 2, then 8, then 18. Junior work never uses more than eight in the outer shell, which is why the third shell is capped at eight.

How deep should isotopes go?

Three points are enough: same protons, different neutrons, and the average atomic mass is a weighted average computed from simple fractions.

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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.