Elements in the same group have the same number of outer-shell electrons, so they react in similar ways. Group 1 metals lose their one outer electron easily; group 17 non-metals gain one easily; group 18 has full shells and hardly reacts.
Lithium, sodium and potassium look nothing alike in a jar, yet drop any of them in water and they fizz across the surface. Why do they behave the same?
Last lesson: electrons fill shells. This lesson uses the outer shell to read the periodic table.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: three fizzing metals
ExploreTry it on the board and compare what happens with your prediction.
Explore: group 1
Click an element to see its shells. The outer electrons are orange.
Try it yourselfYour edits are saved in this browser.
Look at lithium, sodium and potassium. Count the outer electrons of each.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: outer electrons decide
In your book: explain why lithium and potassium react in similar ways, but neon and sodium do not. Use outer electrons in your answer.
Which group does each belong to?
Lithium (2, 1)
Magnesium (2, 8, 2)
Fluorine (2, 7)
Chlorine (2, 8, 7)
Neon (2, 8)
Argon (2, 8, 8)
Potassium (2, 8, 8, 1)
Elaborate TypicalUse the same idea in a new context.
Elaborate: halogens and noble gases
Try it yourselfYour edits are saved in this browser.
Look at fluorine and chlorine (group 17), then helium, neon and argon (group 18).
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: down the group
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Elements in the same group have the same number of outer-shell electrons, so they react in similar ways.
Group 1 (alkali metals): one outer electron, easily lost: very reactive metals.
Group 17 (halogens): seven outer electrons, one short of full: very reactive non-metals.
Group 18 (noble gases): full outer shells: hardly react.
The period (row) tells you how many shells an atom has.
For teachers
Students explain why elements in the same group have similar chemical properties using outer-shell electrons, contrasting group 1, group 17 and the noble gases (AC9S10U06, node 2, a hinge node). About 26 minutes.
I can explain that elements in the same group have the same number of outer-shell electrons.
I can use outer electrons to explain the properties of a group.
I can contrast a reactive group with the noble gases.
Before class
A concept tutorial for introducing or revising the idea, after How are the electrons arranged? (node 1). Set the width with ?width=minimum, typical (default) or wider.
The hinge: students learn group properties as lists ("alkali metals are reactive") without the reason. Every question here asks for the outer-electron explanation.
Board notes: the first 20 elements only, by group (1, 2, 13 to 18) and period. Each element's nucleus shows its commonest isotope. Hydrogen sits above group 1 with a note that it is a non-metal.
Running the lesson
Engage: tally the options; 'same total electrons' is the misconception to address.
Explore: ask a student to read out the shell arrangements of Li, Na and K: the last number is the same.
Explain: the group 17 prediction is the node's checkpoint: position to outer electrons to behaviour.
The hard part (on purpose)
Linking behaviour to outer-shell electrons, not size, state or total electrons. When a student says noble gases don't react because they are gases, ask whether chlorine is a gas.
When students get stuck
A student says noble gases don't react because they're gases. Ask: Is chlorine a gas? Is it reactive? Fix: Yes and yes: state isn't the reason; a full outer shell is.
A student says group number equals the total electrons. Ask: How many electrons does sodium have? Which group is it in? Fix: 11 electrons, group 1: the group matches the outer electrons (for groups 1 and 2; groups 13 to 17 have 3 to 7).
Stretch and support
Stretch: Predict the behaviour of rubidium, below potassium in group 1. What would you expect when it touches water?
Support: Give a blank first-20 table to colour by number of outer electrons.
Where it leads
Next: periodic trends (node 3); predicting the properties of an unfamiliar element from its position is a class task (node 4).
In class: a teacher demonstration of lithium, sodium and potassium in water (video if the school does not hold them).
What each step is for
Engage: three fizzing metals: Students who think similar behaviour comes from similar size or total electrons pick a distractor; the table shows outer electrons as the shared feature.
Explore: group 1: Clicking down a column and seeing the same single orange electron each time makes the group's shared feature visible.
Explain: outer electrons decide: Using outer-electron counts to place and explain whole groups (1, 2, 17, 18) is the bridge from atomic structure to the table's organisation.
Elaborate: halogens and noble gases: A second and third group on the same board, one reactive and one not, show the principle working across the table, not just in group 1.
Elaborate: down the group: A trend within a group (reactivity rising down group 1) extends the model: the outer electron's distance from the nucleus matters too.
Now you can
Explain that elements in the same group have the same number of outer-shell electrons.
Use outer electrons to explain the properties of a group.