Across a period the outer electrons go up from 1 to 8; then a new shell starts and the count goes back to 1, so a new period begins and the properties repeat. The period number is the number of shells. Across a period, elements change from metals to non-metals.
Mendeleev arranged the elements in 1869 and left gaps for ones nobody had found. Years later they were discovered, just as he had predicted. Electrons explain why his pattern worked, but nobody knew about electrons yet.
Elements in a group have the same outer electrons. This lesson asks why the table goes in rows, and why the pattern repeats.
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: after neon
ExploreTry it on the board and compare what happens with your prediction.
Explore: across and down
The colour shows how many outer electrons each element has. Click each element to see its shells; the outer shell is orange.
The colour now shows how many shells each element uses.
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Click across period 2 from lithium to neon, then sodium. Watch the outer electrons.
Use the board. This ticks when you get there.
Now the table is coloured by shells. Click the first element of each period: H, Li, Na, K.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: why it repeats
In your book: draw period 2 as a row of eight boxes with each element's shells (2, 1 up to 2, 8). Under it, write sodium's shells and explain why it goes under lithium.
An element has electrons in shells 2, 8, 8, 2. Which period is it in?
Which group? Use the outer electrons.
2, 1
2, 8, 8, 1
2, 7
2, 8, 7
2, 8
2, 8, 8A full outer shell of 8: a noble gas.
Elaborate TypicalUse the same idea in a new context.
Elaborate: metals to non-metals
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Click across period 3 from sodium to argon.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: Mendeleev's gaps
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Find the two metalloids in the first 20 elements: they sit on the border between the metals and the non-metals.
Use the board. This ticks when you get there.
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
Each period fills one electron shell. The period number is the number of shells an element uses.
Across a period, the outer electrons go up by 1 each time, from 1 to 8 (a full shell). Then a new shell starts and the count goes back to 1.
Because the outer electrons repeat, properties repeat: elements in the same group behave alike.
Across a period, elements change from metals (few outer electrons, easily lost) to non-metals (nearly full shells).
For teachers
Students explain that a new period begins when a new shell starts filling, that properties repeat because outer-electron counts repeat, and read an element's shells from its period and group (AC9S10U06, node 3). About 25 minutes.
I can explain why properties repeat across the periods.
I can use an element's period and group to give its electron arrangement.
Before class
A concept tutorial for introducing or revising the idea. Set the width with ?width=minimum, typical (default) or wider. It builds on Shells and Groups (nodes 1 and 2).
Not a hinge node: it pulls shells and groups together into the table's shape. The prediction catches students who expect outer electrons to keep counting up (sodium with 9).
Board notes: the table shows elements 1 to 20 only, in groups 1, 2 and 13 to 18 (transition metals start in period 4, after calcium). Colour by outer electrons counts helium's 2 as a full shell (with the noble gases). The metalloids shown are boron and silicon. Shells fill 2, 8, 8, 2 for these elements; the 8 in the third shell is a simplification that stops working after calcium.
Running the lesson
Engage: tally 1 / 9 / 0 / 3. The 9s are the misconception the board fixes.
Explore: ask where the colour resets, and what the shell picture shows at that moment.
Explain: the period, group and shells questions are the node's success criteria.
In your own words: the period 2 row with sodium under lithium is the evidence.
The hard part (on purpose)
Seeing why the count resets. Ask how many electrons fit in the shell before you tell them; let them find that sodium needs a new one.
When students get stuck
A student thinks sodium has 9 outer electrons. Ask: How many electrons fit in the second shell? Fix: 8. It is full at neon, so sodium's last electron starts a third shell: 2, 8, 1.
A student mixes up periods and groups. Ask: Which way do rows go, and what do they share? Fix: A period is a row: same number of shells. A group is a column: same number of outer electrons.
A student thinks the table is ordered by mass. Ask: What goes up by exactly 1 each step along the table? Fix: The atomic number (protons, and electrons in a neutral atom). Mass mostly rises too, but not always in step (argon is heavier than potassium).
Stretch and support
Stretch: Argon (atomic number 18) has a larger relative atomic mass than potassium (19). If Mendeleev had ordered strictly by mass, what would have gone wrong? (Argon would land in group 1 and potassium in group 18: the reverse of how they behave.)
Support: Give the frame: 'Period = number of … . Group = number of … electrons. So an element in period … and group … has shells … .'
Where it leads
Next: node 4 (predicting and explaining an element's properties from its structure) is a class task: give students unfamiliar elements in groups 1, 2, 17 and 18 and ask them to predict reactions.
In class: element cards sorted into a table without labels; Mendeleev's predictions for eka-silicon against germanium's measured properties.
What each step is for
Engage: after neon: Asking for sodium's outer electrons after neon's 8 tests whether students expect the count to keep rising (9) or to reset as a new shell starts.
Explore: across and down: Colouring the table by outer electrons, then by shells, shows the two patterns students need: the count rising across each row and resetting, and each new row starting a new shell.
Explain: why it repeats: Tying the period to the number of shells and the group to the outer electrons lets students read an element's position as its electron arrangement, and explain repeating properties.
Elaborate: metals to non-metals: Following period 3 from sodium to argon links metallic character to outer electrons: few to lose on the left, nearly full shells to fill on the right.
Elaborate: Mendeleev's gaps: Mendeleev's table worked as a pattern before electrons explained it: a pattern can be used to predict before it is understood.
Now you can
Explain why a new period starts.
Explain why properties repeat across the periods.
Use an element's period and group to give its electron arrangement.