--- title: "Why does the periodic table repeat?" subtitle: "Years 9 and 10 Chemistry · Atoms and the periodic table · Lesson 12" --- # Why does the periodic table repeat? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** 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. :::teacher-note **In one sentence:** 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). **Students should be able to say:** I can explain why a new period starts. I can explain why properties repeat across the periods. I can use an element's period and group to give its electron arrangement. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **Lithium has 1 outer electron. Each element after it has one more, up to neon with 8 (a full shell). Sodium comes next. How many outer electrons does sodium have?** ☐ 0 ☐ 1 ☐ 3 ☐ 9 My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** 1. Neon's second shell is full, so sodium's eleventh electron starts a third shell: 2, 8, 1. With 1 outer electron, sodium starts a new row and behaves like lithium. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: why it repeats :::keep-together ### New period **Why does a new period (row) start at sodium?** ☐ Each row holds exactly 10 elements ☐ Sodium is the first metal ☐ Its electrons start filling a new shell ::: :::teacher-note **Answer:** Its electrons start filling a new shell Each period fills one shell. When a shell is full, the next electron starts a new one, and a new period begins. ::: :::keep-together ### Period from shells **An element has electrons in shells 2, 8, 8, 2. Which period is it in?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 4 Four shells: period 4. (It is calcium, in group 2 with 2 outer electrons.) ::: :::keep-together ### Group from shells **Which group? Use the outer electrons.** Groups: Group 1, Group 17, Group 18 - 2, 1: {{field-15}} - 2, 8, 8, 1: {{field-15}} - 2, 7: {{field-15}} - 2, 8, 7: {{field-15}} - 2, 8: {{field-15}} - 2, 8, 8: {{field-15}} ::: :::teacher-note **Answer:** 2, 1 → Group 1; 2, 8, 8, 1 → Group 1; 2, 7 → Group 17; 2, 8, 7 → Group 17; 2, 8 → Group 18; 2, 8, 8 → Group 18 ::: :::keep-together ### Shells from position **Sulfur is in period 3, group 16. What is its electron arrangement?** ☐ 2, 8, 8, 6 ☐ 2, 8, 6 ☐ 2, 6 ::: :::teacher-note **Answer:** 2, 8, 6 Period 3: three shells. Group 16: six outer electrons. So 2, 8, 6 (16 electrons, atomic number 16). ::: :::keep-together ### Why it repeats **Why do properties repeat from one period to the next?** ☐ The number of outer electrons repeats, and outer electrons decide how an element reacts ☐ The atoms get heavier in a repeating way ☐ Elements were placed to make the pattern look neat ::: :::teacher-note **Answer:** The number of outer electrons repeats, and outer electrons decide how an element reacts Every period runs from 1 outer electron to a full shell. Elements with the same number of outer electrons, in the same column, react in similar ways. ::: :::typical ## Elaborate: metals to non-metals :::keep-together ### Metal or non-metal **Across period 3, the elements change from metals to non-metals. How do outer electrons explain this?** ☐ Atoms on the right are bigger ☐ On the left, atoms have 1 to 3 outer electrons that are easily lost (metals); on the right, nearly full shells gain or share electrons (non-metals) ☐ Metals have no outer electrons ::: :::teacher-note **Answer:** On the left, atoms have 1 to 3 outer electrons that are easily lost (metals); on the right, nearly full shells gain or share electrons (non-metals) Metals lose outer electrons easily. With more outer electrons, losing them all gets harder, and gaining or sharing to fill the shell takes over. ::: ::: :::wider ## Elaborate: Mendeleev's gaps :::keep-together ### Prediction before explanation **Mendeleev ordered elements by mass and grouped them by how they reacted, leaving gaps. He predicted the properties of a missing element below silicon; germanium was found 17 years later (1886) and matched. What does this show?** ☐ Mendeleev knew about electron shells ☐ The table is just a convenient list with no meaning ☐ A pattern can predict new things before anyone can explain it; electrons later explained why it works ::: :::teacher-note **Answer:** A pattern can predict new things before anyone can explain it; electrons later explained why it works Good scientific patterns predict. The explanation (outer electrons) came decades later and showed why the pattern works, and why the table is ordered by atomic number, not mass. ::: ::: ## 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). ## Exit ticket :::keep-together **Element X has electrons 2, 8, 3. Which period and group is it in, and is it more likely a metal or a non-metal?** {{lines-3}} ::: :::teacher-note **Answer:** Period 3 (three shells), group 13 (three outer electrons): aluminium, a metal. :::