Periodic Reactivity Patterns

Overview

Students link the Bohr model (electron shells) to how elements are organised in the periodic table, and use this to explain why families (groups) show similar reactivity. This lesson builds toward describing repeating patterns across the periodic table by using atomic structure to justify trends.

Learning intentions

Students will be able to:

• explain how atomic number, number of electron shells, and outer-shell electrons relate to an element’s position on the periodic table
• use the Bohr model to describe electron shells and identify how many valence (outer-shell) electrons an atom has
• explain how the position in the periodic table relates to reactivity, with reference to alkali metals and halogens

Success criteria

Students can:

• I can describe how atomic number and electron arrangement place elements in the periodic table
• I can represent an atom using the Bohr model and state the number of outer-shell electrons
• I can explain why elements in the same group have similar reactivity using shell and outer-electron patterns
• I can compare alkali metals and halogens by linking reactivity to outer-shell electron number

Curriculum links

• AC9S10U06: explaining how atomic structure and properties relate to the organisation of elements in the periodic table, using Bohr-model electron shells to connect to element position and properties
• AC9S10I04: selecting and constructing representations (e.g., Bohr diagrams and tables of shell/outer-electron patterns) to organise and process information
• AC9S10I05: analysing patterns in the periodic table (groups) and connecting them to trends in outer-shell electrons, noting any anomalies or limits of the model

Lesson structure (40 minutes)

1. 0–5 min · Hook (pattern prompt). Teacher displays a simple periodic table outline with highlighted Group 1 (alkali metals) and Group 17 (halogens) and asks, “What do you think both groups have in common that could affect reactivity?” Students do a quick think-pair-share and record one guess about similarities.

2. 5–12 min · Direct teach (Bohr model basics). Teacher teaches/recaps: electrons occupy shells; atomic number gives total electrons for neutral atoms; outer-shell electrons are linked to chemical behaviour; use one example (e.g., sodium and chlorine) to build Bohr diagrams step-by-step. Students complete a Bohr diagram for one provided atom template, filling shells and identifying outer-shell electrons.

3. 12–22 min · Guided investigation (group pattern match). Teacher gives each group a “reactivity pattern card set” containing: element name, atomic number, and a partially completed shell/electron data table plus prompts. Students must finish the Bohr diagrams, then fill a table: (atomic number → number of shells → outer-shell electrons → likely relative reactivity trend). Students work in pairs or triads to match elements into columns by group and explain the similarity using outer-shell electrons.

4. 22–30 min · Whole-class reasoning (alkali vs halogen). Teacher facilitates discussion: “Why are alkali metals highly reactive and halogens also reactive, but in different ways?” using the outer-shell electron idea (alkali metals: one outer electron; halogens: near a full outer shell). Students share one explanation sentence each, using the sentence frame: “Because the atom has ___ outer-shell electrons and ___ shells, it is positioned in ___ group, so it shows ___ reactivity.”

5. 30–36 min · Representation-to-claim check (mini task). Teacher provides three blank periodic table squares with atomic numbers only, and students must: predict the group (by estimating number of outer electrons from shell count pattern) and write a short claim about relative reactivity. Students complete individually, then compare answers with a partner.

6. 36–40 min · Exit ticket (assessment). Teacher collects an exit ticket with two prompts:

• Draw a Bohr model for an atom with a specified atomic number and state outer-shell electrons.
• Explain, using shell/outer-electron language, how position on the periodic table relates to reactivity (one alkali or one halogen example). Students answer independently.

Resources

• Printed Bohr model shells templates (e.g., 1–4 shells) and electron counters or drawn electron dots
• Periodic table outline (blank + a version with Group 1 and Group 17 labelled)
• Reactivity pattern card set for small groups (element cards + shell/outer-electron table)
• Sentence starters/frames for explanations
• Mini task periodic table squares with atomic numbers
• Exit ticket slips and pens
• Projector/interactive board to show model steps

Assessment

• Formative: teacher circulates during Bohr diagram construction, checking whether students correctly use atomic number to determine total electrons and whether they correctly identify outer-shell electrons
• Formative: observe group discussions and listen for correct links between outer-shell electron number, group position, and reactivity claims
• Summative within lesson: exit ticket assessing correct Bohr representation and an explanatory paragraph using DO language (reactivity related to position, number of shells, and outer-shell electrons)

Differentiation

• Support: provide worked examples for sodium and chlorine, plus a “shells guide” showing how many electrons fit in each shell for common Year 10 atoms used in the task (e.g., up to 20 or similar set)
• Support for writing: use sentence frames that include: “outer-shell electrons,” “number of shells,” “periodic table position/group,” and “reactivity”
• Extension: ask students to identify one limit of the Bohr model (it predicts electron arrangement but real reactivity can depend on additional factors) and to state what the model does and does not explain
• EAL/SEN: allow oral explanation to be recorded by the student on a partially completed explanation sheet; provide a word bank (reactivity, outer shell, group, valence electrons, atomic number)