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Four carbons: a family of growing chains

With a 3D pen, Grade 8 pupils trace three identical carbon nodes with H rings, two bridges and two terminal H rings. On the track they build methane, ethane and propane, adding a node and moving the terminal H to the new end. They count atoms, record formulae and names, draw the structural formula of propane and derive the general formula of alkanes.

Age
13–15 years
Length
approx. 45 min
Level
Medium
Where you teach
School
Purpose
Educational
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How to run this session

Stages and timing
  1. Node 1: carbon with four bonds
  2. Bridges and terminal H
  3. Socket fit check in pairs
  4. Nodes 2 and 3: pen work ends
  5. Methane: one carbon
  6. Ethane: add a carbon
  7. Propane and a rule for the next alkane
  8. Check yourself without the model
  9. For early finishers: butane with a fourth node

What gets built

A detachable PLA carbon chain (3 nodes, 2 bridges, 2 terminal H rings; 4 nodes and 3 bridges in the extension), repeatedly rebuilt from methane to propane, showing four connections at every carbon. W2 contains four formulae and names, the structural formula of propane, the CH2 increment, a comparison of terminal and internal carbons, and the general formula.

What it practises
  • Recording molecular and condensed structural formulae and names of straight-chain alkanes with up to four carbon atoms.
  • Drawing an alkane's structural formula from a model of its connections.
  • Deriving the general formula of the alkane homologous series from successive formulae.
  • Drawing a notched circle and small rings over a template with a 3D pen without flooding the open socket.

Success criteria
  • Three carbon nodes, two bridges and two terminal H rings lie separately, and the bridge enters the socket without force.
  • Methane, ethane and propane are built in turn on W2, with no socket left empty.
  • W2 contains four molecular and condensed structural formulae with names and the structural formula of propane.
  • The pupil explains what is added whenever the chain is extended and records the formula of an alkane with a given number of carbons without using the model.

What you need
Filament
PLA
Colours
pupil's colour (example)
Also
3D pen with stand (one per pupil), transparent mat, W1: full-size (1:1) node, bridge and terminal H template (provided printout, print at 100 %), W2: chain track and formula sheet (provided printout, A4 landscape, print at 100 %), pencil, ruler (one per pair)

Files

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Version edb2c62d-55e2-4342-9524-481c19c95eb4 Updated 2026-10-14

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