Open with the question on the board: "How do you figure out the shape of something you cannot see?" Hold up a sealed shoebox with an unknown object rattling inside. Shake it, tilt it, roll it. Ask students to shout out guesses about what's inside and, more importantly, WHAT EVIDENCE led to each guess (sound, weight, how it slides). After ~90 seconds of guessing, tell them: "You just did science. In 1911, Ernest Rutherford had the same problem — he wanted to know the inside of an atom, which is a hundred million times smaller than this box. He couldn't open it either. So he threw things at it and watched how they bounced. Today you're doing his experiment." Frame the investigative question written on the board: __If most of your marbles pass straight through but a few bounce back at sharp angles, what must be hidden underneath?__

1. 5m

   Where atomic theory stood in 1911 — the plum pudding model

   Content

   Before Rutherford's experiment, the accepted picture of the atom was J.J. Thomson's plum pudding model, proposed in 1904. Thomson had discovered the electron in 1897 using a cathode ray tube — he showed that the mysterious ray bending toward the positive plate was a stream of negatively charged particles much lighter than any atom. Since atoms overall are neutral, Thomson reasoned there must also be positive charge somewhere. His model: a diffuse, spread-out ball of positive charge with tiny negative electrons embedded throughout, like raisins in a pudding or chocolate chips in a cookie. Crucially, in this model there is no dense center — positive charge and mass are smeared evenly across the whole atom. This matters because it makes a testable prediction: if you shoot something small and fast at an atom, it should pass through with only tiny deflections, because there's no concentrated mass anywhere to bounce off of. This is what Rutherford EXPECTED to see.

   Delivery

   Emphasize that Thomson was not stupid or wrong for its own sake — his model FIT his evidence (cathode rays showed electrons exist; atoms are neutral, so positive charge must be there somewhere). Head off the misconception that "each old scientist was just wrong" — every model was the best available fit for the data at the time. Ask students: "If I shoot a BB at a chocolate chip cookie, what happens?" (goes right through, tiny wobble). That's the prediction we're about to test.

2. 5m

   Pre-lab briefing — how our marble model maps to Rutherford's experiment

   Content

   In today's lab, each group gets a raised platform (a piece of foam board or cardboard supported on books) with an unknown shape taped to its underside — a coin, a washer, a triangle of cardboard, etc. Students cannot lift the platform or peek. They roll marbles from the edge, aim across the surface underneath, and record whether each marble (a) passes straight through, (b) is deflected at a small angle, or (c) bounces back sharply. In Rutherford's real experiment the marbles are alpha particles (helium nuclei, positively charged), the platform is the thin gold foil, and the hidden shape is the nucleus. The mapping: straight-through marble = alpha particle passing through empty space; small deflection = alpha grazing near the nucleus and being pushed by its positive charge; bounce-back = alpha hitting the nucleus nearly head-on. Rutherford famously said it was "as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you." That single observation destroyed the plum pudding model.

   Delivery

   Walk through the procedure verbally — do NOT let students peek under the platform, that ruins the whole point. Emphasize aiming from many different starting positions across the front edge so they sample the whole target area (Rutherford's alphas hit the foil everywhere, not one spot). Remind them: count EVERY roll, including the boring straight-through ones — the ratio of hits to misses is what tells you the size of the nucleus. Assign roles: one roller, one recorder, one observer who calls out the deflection category.

1. 35m

   Marble-and-Hidden-Target Gold Foil SimulationLab

   Setup (teacher does before class): For each group, tape ONE hidden target shape to the underside of the foam board so students cannot see it. Vary the shapes across groups (some coins, some washers, some cardboard triangles) so groups can compare results later. Raise the foam board on two textbook stacks so there is a ~2 cm gap between the board and the desk — marbles will roll UNDER the board. Mark a launch line with masking tape along one edge of the desk. Students do NOT lift the board. Grouping: groups of 3 — one roller, one recorder, one observer. Run: Distribute the handout below. Give students 25 minutes to collect data (50 rolls minimum per group), then 10 minutes to sketch their inferred target and answer the analysis questions. Walk around and check that groups are rolling from DIFFERENT starting positions along the launch line (not all from the same spot) — this is the most common error and it kills their data. If a group finishes early, have them predict what shape their neighbors have based on the neighbors' deflection counts. Student handout: Rutherford's Gold Foil — Marble Simulation Names: __________ Group #: ____ Target letter: ____ Part 1 — The investigative question A hidden shape is taped to the underside of your platform. You cannot look. Using ONLY the pattern of how marbles deflect when rolled underneath, determine the size, shape, and position of the hidden object. Part 2 — Procedure - Line up at the masking-tape launch line. - Roll a marble at a steady speed under the platform toward the far edge. - The observer watches where the marble exits (or if it bounces back). - The recorder marks one tally in the correct column of the data table below. - Move ~2 cm along the launch line and roll again — sample the WHOLE width. - Complete at least 50 rolls before analyzing. Part 3 — Data table - Straight through (no deflection): __________ tallies, total = ____ - Small deflection (< 45°): __________ tallies, total = ____ - Large deflection (> 45°) or bounce-back: __________ tallies, total = ____ - TOTAL ROLLS: ____ Part 4 — Sketch your inferred target On the grid below (draw a rectangle representing the platform viewed from above), mark where you think the hidden shape is and estimate its size and shape. Justify with your data. My inferred shape: __________ My reasoning: __________ Part 5 — Analysis questions 1. What percentage of your marbles passed straight through? Show your calculation. 2. What does that percentage tell you about how much of the platform area is actually covered by the hidden target? 3. In Rutherford's real experiment, about 1 in 8000 alpha particles bounced back sharply. Based on that ratio, was the nucleus a large fraction of the atom or a tiny fraction? Explain. 4. Thomson's plum pudding model predicted that ALL alpha particles would pass straight through with only tiny wobbles. Which of your data — straight-throughs, small deflections, or bounce-backs — is the evidence that most directly disproves plum pudding? Why? 5. Rutherford said the bounce-backs were "as if you had fired a 15-inch shell at a piece of tissue paper and it came back at you." In your own words, why was he so shocked? Part 6 — Reveal and reflect (last 3 minutes) Your teacher will now let you flip the platform. Compare your inferred shape to the real one. - How close were you? __________ - What could you have done to get a better inference? __________

   Materials

   • Foam board or stiff cardboard (~30 cm × 40 cm) per group — 1
   • Textbooks or wood blocks to raise the platform ~2 cm off the desk — 2 per group
   • Assorted hidden target shapes (large coin, metal washer, triangle of cardboard, small rectangle) — 1 per group, taped to underside of platform BEFORE class
   • Glass or steel marbles, 15 mm — 5 per group
   • Masking tape to mark launch line — 1 roll
   • Ruler and protractor per group
   • Student handout (below) — 1 per student

   Example outputs

   • Group with a large coin taped under: ~44 straight-through, ~4 small deflection, ~2 bounce-back out of 50. Inferred: "small round object near the center — because bounce-backs only happened when we rolled from positions 3-5 of 10 along the launch line." Analysis Q1: 44/50 = 88% pass through, meaning the target covers roughly 12% of the platform width in the rolling path.
   • Group with a metal washer: ~40 straight-through, ~7 small deflection, ~3 bounce-back — student notes that small deflections outnumber bounce-backs and infers a ring or hollow shape because "marbles grazed the edge more often than they hit dead center." Analysis Q4 answer: "The bounce-backs prove plum pudding wrong because Thomson said charge was spread out — nothing spread out could push a marble straight back. There has to be something small and hard."

1. In your marble experiment, about 88% of marbles passed straight through, ~8% deflected slightly, and ~4% bounced back. In one or two sentences, what does the 4% bounce-back rate tell you about the hidden target?

   short answerThe bounce-backs mean there is a small, dense, solid object somewhere under the platform. Because only 4% bounced back, the object must be small compared to the total area — most of the space under the platform is empty.

2. Which observation from the real gold-foil experiment most directly disproved Thomson's plum pudding model? A) Most alpha particles passed straight through the foil. B) A few alpha particles were deflected at very large angles or bounced back. C) The gold foil was extremely thin. D) Electrons were detected in the foil.

   multiple choiceB) A few alpha particles were deflected at very large angles or bounced back. Plum pudding predicted that spread-out positive charge could never push a fast alpha particle backward — only a small, dense, concentrated positive nucleus could do that.

3. Rutherford fired 8,000 alpha particles at a gold foil and about 1 was deflected sharply back. If atoms were solid spheres of positive charge (Thomson's model), what result would he have expected instead? What did the actual result force him to conclude about the structure of the atom?

   short answerThomson's model predicted ALL 8,000 particles would pass through with only tiny wobbles — no bounce-backs at all. The single bounce-back forced Rutherford to conclude that (1) most of the atom is empty space, since most particles went straight through, and (2) there must be a tiny, dense, positively charged nucleus at the center capable of repelling an alpha particle straight back.

4. Order these three atomic models chronologically and, for each, name the ONE piece of experimental evidence that led to it: Dalton's solid sphere, Thomson's plum pudding, Rutherford's nuclear model.

   short answer1. Dalton (1808) — solid indivisible sphere; evidence: law of definite proportions in chemical reactions (elements combine in fixed mass ratios). 2. Thomson (1904) — plum pudding; evidence: cathode ray tube experiment showing negative particles (electrons) exist inside atoms. 3. Rutherford (1911) — nuclear model; evidence: gold-foil experiment showing a few alpha particles bounced back, proving a small dense positive nucleus and mostly empty space.

• ☐Foam board or stiff cardboard platforms, ~30×40 cm — 1 per group of 3
• ☐Textbooks or wood blocks to raise platforms — 2 per group
• ☐Hidden target shapes (large coins, metal washers, cardboard cutouts) — 1 per group, pre-taped BEFORE class
• ☐Glass or steel marbles, ~15 mm — 5 per group
• ☐Masking tape — 1 roll
• ☐Rulers and protractors — 1 each per group
• ☐Student handouts printed — 1 per student
• ☐Sealed shoebox with unknown rattling object (for hook)