Atomic models change when evidence challenges predictions
| English | Français |
|---|---|
| alpha particle/ˈælfə ˈpɑːtɪkl/ | particule alpha |
| nuclear model/ˈnjuːklɪə ˈmɒdl/ | modèle nucléaire |
What would explain this observation?
- Most alpha particles 阿尔法粒子 passed straight through thin foil, a few changed direction and very few returned backwards. A useful model must account for all three observations.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Before electrons were discovered, atoms were pictured as tiny indivisible spheres. Electron discovery led to the plum pudding model: negative electrons embedded in diffuse positive charge. Alpha-particle scattering challenged this distribution. Most particles passed through, consistent with an atom that is mostly empty space; rare large deflections showed mass and positive charge concentrated in a tiny central nucleus. Positive alpha particles are repelled by a positively charged nucleus.
- nuclear model 核式模型: An atomic model with mass and positive charge concentrated in a tiny central nucleus; alpha particle: A positively charged particle consisting of two protons and two neutrons.
What best explains rare large deflections of positive alpha particles?
The nuclear model replaced diffuse positive charge with a dense nucleus and electrons outside it. Bohr added electrons at specific distances or energy levels; his theoretical predictions agreed with observations. Later work identified positively charged protons. Chadwick provided evidence for uncharged neutrons in the nucleus about twenty years after the nuclear idea became accepted. A scientific model can develop further without every earlier observation becoming wrong.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- The nuclear model replaced diffuse positive charge with a dense nucleus and electrons outside it. Bohr added electrons at specific distances or energy levels; his theoretical predictions agreed with observations. Later work identified positively charged protons. Chadwick provided evidence for uncharged neutrons in the nucleus about twenty years after the nuclear idea became accepted. A scientific model can develop further without every earlier observation becoming wrong.
- Use supplied scattering observations or a computer-free diagram, not a classroom radioactive source. Match each observation to an inference and compare it with the diffuse-charge prediction. Distinguish observed trajectories from inferred structure. A marble analogue illustrates deflection but cannot establish electric charge or reproduce quantum behaviour. Detailed experimental work behind Bohr and Chadwick is outside this specification requirement.
Which two habits make the investigation or model in this case more defensible?
Use supplied scattering observations or a computer-free diagram, not a classroom radioactive source. Match each observation to an inference and compare it with the diffuse-charge prediction. Distinguish observed trajectories from inferred structure. A marble analogue illustrates deflection but cannot establish electric charge or reproduce quantum behaviour. Detailed experimental work behind Bohr and Chadwick is outside this specification requirement.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: in a fictional sample of 20,000 tracks, 19,800 pass approximately straight, 190 deflect and 10 return backwards. Straight-through percentage = 19,800/20,000×100 = 99%; backwards percentage = 10/20,000×100 = 0.05%. Rare events can be decisive evidence, but these counts are illustrative and are not historical measurements or a direct nucleus-size calculation.
In an illustrative sample, 12 of 24,000 alpha tracks return backwards. Calculate the percentage. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
In an illustrative sample, 12 of 24,000 alpha tracks return backwards. Calculate the percentage.
The result is 0.05 %. Known: in a fictional sample of 20,000 tracks, 19,800 pass approximately straight, 190 deflect and 10 return backwards. Straight-through percentage = 19,800/20,000×100 = 99%; backwards percentage = 10/20,000×100 = 0.05%. Rare events can be decisive evidence, but these counts are illustrative and are not historical measurements or a direct nucleus-size calculation.
Check the conclusion and its limits
- Most particles passing through does not mean atoms have no mass. Backscattering is rare because the dense nucleus occupies little space, not because most nuclei are negatively charged. Shell diagrams are models and do not show electrons following observable miniature planetary tracks.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Only the most common observation matters when testing a scientific model. This claim is false: Most particles passing through does not mean atoms have no mass. Backscattering is rare because the dense nucleus occupies little space, not because most nuclei are negatively charged. Shell diagrams are models and do not show electrons following observable miniature planetary tracks.
Atomic models change when evidence challenges predictions: The nuclear model replaced diffuse positive charge with a dense nucleus and electrons outside it. Bohr added electrons at specific distances or energy levels; his theoretical predictions agreed with observations. Later work identified positively charged protons. Chadwick provided evidence for uncharged neutrons in the nucleus about twenty years after the nuclear idea became accepted. A scientific model can develop further without every earlier observation becoming wrong.
Only the most common observation matters when testing a scientific model.
Most particles passing through does not mean atoms have no mass. Backscattering is rare because the dense nucleus occupies little space, not because most nuclei are negatively charged. Shell diagrams are models and do not show electrons following observable miniature planetary tracks.
An atomic model with mass and positive charge concentrated in a tiny central nucleus: write the technical term.
nuclear model means An atomic model with mass and positive charge concentrated in a tiny central nucleus.