Question

compare two atomic models. how did evidence from one model show the need for a new model?

Explanation:

We can compare J.J. Thomson's "plum pudding" atomic model and Ernest Rutherford's nuclear atomic model. Thomson's model proposed atoms were diffuse, positively charged spheres with embedded electrons. Rutherford's gold foil experiment (1911) provided conflicting evidence: when alpha particles were fired at thin gold foil, most passed through, but a small fraction deflected at large angles or bounced back. This could not be explained by the uniform, diffuse structure of the plum pudding model, proving atoms must have a dense, positively charged central nucleus (with electrons orbiting it) instead.

Answer:

Taking J.J. Thomson's plum pudding model and Ernest Rutherford's nuclear model as an example:

1. Thomson's model described atoms as a diffuse, positively charged sphere with electrons embedded within it, like plums in pudding.
2. Rutherford's gold foil experiment provided evidence that contradicted this model: when positively charged alpha particles were fired at a thin gold foil, most passed through, but a small number were deflected at large angles or even bounced back.
3. This deflection could only be explained if the atom's positive charge and most of its mass were concentrated in a tiny central nucleus, rather than being spread out evenly. This experimental evidence directly showed the need to replace the plum pudding model with a new nuclear atomic model.