The development of the model of the atom
Scientific models change when new experimental evidence cannot be
explained by the old model
Model
What it said
The evidence that led to it
Solid sphere atoms are tiny spheres that cannot be divided
earliest idea, before subatomic particles were known
Plum pudding a ball of positive charge with negative electrons embedded in it
discovery of the electron
Nuclear model mass concentrated in a tiny positive nucleus, electrons around it
alpha particle scattering experiment
Bohr model electrons orbit the nucleus at specific distances (energy levels)
Bohr's calculations agreed with experiment
Protons and neutrons nuclear charge is made of protons; Chadwick found the neutron
later experiments, about 20 years after the nucleus was accepted
The alpha particle scattering experiment : alpha particles were fired
at very thin gold foil
Most went straight through, so the atom is mostly empty space
A few were deflected, so the centre is positively charged
A very small number bounced back, so the mass is concentrated in a
tiny nucleus
These results could not be explained by the plum pudding model, so it was
replaced by the nuclear model
You do not need the details of Bohr's or Chadwick's experiments
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Plum pudding model Nuclear model + + ball of positive charge, spread through the whole atom negative electrons stuck inside it tiny positive nucleus — almost all of the mass electrons orbit at a distance most of the atom is empty space Plum pudding model side by side with the nuclear model, both labelled: positive charge spread through the whole atom vs positive nucleus with electrons outside it
When asked to compare the two models, compare one feature at a time
across both: where the positive charge is, where the electrons are, whether
there is a nucleus. Describing one model and then the other is not a
comparison
Say positive charge , not "positive energy". And in the plum pudding
model the electrons are embedded in the positive ball, not around it
Both models contain electrons; both have no overall charge. A similarity
earns credit too
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