Exercise 8.5
Rutherford found that a few α-particles bounced back sharply. How does this single surprising result completely rule out Thomson’s ʻplum pudding modelʼ of the atom?
Not cross-checked
NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.
Because a thinly spread positive charge can never turn a fast α-particle around — and thin spreading is the whole of Thomson's model.
In the plum pudding model the positive charge is smeared evenly over the entire atom, so an α-particle crossing it meets only a weak push at any one point.
The embedded electrons cannot do it either: they are negative (so they attract, not repel) and far too light to reverse a heavy α-particle.
To send a fast, massive, positively charged particle straight back, it must run almost head-on into something that is very small, very massive and strongly positive all at once.
So even one bounce-back is fatal evidence: it proves such a concentrated centre exists, and Thomson's model contains no such thing anywhere.
This single observation is what forced the idea of the nucleus.