Assertion (A): Rutherford concluded that most of the mass of an atom is concentrated in a small region at the centre called the nucleus. Reason (R): According to Thomson’s model, electrons are embedded in a uniformly distributed positive charge sphere. Choose the correct option: (i) Both A and R are

Question

Accepted Answer

The correct option is (ii) Both A and R are true, but R is not the correct explanation of A. A is true. From the gold foil experiment Rutherford concluded that all the positive charge and most of the mass of an atom sit in a tiny central region, the nucleus. The evidence: only a heavy, concentrated target can turn a fast α-particle through a large angle. R is true. That is a correct statement of Thomson's plum pudding model — electrons embedded in a uniform sphere of positive charge. But R does not explain A. R only tells you what the earlier model said. Thomson's model does not lead to the nucleus — it contradicts it. What actually explains A is the observation : most α-particles passed straight through (so the atom is mostly empty) while a few were deflected through large angles or bounced back (so the mass and charge are concentrated). Why it happens: in assertion–reason questions, check three things in order — is A true, is R true, and does R cause or justify A? Two true statements about the same topic are not automatically an explanation. Here R is the model that the evidence overturned, not the reason the evidence points to a nucleus.

NCERT Solutions for Class 9th Science Chapter 8 .2.2 C. Discovery of the proton — Pause and Ponder