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Asked in GUJCET 2020 · Nuclear radius and density
Given: gold is ¹⁹⁷₇₉Au and an α-particle is a ⁴₂He nucleus.
Idea: a nuclear mass is very nearly the mass number times one atomic mass unit, so two nuclei compare in mass as their mass numbers do.
(197)/4=49.25.
So the gold nucleus is about 50 times heavier than an α-particle — which is why it barely recoils when an α-particle is scattered by it.
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