The ratio of the nuclear radii of elements with mass numbers $216$ and $125$ is

Assertion $(A):$ Forces acting between proton-proton $(f_{pp})$, proton-neutron $(f_{pn})$ and neutron-neutron $(f_{nn})$ are such that $f_{pp} < f_{pn} = f_{nn}$.
Reason $(R):$ Electrostatic force of repulsion between two protons reduces the net nuclear force between them.

Which of the following is most unstable?

The mass number of an iodine nucleus and a polonium nucleus is $125$ and $216$ respectively. The ratio of the radius of the iodine nucleus to that of the polonium nucleus is:

Briefly explain the discovery of neutrons and how it can be demonstrated.

The particles which can be added to the nucleus of an atom without changing its chemical properties are called: