$A$ particle having almost zero mass and exactly zero charge is

Consider the following statements:
$I$. All isotopes of an element have the same number of neutrons.
$II$. Only one isotope of an element can be stable and non-radioactive.
$III$. All elements have isotopes.
$IV$. All isotopes of carbon can form chemical compounds with oxygen-$16$.
Choose the correct option regarding an isotope.

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

The graph of $\ln \left(\frac{R}{R_0}\right)$ versus $\ln A$ is,where $R$ is the radius of a nucleus,$A$ is its mass number,and $R_0$ is a constant.

The radius of a nucleus with atomic number $Z = 50$ is $9 \times 10^{-13} \ m$. The electric potential at its surface is:

The atomic number of a nucleus is $Z$ and the atomic mass number is $M$. The number of neutrons is