$1 \, \text{atomic mass unit (amu)}$ is equal to:

$(a)$ Two stable isotopes of lithium $_{3}^{6} Li$ and $_{3}^{7} Li$ have respective abundances of $7.5 \%$ and $92.5 \%$. These isotopes have masses $6.01512 \; u$ and $7.01600 \; u$,respectively. Find the atomic mass of lithium.
$(b)$ Boron has two stable isotopes,$_{5}^{10} B$ and $_{5}^{11} B$. Their respective masses are $10.01294 \; u$ and $11.00931 \; u$,and the atomic mass of boron is $10.811 \; u$. Find the abundances of $_{5}^{10} B$ and $_{5}^{11} B$.

The distance between two nucleons for which the potential energy is minimum, is (in $fm$)

Two protons are kept at a separation of $40 \text{ Å}$. $F_{n}$ is the nuclear force and $F_{e}$ is the electrostatic force between them. Then:

The composition of an $\alpha$-particle can be expressed as:

Who discovered the neutron?