8.(a)From Rutherford alpha - particle scattering experiment give the experimental evidence for deriving the conclusion that

(i) Most of the space inside the atom is empty

(ii) The nucleus of atom is positively charged

(b) An element has mass number 31 and atomic number 15 find

(i) The number of neutron in element

(ii) The number of electron in element

(a)
(i) In Rutherford-$\alpha$-scattering experiment, most of the fast moving $\alpha$-particles passed straight through the gold foil. So, as most $\alpha$-particles passed through the gold foil without any deflection shows that most of the space inside an atom is empty.

(ii) Also Rutherford-$\alpha$-scattering experiments, few $\alpha$-particles were deflected through the foil by small angles. That is very small number of particles suffered a deflection from their original path. This shows that positive charge occupies very little space inside an atom.

(b)
(i) It is given that $A=31 and Z=15$
The number of neutron, atomic number and the atomic mass is related as,
 $$\begin{gathered} A=Z+N \\ or, N=A-Z \end{gathered}$$
On substituting the values we get the number of neutrons in the atom as,
 $$\begin{gathered} N=31-15 \\ N=16 \end{gathered}$$

(ii) We know that in an atom, the number of proton is equal to the number of electron and atomic number. Thus, the number of electron in the respective atom will be $e=Z=15$