$(a)$ Why did Rutherford choose gold and not any other lighter element (such as aluminium) for his experiment? Explain your answer giving two reasons.
$(b)$ Write the name and symbol of the particle chosen by Rutherford for bombardment against the gold foil in his experiment.

(N/A) Rutherford chose gold for his experiment for the following two reasons:
$1$. Gold is highly malleable,which allowed him to obtain an extremely thin foil (approximately $1000$ atoms thick). $A$ thinner foil minimizes the probability of multiple scattering events,ensuring that the observed deflections are primarily due to single interactions with the nucleus.
$2$. Gold has a high atomic mass (high atomic number $Z = 79$),which provides a large,dense,and positively charged nucleus. This makes the electrostatic repulsion between the incident alpha particles and the gold nuclei significant,leading to observable and measurable deflections.
$(b)$ The particle chosen by Rutherford for bombardment was the alpha particle. Its symbol is $\alpha$ (or $^4_2He^{2+}$).