Q.Explain why electric charge is said to be quantised, stating the value of the elementary charge $e$. Why is it impossible, in practice, to observe a free charge of $0.5\,e$?

Electric charge is a fundamental property of matter that obeys three properties verified in every experiment: additivity (the net charge of a system is the plain algebraic/scalar sum $Q=\sum q_i$ of the individual charges, signs included), conservation (the total charge of an isolated system never changes -- charge can be transferred between bodies but never created or destroyed, e.g. rubbing a rod with wool transfers electrons so the rod-wool system's total charge stays exactly zero throughout), and quantisation (every observable charge is an integral multiple of the elementary charge, $q=ne$ with $e=1.6\times10^{-19}\ \text{C}$; this discreteness is unnoticeable at the macroscopic scale because ordinary charges involve enormous numbers of elementary charges). The SI unit of charge is the coulomb (C). Like charges repel, unlike charges attract.