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# Rutherford proposed the atomic model after his most striking experiment on $\alpha$ – scattering leading to discovery of nucleus. Bohr later on modified the atomic model on the basis of Planck’s quantum theory of light and proposed the concept of stationary circular orbits of quantised energy $\large\frac{nh}{2\pi}$. The collection of fine lines in line spectrum led Sommerfeld to gave the idea of elliptical orbits. He successfully explained the existence of subshells and their number in a shell. The angular momentum of subshells was propped as $\sqrt{l(l+1}\times\large\frac{h}{2\pi}$. The emission of a spectral line in atomic spectra was supposed to be due to the jump of electron from one energy level to other. An oil drop has $-6.39\times10^{-19}$ coulomb charge. The number of electrons in this oil drop is

$(a)\;4\qquad(b)\;3\qquad(c)\;2\qquad(d)\;1$

Charge on one electron = $-1.60\times10^{-19}$
$\therefore$ Number of electron = $\large\frac{-6.39\times10^{-19}}{-1.60\times10^{-19}}$ = 4