I don't really know any quantum mechanics. But in our class, we were introduced to Bohr's model of the atom with his postulate that the angular momentum of an electron in the $n$-th orbit is $\frac{nh}{2\pi}$
Recently I read that electrons could jump from one orbit to another, by absorbing energy (through light or heat). I'm wondering that, if the electron jump from orbit $n_1$ to orbit $n_2$, then it's angular momentum about the nucleus should change by $\frac{(n_2-n_1)h}{2\pi}$ which is against the law of conservation of angular momentum since the only force acting on the electron is the Coulombic attraction towards the nucleus which provides no torque. How then, does the angular momentum change without a torque? Does this have something to do with spin angular momentum that the electron also has? Or is it that these laws don't hold good at such scales? Or is it a flaw of Bohr's model altogether?
Best Answer
When an electron moves from a higher orbital to a lower orbital, the atom emits a photon. This photon carries away the energy difference. However a photon always has an angular momentum of $\hbar$, which is exactly the size of the angular momentum difference for which you are looking for.