The key feature of Bohr's theory of spectrum of hydrogen atom is the quantization of angular momentum when an electron is revolving around a proton. We will extend this to a general rotational motion to find quantized rotational energy of a diatomic molecule assuming it to be rigid. The rule to be applied is Bohr's quantization condition.
Consider a gold nucleus to be a sphere of radius $$6\cdot\ 9$$ Fermi in which protons and neutrons are distributed. Find the force of repulsion between two protons situated at largest separation. Why do these protons not fly apart under this repulsion ?
In a set of experiments on a hypothetical one-electron atom, the wavelengths of the photons emitted from transitions ending in the ground state $$(n=1)$$ are shown in the energy diagram above. 

$$ \propto $$-particles are scattered by the gold nucleus but not by the electrons.
It is, because 

Which is de-Broglie wavelength of a $$3\ kg$$ object moving with a speed of $$2\ m/s$$?