This article is Episode 1 of the Satya–Chandra series Why Matter Exists.
Electrons repel electrons. Protons repel protons. How is matter stable then?
Oh! I see. It’s because a proton attracts an electron.
But… if so, then why doesn’t electron fall into the nucleus?
Umm… according to classical physics it should collapse.
Yet, here we are.
An orbiting electron is an accelerating charge. Accelerating charges radiate energy. Radiating energy means losing energy. Losing energy means spiralling inward.
Ah…. Bohr…..
Exactly, Bohr proposed something outrageous. He proposed that an electron could occupy only certain allowed orbits. Only those orbits whose angular momentum is an integral multiple of 
are allowed, i.e. 
And remember, 
Therefore, there is no allowed orbit below the ground state.
Oh! I see. That means an electron is not allowed to possess just any energy. Only certain discrete energy levels are permitted. It can jump only between these allowed levels.
So there is a lowest allowed energy state n=1, called ground state. There is simply no allowed state corresponding to n=0.
But why is there a lowest one at all?
Why couldn’t nature have allowed one more orbit, even closer to the nucleus?
An excellent question.
Bohr answered where the electron can exist.
He never really explained why those orbits exist in the first place.
That question would eventually lead physicists to one of the deepest principles in quantum mechanics.
We will chase that mystery in our next conversation.
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