As early as 1913, Niels Bohr proposed what is now called the Bohr model of the atom, and suggested that electrons could only have certain classical motions.

The model describes the atom as a small, positively charged nucleus, surrounded by electrons travelling in circular orbits around it.  In a way, the concept is similar in structure to the Solar system, but with attraction provided by electrostatic forces, rather than by gravity.  Although the Bohr model is now obsolete, the quantum theory at the heart of it is still regarded as valid.

The Bohr radius for the hydrogen atom remains an important physical constant.  The Bohr radius  corresponds to the radius of the lowest energy electron orbit predicted by the Bohr model of the atom.

The radius of an atom is over 10,000 times the radius of its nucleus, and less than 1/1000 th  of the wavelength of visible light.  The Bohr model only applies to atoms and ions with a single electron, such as singly ionized helium He II, positronium Ps, and of course hydrogen H.

So, the size of a hydrogen atom in its ‘ground state’ is of order .

That’s MIGHTY small !!