Let us model the Hydrogen atom, at time t=0, as (a) an electron at x=-re, (b) a proton at x=rp (with rp<<re)n. re and ve are to be determined.
First determine the desired level, N (with N=1 being the ground level).
For brevity we write 1/4πε0 as k:
k=1/4πε0. (1)
Then the net Coulomb force on the electron has the magnitude
Fcoulomb=ke2/re2 (2)
There is also a Newtonian force on the electron:
Fnewton=mve2/re. (3)
Equating the Coulombic and Newtonian forces produces
mve2=ke2/re (4)
or
ve2re=ke2/m. (5)
By deBroglie the electron is associated with a wavelength λ:
λ=h/mve. (6)
In the N’th state we assume that N of these wavelengths fits into the electron orbital:
λ=2πre/N. (7)
This being the case,
h/mve=2πre/N, (8)
and
vere=Nh/2πm. (9)
Dividing Eq. 5 by Eq. 9 produces
ve=2πΝke2/Nh. (10)
And,
re= Nh/2πmve.