The electron in an atom is restricted to revolve in certain allowed circular orbit known as stationary orbit. The centripetal force for its motion is provided by electrostatic force of attraction between positively charged nucleus and negatively charged electron.
The formula for radius r in the nth orbit of an electron
rn = n2h2 / 4π2 Kme2 Z
rn = radius of nth orbit
h= Planck’s constant (6.67 x 10–34 J-s)
n is a positive integer
m= mass of an electron (9.1 x 10–31 Kg)
Z= atomic number (No. of Protons)
K = 1/(4πε0) = constant = 9 x 109 N m2 C–2
e = Charge on an electron (1.6 x 10–19 C)