The kinetic energy of the electron in an orbit of radius `r` in hydrogen atom is (`e =` electronic charge)

Explanation

Hydrogen has one proton (+e charge) at the centre . Electron (m,-e) rotates in a circular path using electrostatic force of attraction as centripetal force . Hence , for the electron we can write the following :
`F=1/(4piepsilon_0)e^2/r^2=(mv^2)/r`
K.E.=`1/2mv^2=1/(4piepsilon_0)e^2/(2r)`