According to Bohr’s postulate, the angular momentum of an electron is quantised, that is,
mvr =
⇒ v =
Now, substituting the above expression in the kinetic energy expression
()mv2, we get
K.E. =
m()2 (1)
where r is the radius of Bohr’s orbit, that is, r =
a0×n2∕Z. For the second Bohr’s orbit of a hydrogen atom, substituting Z = 1 and n = 2, we get r =
4a0 where
a0 is a constant. Now, substituting this value of r in Eq. (1), we get
K.E. =
×m() =