For a hydrogen atom
it is given by
r
n
m
h
e
n = 





2
2
20
2
4
π
πε
The total energy is also quantised:
4
2
2
2
0
8
n
me
E
n
εh
= −
     = –13 6 eV/n2
The n = 1 state is called ground state In hydrogen atom the ground
state energy is  –13 6 eV