Determine the value of the de Broglie wavelength associated with the electron orbiting in the ground state of hydrogen atom (Given eV and Bohr radius Å). How will the de Broglie wavelength change when it is in the first excited state?

In the ground state, the kinetic energy is

We know the de Broglie wavelength() is given by

In the first excited state, the energy increases, or becomes less negative. The kinetic energy thus decreases. So the de Broglie wavelength increases.