Frequency of photons emitted from electrons

[Abidal Sala]

Homework Statement

The energy of the electron in the first and forth levels in a hydrogen atom are: (21.76*10^-19), (-0.85) e.V.. the electron moved from the forth level to the first level with one leap. Calculate:
The frequency of the emitted photon from the atom

Homework Equations

ΔE = hf

The Attempt at a Solution

f = ΔE / h
f = [(-0.85) - (21.76*10^-19)] / 6.6*10^-34
f = -1.2878*10^33 Hz ..
I get that answer while the answer given in the book is 3.09*10^15 Hz, I'm confused..

 

[hadi_1367]

i think that energy in the first level is given in Joule (-21.76*10^-19) and the second is in eV.so you have to convert both energy levels to Joule.since electron's charge is negative, it's energy in presence of positive charge's potential is negative. so f=(-.85*1.602*10^-19--21.76*10^-19)/(6.6*10^-34)=3.09*10^15 Hz