10 C atoms, therefore 10 p -electrons.

Ground state; levels 0, 1, 2 are full

Therefore the electron goes from n = 2 to n = 3

E_photon = E₃ -E₂ = (3² - 2²)h²/2ml² = [5(6.63X10^-34 Js)]/[2(9.1X10^-31 kg)(10X0.15X10^-9 m)]

= 5.37X10^-19 J