(a) Write an expression for the energy of a single electron in a hydrogen-like orbit. Your expression should take into account a nuclear charge of Z and quantum number n.(b) Express your answer in Hartrees if the electron is in a hydrogen 1s orbital. (c) Calculate the energy in Hartrees of an electron in a helium 1s and a helium 2s orbital. (d) Neglecting the r12 term in the Hamiltonian, calculate in Hartrees the energy of helium excited state having one electron in a 1s orbital and the other electron in a 2s orbital.

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it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me

there are no contact forces mentioned on the list of choices that you've provided.

one "light year" is the distance that light travels, through vacuum, in one julian year.   that's 365.25 days, of 24 hours each.