Which energy level has the least energy? n=1 n=3 n=5 n=7

As we go farther from the nucleus, the energy of orbitals increase. Also, the farther an electron is from the nucleus, the more repulsion it has to face because of other electrons(more screening effect) and hence it is easier to remove that electron.so n=1 which is very close to the nucleus has least energy and n=7 which is far away from nucleus has higher orbital energies.

n=1 has the least energy.

hope it helps:)

pure substance: iron,copper

impure substance: water,charcoal