It's often helpful to draw a diagram showing the energy levels for the particular element you're interested in. The diagram for hydrogen is shown above.
The n = 1 state is known as the ground state, while higher n states are known as excited states. If the electron in the atom makes a transition from a particular state to a lower state, it is losing energy. To conserve energy, a photon with an energy equal to the energy difference between the states will be emitted by the atom.
Consider the photon emitted when an electron drops from the n=4 to the n=2 state to the photon emitted when an electron drops from n=3 to n=2. Which photon has the longer wavelength?
The photon emitted in the n=4 to n=2 transition The photon emitted in the n=3 to n=2 transition The smaller the energy the longer the wavelength. The photon has a smaller energy for the n=3 to n=2 transition.
In the hydrogen atom, with Z = 1, the energy of the emitted photon can be found using:
E = (13.6 eV) [1/nf2 - 1/ni2]
Atoms can also absorb photons. If a photon with an energy equal to the energy difference between two levels is incident on an atom, the photon can be absorbed, raising the electron up to the higher level.
If we did not have an atmosphere, we would fly out into space. We would also not be protected from harmful rays from the sun, and astroids. Human life would not live very long.
