The mercury atoms in a fluorescent tube are excited and then emit photons in the ultraviolet region of the electromagnetic spectrum. Explain (i) how the mercury atoms become excited and (ii) how the excited atoms emit photons.

(i) As they pass through the fluorescent tube, they collide with the mercury atoms within it and this collision transfers energy to the atom. This energy transfer allows the atom's orbiting electrons to move to a higher energy state thus exciting the atom. (ii) Shortly after the collision, the atom proceeds to de-excite. The higher-energy electron returns to the ground state and a photon of equal energy to the difference in energy levels the electron travels through is emitted to conserve energy.