The pi electrons of metal porphyrins - such as the iron-heme of hemoglobin, the magnesium-porphyrin in chlorophyll, and the cobalt-porphyrin in vitamin B₁₂ - can be visualized using a simple model of free electrons in a two-dimensional box. Construct the energy-level diagram for a free electron in a two-dimensional square box of length. Set E₀ = h²/8ma² and label the energy of each level in units of E₀.