In the chlorophyll head, an atom of magnesium is coordinated with the:

Explanation

The magnesium atom in the chlorophyll head is coordinated with the nitrogen atoms of the porphyrin ring. These nitrogen atoms form a square planar arrangement around the magnesium, stabilizing it in the center of the ring. This coordination is essential for the porphyrin ring to absorb light effectively, while the phytol tail anchors the molecule into the membrane and hydrogen and carbon atoms do not participate directly in magnesium coordination.