18. Aromaticity

For the molecule benzene ($C_6{H}_6$), how many $\pi$ electrons are present in its conjugated system?

Which of the following molecules contains exactly $6$ pi electrons?

Chlorophyll is the general name for a family of compounds present in algae and green plants. These molecules use the energy in sunlight to convert carbon dioxide and water into carbohydrates and other energy sources. At the heart of chlorophyll (shown below) is a large-ring magnesium complex called a chlorin. Circle each double bond in the large cyclic conjugated pi system that makes it aromatic. How many pi electrons are in this aromatic system?