English: The side chain of histidine is aromatic, because the ring atoms have a total of 3 pairs of p-orbital electrons in addition to being planar (also every atom of the ring is sp2-hybridized); the third pair (in addition to 2 pairs, which are half of the two double bonds' electrons) comes from the RED-depicted sp2-hybridized nitrogen that has its free electron pair in its p-orbital. The BLUE-depicted nitrogen, on the other hand, has its electron pair on its sp2-orbital because its p-orbital is already used for double-bonding, which cannot hybridize with the adjacent p-orbitals to form the ring-encompassing pi-elctron cloud reguired for a compound to be aromatic. Finnish texts: "the electron pair of nitrogen on a sp2-orbital" above and "the electron pair of nitrogen on a p-orbital" below. Red color corresponds to aromaticity-contributing bonds/atoms/electron pair on nitrogen.