Chlorophyll a is not called 680. The name "680" refers to photosystem II, specifically to the special pair of chlorophyll a molecules at the reaction center of photosystem II.

Here's why:

* Photosystem II: This is a complex of proteins and pigments that plays a crucial role in photosynthesis. It captures light energy to split water molecules and generate oxygen.

* Special Pair: At the heart of photosystem II lies a pair of chlorophyll a molecules, which absorb light energy most efficiently at a wavelength of approximately 680 nanometers. This wavelength is within the red portion of the visible light spectrum.

* Naming Convention: Because of this specific light absorption property, this special pair is often referred to as P680.

Therefore, "680" refers to a specific chlorophyll a pair within photosystem II, not to chlorophyll a itself.