Last electron acceptor.

The electron acceptor is molecular oxygen O2. Oxygen accepts two electrons in cellular respiration and two H+ ions to form water, a stable substance that organisms can easily transport or reuse. In fact, each atom of molecular oxygen accepts one electron each.

Fluorine has an oxidation state of -1 in compounds because it occurs in its ionic form in the human body (F-). Similarly, it can accept only one electron to make it stable, not two. Therefore, it cannot be the final electron acceptor because then where would the extra electron go that is coming from the electron transport chain? It would be lost.

Make sense?