what are some unique physical characteristics of a scorpion

Two tails

On rare occasions, scorpions can be born with two metasoma (tails). Two-tailed scorpions are not a species, but rather a genetic abnormality.

Scorpion venom

All scorpion species possess venom. In general, scorpion venom is described as neurotoxic in nature. One exception to this however is Hemiscorpius lepturus which possesses cytotoxic venom. The neurotoxins consist of a variety of small proteins as well as sodium and potassium cations, which serve to interfere with neurotransmission in the victim. Scorpions use their venom to kill or paralyze their prey so that it can be eaten; in general it is fast acting, allowing for effective prey capture.

Scorpion venoms are optimized for action upon other arthropods and therefore most scorpions are relatively harmless to humans; stings produce only local effects (such as pain, numbness or swelling). A few scorpion species, however, mostly in the family Buthidae, can be dangerous to humans. Among the most dangerous are Leiurus quinquestriatus, otherwise dubiously known as the deathstalker, which has the most potent venom in the family, and members of the genera Parabuthus, Tityus, Centruroides, and especially Androctonus, which also have strong venom. The amount of venom typically injected by Androctonus australis, or fat-tailed scorpion of North Africa, results in this scorpion being the most deadly, but its toxicity is roughly half that of Leiurus quinquestriatus, which results in fewer deaths. These scorpions, and others in the family Buthidae, have been responsible for many human deaths. Scorpions are generally unable to deliver enough venom to kill healthy adults; deaths normally occur in the young, elderly or infirm. One symptom of a scorpion sting can include numbing at the injection site, sometimes lasting for several days. Unless provoked, scorpions are generally harmless and timid and use their sting only for killing prey. Generally, they will run from danger or remain very still. Also in some species, it has been recorded that the sting can kill a human if they suffer from an allergy to it.