Arsenic exists in two main allotropic forms:

* Grey arsenic (α-arsenic): This is the most common form and has a rhombohedral crystal structure. It is similar to the structure of phosphorus, with each arsenic atom bonded to three others in a trigonal pyramidal arrangement.

* Yellow arsenic (β-arsenic): This is a metastable form that has a monoclinic crystal structure. It is similar to the structure of white phosphorus.

Other forms of arsenic:

* Black arsenic: This is a less common form that has a layered structure.

* Amorphous arsenic: This is a non-crystalline form of arsenic.

The specific crystal structure of arsenic can vary depending on the temperature and pressure conditions. However, all forms of arsenic exhibit some degree of crystalline order.