I notice that out of all of group 16 oxygen is the only element that can form a double bond. Why is that because shouldn't all of group 16 be able to form a double bond and be diatomic because they all have six valence electrons?
I notice that out of all of group 16 oxygen is the only element that can form a double bond.
This premise is not true. Sulfur in fact forms double bonds in many organic compounds. Selenium and tellurium both form double bonds with triphenylphosphine as triphenylphosphineselenide and triphenylphosphinetelluride.
The larger chalcogenides do have difficulty in forming double bonds in organic molecules due to lower bond strength, higher reactivity, preference of molecules for oxygen, and steric bulk. They can however indeed form double bonds; it's just not as common.
Sulphur atoms are larger then oxygen atoms. This means that 3p orbitals in sulphur can't overlap efficiently or enough to create a pi bond (which is the "second" bond in the double bond) between two sulphur atoms. Oxygen, however, is small enough that its 2p orbitals can overlap to form a stable pi bond. The size of the atoms only increase down group 16, and so anything larger than oxygen will not form stable double bonds.