There are a large numbers of allotropes of the sulfur. In this respect, sulfur is second only to carbon. The most common form found in nature is yellow orthorhombic α-sulfur, which contains puckered rings of S8. Chemistry students may have seen "plastic sulfur"; this is not an allotrope but a mixture of long chain polymeric sulfur forms, two of which have been identified as allotropes. In addition there are other solid forms that contain sulfur rings of 6, 7, 9–15, 18 and 20 atoms. There are also gases, S2, S3; some species only detected in the vapour phase, S4 and S5 and perhaps five or more high-pressure forms, two of which are metallic form.