- Each of these allotropes of phosphorous also has its own subset of forms.
- Incredibly lightweight carbon nanofoams, essentially a fifth major allotrope of carbon, were discovered several years ago.
- Arsenic has two allotropes, yellow arsenic and metallic arsenic, which is brittle.
- Two allotropes of the element exist, accounting for the slight variation in densities.
- A third allotrope occurs as an amorphous brown powder with a density of 2.350 grams per cubic centimeter.