Final answer:
Graphite is the crystalline carbon allotrope with a hardness level of '2' on the 1 to 4 scale in the U.S., being soft and commonly used as pencil lead.
Step-by-step explanation:
The crystalline carbon allotrope that is measured on a hardness scale of 1 to 4 in the U.S., with '2' being the most common, is graphite. This soft, black solid is commonly found in pencil lead. Allotropes of carbon, such as graphite and diamond, have very different properties due to the varied arrangements of their carbon atoms, but they all conform to the octet rule by having each carbon atom forming four bonds. While diamond is one of the hardest-known substances, graphite is much softer, which explains its lower position on the hardness scale.
The crystalline carbon allotrope that is measured on a hardness scale of 1 to 4 in the U.S., with '2' being the most common, is diamond. Diamond is one of the hardest-known substances. Graphite, on the other hand, is a soft solid that is commonly used as pencil lead.