Single most (what?) substance
---common?
---important?
---fragile?
This question is not formed properly and thus can not be answered
carbon
Carbon.
a substance
An element is a single kind of matter that is pure. It consists of atoms with the same number of protons in their nucleus. Examples of elements include oxygen, gold, and carbon.
a pure substance! ;)
If a single atom of a hunk of matter is still the same substance as the matter, then that matter is an element. (Gold, sodium, carbon, uranium, and so forth) If the matter can only be broken down to a molecule, but not an atom, then it is not an element. (Salt, ammonia, sugar, alcohol, and so forth)
mixture bobsaysftw YouTube
substance! (:
Water would be the most abundant substance but if you're asking about elements then oxygen would make up the greatest percentage of body mass.
H20