You can tell whether oxygen is an elementary substance or a compound by counting the number of atomic symbols in its chemical formula. An atomic symbol consists of either one capital letter or a capital letter followed by one or two lowercase letters.
The chemical formula for oxygen, O2, contains one atomic symbol: O. So, the formula tells you that oxygen is composed of only one chemical element.
Since oxygen is composed of only one chemical element, oxygen is an elementary substance.
Complete the statement.
Oxygen is ().
A: a compound
B: an elementary substance