The element with a formula that doesn't follow its symbol, atomic number, or mass number is Mercury.
The symbol for mercury is Hg, which is derived from its Latin name "hydrargyrum." The atomic number of mercury is 80, indicating that it has 80 protons in its nucleus. However, the mass number of mercury is not equal to its atomic number because it has several isotopes with different numbers of neutrons. The most common isotope of mercury is mercury-202, which has a mass number of 202.
So, while the symbol for mercury is Hg and its atomic number is 80, its most abundant isotope has a mass number of 202, which doesn't match either its symbol or atomic number.