What two elements whose atomic mass order does not match their atomic number order?

The two elements that exhibit a discrepancy between Atomic Mass order and atomic number order are potassium (K) and argon (Ar). Potassium has an atomic number of 19 and an atomic mass of about 39.1, while argon has an atomic number of 18 and an atomic mass of approximately 39.9. This means that argon, which is lighter in terms of atomic mass, appears after potassium in the Periodic Table based on atomic number.