Question

argon (atomic number 18) has a mass of 39.95 amu. potassium (atomic number 19) has a mass of 39.10 amu. which element would have appeared first on mendeleevs table? potassium they were separated by one element between them. argon they were interchangeable.

Explanation:

Mendeleev's periodic table was arranged by atomic mass. Potassium has a lower atomic mass (39.10 amu) compared to argon (39.95 amu). So potassium would have appeared first.

Answer:

potassium