Answer;
Potassium
Step-by-step explanation;
The Russian chemist Dmitri Mendeleev was the first scientist to make a periodic table similar to the one used today. Mendeleev arranged the elements by atomic mass, corresponding to relative molar mass.
When the elements are arranged in order of increasing atomic mass, certain sets of properties recur periodically;
In this case; P- 31. S- 32, Cl-35 , K-39, Ca- 40 and so forth.