In the notation 40/19 K, what does the subscript represent?

In the notation 40/19 K, the subscript "19" represents the atomic number of the element potassium. The atomic number indicates the number of protons present in the nucleus of an atom of that element. Since the identity of an element is determined by its number of protons, this subscript is crucial for distinguishing potassium from other elements.

The mass number, represented by the superscript "40" in this case, refers to the total number of protons and neutrons in the nucleus, while the electron count typically matches the number of protons in a neutral atom but is not directly indicated in the notation provided. Understanding the roles of the atomic number and mass number is fundamental in the study of chemical elements and their behaviors.