Each chemical element is made up of one kind of atom, so the answer is A
Cathode rays carry electronic currents through the tube. Electrons were firstdiscovered as the constituents ofcathode rays. J.J. Thomson used thecathode ray tube to determine that atoms had small negatively charged particles inside of them, which he called “electrons.”
The atomic number is the number of protons, while the atomic mass is the number of protons added to the number of neutrons. By definition, potassium (K) has atomic number 19, so it should have 19 protons. If it has a mass of 42, then the number of neutrons is 42 - 19 = 23 neutrons. This is choice (1).
