Question

The nucleus of an atom of K-42 contains

(1) 19 protons and 23 neutrons
(2) 19 protons and 42 neutrons
(3) 20 protons and 19 neutrons
(4) 23 protons and 19 neutrons

Answer 1

Number of proton K => 19
therefore, 42 - 19 = 23.

So the answer is 19 protons and 23 neutrons.

Answer 2

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).