Question

Consider two isotopes of the element potassium. The atomic number of potassium is 19. One isotope has the atomic mass 39 while the other isotope has the atomic mass 40. Which of the following statements about the isotopes is correct?

One isotope contains 39 protons while the other contains 40 protons.
One isotope contains 20 protons while the other isotope contains 19 protons.
Both isotopes contain 20 protons.
One isotope contains 20 neutrons while the other isotope contains 21 neutrons.

Answer 1

The correct statement is: One isotope contains 20 neutrons while the other isotope contains 21 neutrons. This is because both of the potassium isotopes have an atomic number of 19, meaning that they each contain 19 protons in their nucleus. The difference between them lies in how many neutrons are present; one has 20 and the other has 21.