Which are isotopes? An atom that has an atomic number of 20 and a mass number of 42 is an isotope of an atom that has
A)an atomic number of 21 and a mass number of 42.
B)an atomic number of 20 and a mass number of 40.
C)22 neutrons and 20 protons.
D)22 protons and 20 neutrons.

Question

Quizplus · Accepted Answer

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, resulting in different mass numbers. Therefore, the isotope with an atomic number of 20 and a mass number of 42 must have the same number of protons as the atom with an atomic number of 20, but a different number of neutrons. 
Option A has a different atomic number, so it cannot be an isotope of the given atom. 
Option C and D describe the composition of the isotope, but they do not specify the element, so they cannot be considered as the correct answer. 
Option B is the only one that satisfies the conditions mentioned. The most abundant isotope of Calcium, having an atomic number of 20, has a mass number of 40. Therefore, the given isotope with a mass number of 42 and an atomic number of 20 is an isotope of Calcium-40.

Which Are Isotopes? an Atom That Has an Atomic Number