Question

isotopes of an element differ in the composition of the nucleus. the varying numbers of ____ cause its ____ of each isotope to also vary?
a protons, shape
b neutrons, mass number
c baryons, atomic number
d electrons, molecular geometry

Explanation:

Isotopes of an element have the same number of protons (so same atomic number) but different numbers of neutrons. The mass number is the sum of protons and neutrons, so varying neutrons cause varying mass numbers. Option A is wrong as protons don't vary for isotopes of the same element, and shape is irrelevant. Option C is wrong as baryons (protons + neutrons) - but atomic number (protons) is same, so atomic number doesn't vary. Option D is wrong as electrons and molecular geometry are not related to isotope nucleus composition.

Answer:

B. neutrons, mass number