Mass number is the sum of protons and neutrons in an atom’s nucleus. Isotopes are atoms of the same element with different numbers of neutrons.  How to solve  Calculate the mass number for each species by adding the number of protons and neutrons. Then, determine the relationship between the two species based on their atomic and mass numbers.  Step 1 . Calculate the mass number of X  The mass number is the sum of protons and neutrons.  Mass number of X = 6+6=12.  Step 2 . Calculate the mass number of Y  The mass number is the sum of protons and neutrons.  Mass number of Y = 6+8=14.  Step 3 . Determine the relationship between X and Y  Both X and Y have the same number of protons 6, so they are the same element.  They have different numbers of neutrons, so they are isotopes.  Solution  The mass numbers of X and Y are 12 and 14 respectively, and they are isotopes of the same element.