Final answer:
Changing the atom's mass number results in different isotopes of the element but does not alter the element's chemical properties or its position on the periodic table. This change only affects the number of neutrons and thus the atomic mass.
Step-by-step explanation:
When you change the mass number of an atom, what essentially changes is the number of neutrons in the nucleus, creating different isotopes of the element. The chemical properties of an element are determined by the number of protons (and electrons), so by altering the number of neutrons, you do not change the element's chemical behavior, as seen with carbon-12 and carbon-13 which have the same chemical properties. Isotopes of the same element have identical atomic numbers but different mass numbers. This variation in neutron number may affect the atomic mass of the element, but not its position on the periodic table or its chemical reactivity. When the atomic mass is calculated for an element that has several isotopes, we use the weighted average of the mass numbers of all naturally occurring isotopes of that element. For example, the atomic mass of chlorine is 35.45 amu because it includes isotopes with mass numbers of 35 and 37.