WJEC Chemistry Practice Exam

Atomic mass is defined as the weighted average mass of an element's isotopes, taking into account the relative abundance of each isotope in nature. This means that each isotope contributes to the overall atomic mass based on both its mass and how common it is compared to the other isotopes.

When you have multiple isotopes of an element, each isotope has a unique number of neutrons and therefore a different mass. The atomic mass listed on the periodic table reflects this average, calculated with the abundances of these isotopes factored in. Thus, the relationship between atomic mass and isotopes is that atomic mass represents a blend of all isotopic contributions, making the first choice correct.

This understanding also clarifies why the other options don't hold true. The second choice inaccurately suggests that atomic mass is based solely on the heaviest isotope, which overlooks the contributions of lighter isotopes. The third choice implies that the number of isotopes does not impact atomic mass, which contradicts the foundational concept of how atomic mass is derived. Lastly, the fourth option suggests a simple addition of isotope masses without regard for abundance, which also misrepresents how atomic mass is calculated.