Final answer:
The average atomic mass of an element must be calculated using a weighted average to account for the different masses and natural abundances of the element's isotopes, which directly influence the element's properties and are vital for chemical calculations.
Step-by-step explanation:
When calculating the average atomic mass of an element, it is essential to use a weighted average because elements naturally occur as a mixture of isotopes, each with a different mass. The average atomic mass of an element is the weighted average of the atomic masses of the naturally occurring isotopes of that element, factoring in the percent abundance of each isotope. To calculate this, you can use the formula:
average mass = (fractional abundance × isotopic mass)
For example, if you have two isotopes of an element with masses of 10 u and 11 u, and their fractional abundances are 50% each, the calculation would be (0.5 × 10) + (0.5 × 11), resulting in an average atomic mass of 10.5 u. It's important to make this calculation because the atomic mass affects the element's physical and chemical properties and is used in many calculations in chemistry.