Final answer:
The element with isotopes weighing approximately 10 amu and 11 amu is boron. Its average atomic mass is calculated using a weighted average based on the isotopes' masses and their relative abundances.
Step-by-step explanation:
The element with two naturally occurring isotopes, one with an atomic mass of about 10 amu and the other with an atomic mass of about 11 amu, is boron. Boron's atomic mass is calculated as a weighted average of its isotopes' mass and their relative abundance. For instance, in a sample of boron, 20% are B-10 isotopes with a mass of 10 amu, and 80% are B-11 isotopes with a mass of 11 amu. To find the average atomic mass of boron, you multiply each isotope's mass by its relative abundance, add these products together, and then divide by 100 to account for the percentage. This results in the atomic mass of boron, which is slightly higher than 10.8 amu.