The relationship between the abundances of the isotopes and their masses can be written as:

$\text{Average atomic mass} = (\text{abundance1} \times \text{mass1}) + (\text{abundance2} \times \text{mass2})$

This formula essentially states that the average atomic mass of lithium is the weighted average of the masses of its isotopes, where the weights are the relative abundances.

We are asked to find the relationship between abundance1 and abundance2.

Since abundance1 is the fraction of the first isotope, and abundance2 is the fraction of the second isotope, they must sum to 1, because the total abundance must account for all isotopes of lithium:

$\text{abundance1} + \text{abundance2} = 1$

This is equivalent to 100% if expressed as a percentage, but in fractional terms it simplifies to:

$\boxed{D. \, \text{abundance1} + \text{abundance2} = 1}$

Would you like further clarification or details on the calculation?

Here are 5 questions to expand on this:

1. How do you calculate the individual abundances of isotopes using the average atomic mass?
2. Why do isotope abundances sum to 1?
3. How can you use algebra to solve for abundance1 or abundance2 in this type of problem?
4. What is the significance of average atomic mass in chemical reactions?
5. How do the masses of isotopes affect the physical properties of elements?

Tip: When solving for isotope abundances, always check that the sum of the relative abundances equals 1 or 100%.