Final answer:
The average atomic mass of rubidium is calculated by multiplying the isotope masses by their abundances and summing the results. Bromine's average atomic mass can be estimated at around 80 amu, considering its isotopes have nearly equal abundances and similar masses. Fluorine has an average atomic mass of 19 amu, as it has only one naturally occurring isotope.
Step-by-step explanation:
To calculate the average atomic mass of an element with various isotopes, we multiply the mass of each isotope by its fractional abundance and then sum the products. For rubidium, with isotopes rubidium-85 (72.2% abundance) and rubidium-87 (27.8% abundance), the calculation involves multiplying the mass numbers of each isotope by their respective abundances, then adding the results.
To estimate the average atomic mass of bromine without a calculator, we consider bromine-79 and bromine-81 with nearly equal abundances of roughly 50%. Since their atomic masses are close and their abundances are nearly the same, the average atomic mass of bromine will be about halfway between 79 and 81, so around 80 amu.
For fluorine, since there is only one naturally occurring isotope, fluorine-19, the average atomic mass is simply the mass of that isotope, which is 19 amu.