* Atomic Mass vs. Isotope Mass: The atomic mass listed on the periodic table is a weighted average of the masses of all naturally occurring isotopes of that element. It's not the mass of any single isotope.

* Isotopes: Isotopes of an element have the same number of protons but different numbers of neutrons. This means they have different atomic masses.

* Bromine's Isotopes: Bromine has two main isotopes:

* Bromine-79: Accounts for about 50.69% of naturally occurring bromine. It has 35 protons and 44 neutrons.

* Bromine-81: Accounts for about 49.31% of naturally occurring bromine. It has 35 protons and 46 neutrons.

Why 79.9 amu?

The atomic mass of bromine (79.9 amu) is calculated by taking the weighted average of the masses of these two isotopes:

* (0.5069 * 79 amu) + (0.4931 * 81 amu) = 79.9 amu

The Key Takeaway: The periodic table lists the average mass of an element, not the mass of any specific isotope. While bromine has isotopes with masses close to 79 and 81 amu, there isn't a naturally occurring isotope with exactly 80 amu.