Here's why:

* Atomic Number: The atomic number of an element defines the number of protons in its atom. Lead (Pb) has an atomic number of 82.

* Isotopes: The "206" in lead-206 refers to the mass number, which is the total number of protons and neutrons in the nucleus. While the number of neutrons can vary in isotopes of the same element, the number of protons always remains the same.