The element with an atomic mass number of 207 is lead (Pb).

Here's why:

* Atomic mass number is the total number of protons and neutrons in an atom's nucleus.

* Lead has an atomic number of 82, meaning it has 82 protons.

* To have an atomic mass number of 207, lead must have 125 neutrons (207 - 82 = 125).

While lead has several isotopes, the most common and stable isotope is lead-207, which is what you're likely referring to.