The position of elements on the periodic table is primarily determined by their atomic number rather than atomic mass. However, there is one notable example of neighboring elements whose positions would be reversed if the elements were arranged solely by atomic mass instead of atomic number. These elements are iodine (I) and tellurium (Te).
In the periodic table, iodine (atomic number 53) is located to the left of tellurium (atomic number 52). However, if the elements were arranged solely by atomic mass, the positions would be reversed. This is because the atomic mass of iodine (around 126.90 atomic mass units) is greater than that of tellurium (around 127.60 atomic mass units).
It's important to note that the periodic table is primarily organized by atomic number, as it reflects the number of protons in an atom's nucleus, which determines its chemical properties. However, atomic mass is still an important characteristic of elements and is often listed alongside the atomic number.