The two neighboring elements on the periodic table whose positions would be reversed if arranged by atomic mass instead of atomic number are iodine (I) and tellurium (Te).
To find element number 5 on the periodic table, we need to consider the atomic number. The atomic number corresponds to the number of protons in the nucleus of an atom, and it determines an element's position on the periodic table.
Element number 5 is boron (B). It has an atomic number of 5, which means it has five protons in its nucleus. Boron is located in Group 13 and Period 2 of the periodic table.