Final answer:
Oxygen (O), which is at the top of group 6 in the periodic table, has the highest first ionization energy among the elements in that group.
Step-by-step explanation:
To determine which element in group 6 has the highest first ionization energy, we should remember that ionization energy generally increases across a period (left to right) and up a group (from bottom to top). The elements of group 6 include Chromium (Cr), Molybdenum (Mo), Tungsten (W), and Seaborgium (Sg), with Seaborgium being artificially synthesized and less commonly discussed in educational contexts.
Looking at the periodic table, the first ionization energy will be highest for the element at the top of this group, which is Oxygen (O). Although Oxygen is not a metal like the others listed, it is part of the chalcogen family and resides in group 6. The misconception regarding the rectangle formed by two far-left columns referenced in the question does not apply to group 6 elements, but it illustrates how ionization energy trends work within the periodic table.