The correct answer is outer electrons. These are sometimes called 'valence electrons', and they are responsible for the element's properties and how it forms bonds. For example, elements in Column 2 are likely to form ionic bonds with elements in Column 16. This is because elements in Column 2 have 2 outer electrons, while elements in Column 16 have 6 outer electrons. If the element in Column 2 gives its 2 electrons to the element in Column 16, then the former will be stable because the energy level underneath will be complete, and the latter will be stable because it will have a complete outer energy level with 8 electrons.
