Discuss how the ionization energy of an atom was determined in the late 30's by bombarding ground state elements in the vapor phase with electrons of varying energy. The energy needed to "knock" an electron out of its valence orbit could be recorded for each of the elements. This energy became the ionization energy of the atom.
Introduce the energy which holds an electron added to the valence orbit. Define this as the electronegativity of the atom. View from a table the electronegativity values of both metal and non metal atoms. Elicit why non metals atoms will possess a higher value than metal atoms will.