What is the best explanation for the fact that the ionization energy of boron is lower than that of beryllium? 1. the increased repulsion experienced by the electrons in beryllium 2. boron has a larger effective nuclear charge than beryllium. 3. the radius of boron is smaller than that of beryllium making it easy to remove an electron 4. the quantum mechanical stability of the filled s subshell of beryllium

Ionization energy (EI) is the energy that must be supplied to a neutral, gaseous and fundamental state atom to tear out the outermost electron, which is most weakly retained, and convert it into a gaseous monopositive cation.

it is becoming increasingly difficult to extract electrons that are closer to the nucleus than those that are further away from it. As an increase in energy implies difficulty, nonmetals (located on the right side of the periodic table) have the highest energies, so the ionization potential increases as one advances in a period; while as an atom grows larger it is easier to extract its furthest electron, since it feels less attracted to the nucleus (check the sections corresponding to atomic radius and ionic radius and electronegativity and electroaffinity), so ionization energy decreases as it drops in a group .

So boron has the full 2s orbital, this gives it greater protection than beryllium against ionization energy. The additional stability inherent in being filled or a sublayer filled in half, such as the sublayer filled with s of beryllium, increases the energy required to remove an electron.