What primarily influences the trends of the first ionization energy in the first 20 elements?

The trend of first ionization energy among the first 20 elements is primarily influenced by the electron configuration, specifically the sub-shells. Ionization energy is defined as the energy required to remove the outermost electron from an atom in its gaseous state. As we move across a period in the periodic table, the number of protons in the nucleus increases. This increase in nuclear charge exerts a greater attractive force on the electrons, thus generally increasing the ionization energy.

Furthermore, the distribution of electrons in the various sub-shells—s, p, d—also plays a critical role in determining how tightly electrons are held by the nucleus. For instance, electrons in filled or half-filled sub-shells often require more energy to remove due to stability associated with these configurations. Therefore, the configuration dictates how the electrons are distributed, influencing their distance from the nucleus and the strength of the attraction they experience. This electronic structure explains why certain elements have higher ionization energies than others, reflecting the periodic trends observed, particularly across periods and down groups.

The other factors mentioned, such as the number of neutrons, the number of protons, and atomic mass, do influence atomic properties but do not play as direct a role in the immediate trends