Chemistry

First ionization energy reflects how tightly an atom holds its outermost electrons—the stronger the nuclear pull relative to shielding, the higher the energy needed to remove one. The modest plateau from K to Zn occurs because you're filling the $d$-orbitals, which add nuclear charge without dramatically improving the shielding of the valence $s$ electrons. The steep rise from Ga onward marks the transition to filling $p$-orbitals, which are closer to the nucleus and experience much less shielding, so each additional proton makes a bigger impact. The peak at Kr and dip at Rb showcase the competition between nuclear charge and electron configuration: Rb's single $5s^1$ electron is farther out and more easily removed than Kr's stable, filled $4p^6$ shell.

First ionization energy reflects how tightly an atom holds its outermost electrons—the stronger the nuclear pull relative to shielding, the higher the energy needed to remove one. The modest plateau from K to Zn occurs because you're filling the $d$-orbitals, which add nuclear charge without dramatically improving the shielding of the valence $s$ electrons. The steep rise from Ga onward marks the transition to filling $p$-orbitals, which are closer to the nucleus and experience much less shielding, so each additional proton makes a bigger impact. The peak at Kr and dip at Rb showcase the competition between nuclear charge and electron configuration: Rb's single $5s^1$ electron is farther out and more easily removed than Kr's stable, filled $4p^6$ shell.