Final answer:
The 4s2 electrons have the lowest binding energy due to the shielding effect by inner electrons, which reduces the electrostatic pull of the nucleus, making their removal easier. Higher principal quantum numbers also imply that these electrons are further from the nucleus on average, contributing to their higher energy levels and lower binding energy.
Step-by-step explanation:
The 4s2 electrons have the lowest binding energy primarily because they are shielded by inner electrons (A). In multielectron atoms, such as those on the periodic table, inner electrons effectively shield the outer electrons from the full attractive force of the positively charged nucleus. This shielding effect makes it easier for an electron to be removed from its orbital as it experiences less electrostatic pull towards the nucleus. Despite being closer to the nucleus in terms of the principal quantum number, other inner electron shells and subshells screen the 4s electrons, making their binding energy lower.
Moreover, as electrons occupy higher energy levels indicated by the principal quantum number (n), such as the 4s orbital, these electrons spend more time farther from the nucleus, resulting in a weaker attraction to the nucleus and thus a higher energy level (less stabilized) which also contributes to lower binding energies (C). In this case, another factor that may seem relevant but is not the primary reason for lower binding energy is the principal quantum number. The principal quantum number indicates the energy level or shell of an electron, with higher numbers correlating with electrons that are on average farther from the nucleus; however, for 4s electrons, the shielding by other electrons is more significant in reducing the binding energy than the principal quantum number itself (D).
It's also important to understand that not all electrons shield equally. Electrons in the same principal shell, such as 4s, are less effective at shielding one another from the nuclear charge, meaning s electrons shield p electrons within the same energy level, but not vice versa. This internal shielding within the same principal energy level also contributes to the lower energy of the 4s electrons.