In which shell do electrons experience the greatest difficulty in being removed?

Electrons in the inner shell experience the greatest difficulty in being removed due to their proximity to the positively charged nucleus and the strong electrostatic forces acting on them. These inner shell electrons are shielded by the outer electrons, which means they feel a stronger effective nuclear charge, making it more challenging for external forces to remove them from the atom.

In contrast, electrons in the outer shell or valence shell are farther from the nucleus and experience less attraction. They are more involved in chemical bonding and are generally easier to remove. The electron cloud is a concept related to the probabilistic nature of electron locations but does not refer to a specific shell where electrons can be categorized in terms of ease of removal.