Final answer:
When an electron is removed, the nucleus' hold on remaining electrons strengthens due to the electrostatic force of attraction from protons. Inside the nucleus, the strong nuclear force keeps nucleons tightly bound, so as the nucleus loses nucleons, it becomes more challenging to remove additional ones.
Step-by-step explanation:
Each time an electron is removed, the nucleus holds the remaining electrons even more tightly. The protons in the nucleus attract the negatively charged electrons via the electromagnetic force. However, inside the nucleus, the strong nuclear force, significantly stronger than the electromagnetic force, is responsible for holding protons and neutrons (nucleons) together.
This strong nuclear force is a short range force that binds nucleons into a densely packed structure and makes it energetically costly to remove a nucleon from the nucleus. Just as nucleons resist being pushed inside one another, they become harder to remove as the nucleus gets smaller because the binding energy becomes stronger when there are fewer nucleons to share it.