Answer:
Because the attraction force of the protons is higher in the ion.
Explanation:
The atom is composed of nuclei with protons (positively charged) and neutrons (with no charge) and by an electrosphere with electrons (negatively charged). Because of the opposite charges, the protons and electrons are attracted by an electrostatic force.
The electroshpere is formed by levels, and as far from the nuclei is the electrons, as weak is the electrostatic force. When an atom loses one electron, it comes from the last level of the electrosphere, thus, the electrons that remain will be closer to the nuclei, and there'll be more protons than electrons, so the electrostatic force will be greater, and the electrons intends to approximate of the nuclei. Thus, the radius is smaller.
