Why are some ions larger than their parent atoms?

Some ions are larger than their parent atoms because they gain electrons, increasing electron-electron repulsion and expanding the electron cloud.

In more detail, the size of an ion compared to its parent atom is determined by the number of electrons it has in relation to the number of protons. When an atom becomes an ion, it either loses or gains electrons. If an atom loses electrons and becomes a positive ion (or cation), it usually becomes smaller than its parent atom. This is because there are now fewer electrons being attracted by the same number of protons, causing the electron cloud to contract.

On the other hand, if an atom gains electrons and becomes a negative ion (or anion), it usually becomes larger than its parent atom. This is because there are now more electrons being attracted by the same number of protons. The increased number of electrons leads to increased electron-electron repulsion, which causes the electron cloud to expand. This expansion results in a larger ionic radius, making the ion larger than its parent atom.

For example, consider the case of a neutral chlorine atom gaining an electron to become a chloride ion. The chlorine atom has 17 protons and 17 electrons, while the chloride ion has 17 protons and 18 electrons. The extra electron in the chloride ion increases electron-electron repulsion, causing the electron cloud to expand and making the chloride ion larger than the chlorine atom.

It's also worth noting that the nuclear charge (the total charge of all the protons in the nucleus) has a significant effect on the size of an ion. The greater the nuclear charge, the stronger the attraction between the nucleus and the electrons, and the smaller the ion. However, when an atom becomes an ion by gaining electrons, the increase in electron-electron repulsion usually outweighs the effect of the nuclear charge, resulting in a larger ion.