When comparing the size of different ions, the general radii trend discussed in Chapter 2 is usually not very useful. What do you concentrate on when comparing sizes of ions to each other or when comparing the size of an ion to its neutral atom?

Atomic radius is the distance between the nucleus of an atom and its outermost electron orbital. Ionic radius is the distance between the nucleus of an ion (an atom with a positive or negative charge) and its outermost electron orbital. In essence, the atomic radius is the size of a neutral atom, while ionic radius is the size of a charged atom (ion).

There are three main factors that affect the size of ionic radii: 1. Charge of the ion (cation/anion): When an atom loses an electron (forming a cation), its nuclear charge increases, causing the remaining electrons to experience stronger attraction to the nucleus, thus resulting in a smaller ionic radius. In contrast, when an atom gains an electron (forming an anion), the increased number of electrons causes a decrease in effective nuclear charge and results in a larger ionic radius. 2. Atomic size of the neutral atom: The larger the atomic size of a neutral atom, the larger its ionic radius will be when it becomes an ion. 3. Electron configuration and shielding effect: The electron configuration of an ion determines the number of electron shells and their effective shielding. Higher shielding reduces the net nuclear charge experienced by the outermost electrons, resulting in a larger ionic radius.

When comparing the sizes of ions, focus on the following aspects: 1. Ions with the same charge and similar electron configurations can be compared based on the atomic size of their neutral atoms. For example, when comparing Na^+ and K^+ ions, K^+ will have a larger ionic radius as K has a larger atomic size than Na. 2. When comparing cations and anions, remember that cations are generally smaller than their neutral atoms, and anions are generally larger. For example, Na^+ is smaller than Na (neutral atom), and Cl^- is larger than Cl (neutral atom). 3. When comparing cations or anions with different charges, a higher charge will typically result in a smaller ionic radius due to the increased effective nuclear charge. For example, comparing Mg^2+ and Al^3+ ions, Al^3+ has a smaller ionic radius than Mg^2+. 4. When comparing ions with different electron configurations, the shielding effect and effective nuclear charge play a significant role in determining ion size. A more significant shielding effect will result in a larger ionic radius.