How do atomic and ionic radii differ?

Atomic radius refers to the size of a neutral atom, while ionic radius refers to the size of an ion.

Atomic radius is a term used to describe the size of an atom. It is typically measured from the centre of the nucleus to the boundary of the surrounding electron cloud. This boundary is not a fixed point, but rather a fuzzy region where the probability of finding an electron decreases rapidly. The atomic radius can vary depending on the atom's state and what other atoms or ions it is bonded to.

On the other hand, the ionic radius refers to the size of an ion, which is an atom or a group of atoms that has gained or lost one or more electrons, thus acquiring a charge. When an atom loses an electron to become a positively charged cation, its radius typically decreases. This is because the electron cloud shrinks due to the increased attraction between the remaining electrons and the nucleus, which now has a greater positive charge. Conversely, when an atom gains an electron to become a negatively charged anion, its radius usually increases. The added electron increases the electron-electron repulsion in the cloud, causing it to expand.

It's important to note that the atomic and ionic radii of an element can differ significantly. For example, the atomic radius of chlorine is smaller than its ionic radius. This is because when chlorine gains an electron to become a chloride ion, the added electron increases the electron-electron repulsion, causing the electron cloud to expand and the ionic radius to be larger than the atomic radius.

In summary, while both atomic and ionic radii refer to the size of an atom or ion, they are influenced by different factors. The atomic radius is determined by the electron cloud surrounding a neutral atom, while the ionic radius is influenced by the gain or loss of electrons and the resulting change in charge.