Because nonmetals frequently produce anions, their ionic radius is bigger than their atomic radius. This is especially obvious with halogens. For example, iodine has an ionic radius of about 130 pm while its atomic radius is only about 70 pm. Chlorine has an ionic radius of about 120 pm while its atomic radius is only about 60 pm.
Ions are atoms that have lost or gained electrons. Ionic radii are the average distances between adjacent ions in a crystal. Atomic radii are the distances between nuclei. Atoms can lose or gain electrons which affects how many electrons there are to fill the nucleus. Electrons also affect chemical properties such as color and toxicity. Ions are usually composed of one neutral atom and one or more negative ions. Ions are important in chemistry because they provide a way to extract electrons from molecules to form compounds. This allows molecules with empty electronic shells to be stable.
When comparing ions with atoms, it's helpful to think about how much space an ion takes up. Ions are large because they contain several atoms bonded together. An ion has a positive charge so it needs to have at least one positively charged atom, but it can have more than one if it wants to.
When compared to a metal of the same period, a nonmetal has a lower ionic radius. The logic for this is that a metal normally loses an electron when it becomes an ion, but a nonmetal acquires an electron. Thus, the nucleus of a nonmetal such as oxygen or nitrogen is larger than that of a metal such as chlorine or bromine.
Ionic radii are usually given in millimeters (mm) and can be used to compare the size of various atoms. However, since electrons are not completely removed from atoms except in a vacuum, some residual charge remains even after atomic separation. This leaves an ion with a positive charge and an atom with a negative charge, which causes them to be attracted to each other. Therefore, to estimate the true distance between ions, physicists use the concept of effective nuclear charge, which takes into account the electrostatic interaction between the nuclei.
Ionization energies are the energy required to remove one or more electrons from an atom. These energies are also called vacancy energies because they represent the energy needed to create a vacancy in an otherwise full valence shell. For example, if an atom needs to be stripped of its two outer electrons, then it will require 24 kiloercentigrades (Kelvin) for its sodium atom to become an ion.
The atomic and ionic radius of a neutral atom are the same, however many elements exist as anions or cations. Because the atom loses an electron energy shell when it loses its outermost electron (positively charged or cation), the ionic radius is lower than the atomic radius. Ions can be either small, medium, or large compared to atoms. The radius of a sodium ion is about 5% smaller than that of a neutral atom. The radius of a magnesium ion is about 15% smaller than that of a neutral atom.
Ions are formed by the loss of electrons from the nucleus of an atom. Ions may have multiple losses and may be composed of different elements. For example, oxygen ions contain two electrons in their outer shell and so they are called doubly-charged ions; hydrogen ions have 1 electron in their outer shell and are called singly-charged ions. Ions are important in chemistry because they play a role in chemical reactions. For example, enzymes can catalyze certain reactions by using positively charged ions as "navigators" to find suitable partners for merging with. Then, once the partners are near enough, the enzyme can escape without being attached to itself.
Ions are also important in physics. They provide one way for scientists to describe the behavior of electrons within an atom. Ions can be divided into two main types: positive ions and negative ions. Ions can lose or gain electrons, changing type but not charge.