Ionic radius

The ionic radius refers to the effective size of a monatomic ion in an ionic lattice. It is assumed simplified in that they are rigid spheres whose radii are independent of the ionic compound ( provided that the coordination number of matches ). To determine the ionic radii, one first determines the spacing of the ions encountered in the crystalline lattice. For these cation-anion distances initially obtains the sum of radii rA rK for various ion combinations. Thus, the radii of the individual ions can be determined, the radius of an ion involved must be known independently at least.

Pauling has the value of 140 pm for an O2 - ion determined theoretically; this value and the calculated additional ionic radii apply to the coordination number 6

A function of the ionic radii of the co-ordination number

The ionic radii are always directly related to the coordination number, because increasing the number of neighboring ions, thus increasing also the repulsive forces between the electron shells of ions, the consequence of this is that the equilibrium distance increases. From experimentally determined ionic radii shows that the relative changes of the individual ions are individually and you can specify only an average approximation. It results in the following dependence:

The table shows that the ionic radii of one and the same ion behave as 1,1:1,0:0,8 for coordination numbers 8, 6, 4 From these values ​​it can be concluded that the ionic radii of the equilibrium distance resulting in a crystal and the image of a rigid sphere for an isolated ion is not the case. An ion behaves in different compounds, therefore, only as a rigid sphere with radius approximately constant when the number of nearest neighbors, called the coordination number does not change.

Relationship between ionic radius and atomic radius

Ionic radii and atomic radii are related:

• With cations, ie positively charged ions, the ionic radius is smaller than the atomic radius. The greater the positive charge is, the smaller the ion radius.
• With anions, negatively charged ions that is, the ionic radius is greater than the atomic radius. The higher the negative charge, the greater is the ion radius.

What are the ionic radius depends?

A) Within a group (ie, in the periodic table from top to bottom ) to take the ionic radii, since in each period, a new atomic orbital is present and the distance of the valence electrons increases towards the nucleus. Within one period (that is, in the periodic table from left to right ) of the ion radius decreases as the atomic number increases, so that the attraction force on the electrons of the nucleus is larger and the ionic radius decreases.

B) on the number of valence electrons. The following rule applies: Cations are always smaller than the anions of the same period. Some cations ( K , Rb , Cs , NH4 , Ba2 ) is larger than the smallest anion ( F- ). Explanation: from element to element takes within a period, the atomic number ( = number of protons ) to. That is, the valence electrons are increasingly attracted by more and more protons. It follows: The radius decreases slightly.

For ions of the transition metals, the radius also depends on the spinning condition (see the ligand field theory).

The ratio of the ionic radii of cations / anions determined by how many ions they are surrounded ( coordination number ), making it among other things, responsible for the crystal structure or the crystal structure type.