Ion radius is a parameter describing the size of ions.Depends on the charge of the ionElectron distributionAnd crystal structure type.Set r positive asCationic radius, r negative isAnion radius。R Yang+r Yin=Key length。R Yang/r Yin andCrystal typeofThe ionic radius can be calculated from the bond length.Generally adoptedGoldschmidt radiusandPauling radius, both of which are of NaCl type structurecoordination numberIs 6.Shannon considered coordination number and electronspinTwo sets of latest data are obtained for the influence of status, one of which is referred toElectron cloud density map，cationRadius ratioTraditional data is 14pm large,anion14pm smaller, closer to the actual crystal.
The ion radius reflects the size of the ionphysical quantity。Ions can be approximately regarded as spheres. The derivation of ion radius is equal to the sum of positive and negative ion radii equal to the ionic bondKey lengthBased on this principleX-raycrystal structureThe ion radius is derived from the measured bond length.The size of the ion radius mainly depends on the charge carried by the ion and theElectron distribution, but we have to sufferIonic compoundStructure type (e.gcoordination numberThe ion radius is generally determined bysodium chlorideWhen the coordination number is 8, the radius increases about 3%;When the coordination number is 4, the radius value decreases by about 5%.The negative ion radius is generally large, about 1.3~2.5 angstroms;The positive ion radius is small, about 0.1~1.7 angstrom.The sum of the radii of positive and negative ions andRadius ratioThese are two important factors to clarify the properties and structural types of ionic compounds.
For example: Na+staycoordination numberWhen it is 4, 6, 8, 9, 12, the ion radius is 99, 102, 118, 124, 139 pm respectively;O2-When the coordination number is 2, 6 and 8, the ion radius is 121, 140 and 142 pm respectively.
Judgment method of ion radius
(1) The larger the number of electrons, the larger the radius of particles of the same element.For example, sodium atom>sodium ion, chlorine atom<chlorine ion, ferrous ion>iron ion;
(2) In the same period, the anion radius of the element particles is larger than the cation radius.For example, oxygen ion>lithium ion;
(3) For ions (single nuclei) with the same electronic layer structure, the smaller the number of nuclear charges, the larger the radius.For example, oxygen ion>fluoride ion>sodium ion>magnesium ion>aluminum ion sulfur ion>chloride ion>potassium ion>calcium ion;
(4) The radius of the high valence cation of the same element is smaller than that of the low valence cation and the atomic radius of the metal.For example, copper ion<cuprous ion<copper atom negative divalent sulfur>sulfur atom>tetravalent sulfur>hexavalent sulfur.[1]
Determination of ionic radius
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The distance between anion and cation can be measured by the method of one ray crystal analysis. The ion radius can be calculated from the distance between ions, the ions with the same type of electronic structure, and the contact conditions of several crystals and different positive and negative ions.Pauling once studied the problem of ion radius from another angle.The ionic radius is different due to the different coordination number. When the coordination number is a, the coordination number is 6 as the standard to calculate: ra, ra=
, n is the Bonn index (when the coordination number is 12, 8, 6, 4, the corresponding division coefficient is 1.12, 1.03, 1.0, 0.94), but it does not seem to fully conform to the formula when back calculated from the actual data.From the definition of ion radius and the determination of its value, it can be seen that the crystal formed by the actual element ions is different from the NaCI type ion crystal;X-ray measurement requires well crystallized crystals, but there are often ion crystals of many elements that are difficult to prepare or poorly crystallized, so it is not surprising that the ion radius values provided by predecessors are different from each other, and some valence states do not have ion radius values.[2]
