What Is the Atomic Radius?

Atomic Radius ^[1] ( Atomic Radius ) is one of the parameters describing the size of the atom.

There are three factors affecting the atomic radius: one is

Atoms do not have an exact size. The so-called effective size refers to the atomic distance shown by the atoms in chemical motion, that is, the equilibrium distance between attraction and repulsion.

Depending on the interaction force, there are several effective radii:

Van der Waals radius, metal radius, ion radius, and covalent radius.

Divide the closest contact distance between the atoms by 2 to get the metal atom radius.

When two electrons of the same element are connected by a covalent bond, 1/2 of the distance between their cores is called the covalent radius of the atom (such as H2, O2)

In ionic crystals, the nuclear distance between adjacent ions is equal to the sum of the radii of the two ions ^[7] .

Determination of ionic radius

Lande ion radius

In 1920, after comparing the unit cell parameters of the compounds with a NaCl structure in the following table (the numbers in parentheses in the table are more accurate measurements later), Lande considered that The unit cell parameters are almost equal, meaning that negative ions and negative ions are already in contact in the crystal. He used simple geometric relationships to derive the ionic radii of S2- and Se2-.

Wasastjerna ion radius

Wasastjerna (Wassastjerna) in 1925 according to the molar refraction of the ion proportional to its volume method. Divide the size of the ions. Obtain 8 positive ions and 8 negative ions radius. Including F- (133pm) and O2- (132 Pm).

Goldschmidt ion radius

In 1927, Goldscbmidt used the ionic radius data of F- and O2- of Wasastjerna, and based on the experimentally measured data of the contact distance between ions in the ionic crystals, the radii of more than 80 ions were derived ), Which is still in use today.

Pauling ion radius

In 1927, Pauling used semi-empirical methods to derive a large number of ionic radii based on data on the distance between the nuclei of five crystals (NaF, KCl, RbBr, CsI, and Li2O). Because the size of an ion is determined by its outermost electron distribution, the outermost electron distribution is inversely proportional to the effective nuclear charge.

Shannon ion radius

Shanon et al. Deduced the ion radius according to the contact distance under different conditions according to the coordination number of the ion, the geometric configuration of the coordination polyhedron, and the spin condition of the ion. One set starts with an O2-radius of 140pm and the other set starts with an O2-radius of 132pm. Two effective ion radii are obtained.

Ion radius has the following trends

1) The same main family, the same charged ions, the radius increases from top to bottom.

Li + <Na + <K + <Rb + <Cs +; F- <Cl- <Br- <I-

2) Positive ions with the same number of electrons outside the nucleus of the element in the same period significantly decrease with the increase of the positive charge number.

3) Ions of various valences outside the same element nucleus. The more electrons, the larger the ion radius.

4) Negative ion pairs with the same number of electrons outside the nucleus. As the negative electricity price increases, the radius slightly increases, but the increase is not large.

5) The radius of lanthanide trivalent positive ions decreases from La3 + to Lu3 +, which is caused by the shrinkage effect of lanthanide