The concept of atomic radius

5.
4.
1 Atomic radius

1.
The concept of atomic radius

According to the different forces between atoms, atomic radii are generally divided into three types: covalent radius, metal radius and van der Waals radius
.

Covalent radius refers to half of the distance between two nuclei when two atoms of the same element form a covalent single bond
.

The metal radius refers to half of the nucleus distance between two tangent atoms in a metal crystal

2.
The changing law of atomic radius

In the same periodic element, as the number of nuclear charges increases, the attraction of the nucleus to the electrons increases, and the atomic radius tends to decrease; the reason is that the increased nuclear charge cannot be fully shielded by the increased electrons and the effective nuclear charge gradually increases
.

Among elements of the same family, the atomic radius tends to increase from top to bottom due to the increase in the number of electron layers

1) The changing law of the atomic radius of the main group elements

In the main group elements of the same period, the atomic radius decreases more obviously from left to right
.

The electrons are added to the outer orbital, and the positive charge of the nucleus is less neutralized, and the effective nuclear charge Z* increases more, so the amplitude of the reduction of the atomic radius is large

In the same main group elements, the atomic radius increases from top to bottom, and the increase is large
.

The reason is that as the number of electron layers increases, the orbit of the electron becomes farther from the nucleus, so the atomic radius tends to increase

Table 5-5 Atomic radii of some main group elements

2) The changing law of the atomic radius of the subgroup elements

For the elements of the subgroups of the same period, the atomic radius decreases from left to right to a small extent
.

The reason is that newly added electrons are filled into the d orbital of the secondary outer layer, and the shielding effect of the secondary outer layer electrons on the outermost electrons is much greater than the shielding effect between the electrons in the outermost electron layer

Table 5-6 Atomic radii of some subgroup elements

Lanthanides and lanthanide elements (generally referred to as internal transition elements) of the same period have a smaller radius decrease (Table 5-7)
.

Since the last electron is generally filled into the (n-2) layer f orbital, the shielding effect on the outermost electrons is large, and the effective nuclear charge increases less

Table 5-7 Atomic radii of lanthanides

It can be seen from Table 5-7 that the atomic radius (according to the metal radius) of the 15 lanthanide elements decreases by 9 pm in total, and this phenomenon is called lanthanide shrinkage
.

Due to the contraction of the lanthanide series, the atomic radii of the transition elements after the lanthanide series are reduced accordingly, resulting in the atomic radii of the fifth and sixth period transition elements of the same subgroup are very close, very similar in nature, and difficult to separate, such as Zr With Hf, Nb and Ta, Mo and W, etc.
; at the same time, the 15 elements of the lanthanide series have close atomic radii, similar in nature, and difficult to separate

For elements of the same subgroup, the atomic radius of the fifth period element is larger than that of the fourth period element, which is the result of the increase in the number of electron layers
.

The atomic radius of the elements of the fifth period is very similar to that of the elements of the sixth period, which is mainly caused by the shrinkage of the lanthanide series (Table 5-6)