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Transition elements are the chemical elements in the periodic table from group ⅢB to group VⅢ (some places include all subgroup and group VIII elements as transition elements). The common characteristic of the atomic structure of these elements is that the valence electrons are sequentially filled in the d orbital of the lower outer shell, so the lanthanides and actinides are sometimes included in the transition elements. Group I B elements (copper, silver and gold) also use d electrons in the formation of +2 and +3 valence compounds. Group ⅡB elements (zinc, cadmium, mercury) are similar to the traditional transition elements in their ability to form stable coordination compounds, so group ⅠB and ⅡB elements are often included in the transition elements.
Transition elements property characteristics
The characteristic properties of transition elements are:
(1) They are all metals, with high melting point, high boiling point, high hardness, high density and other characteristics, and metal luster, ductility, electrical conductivity and thermal conductivity are very good, a variety of alloys can be formed between different transition metals.
(2) There may be a single d electron in the atom or ion of the transition metal. The spin of the electron determines the magnetic properties of the atom or molecule. Thus, many transition metals are paramagnetic, and ferromagnetism can be observed in three metals: iron, cobalt, and nickel. It can be used as magnetic material.
(3) The d electrons of transition elements participate in the formation of chemical bonds during chemical reactions and can exhibit various oxidation states. The highest oxidation states range from +3 of scandium, yttrium and lanthanum to +8 of ruthenium and osmium. The transition elements form ionic bonds and hydrate easily when they form compounds with low oxidation states. When you form a high oxidation state compound, you form a covalent bond.
(4) Hydration ions of transition elements in the compound or solution mostly show a certain color, which is due to the unsaturated or irregular electron layer structure.
(5) Transition elements with vacant d orbitals for bonding and high charge/radius ratio are easy to form stable coordination compounds with various ligands. Most transition metals have their own unique production methods: electrolysis, metal thermal reduction, hydrogen reduction and iodide thermal decomposition.
Radius of atom
Compared with group ⅠA and ⅡA of the same period, the transition elements have smaller atomic radius.
In each period, the atomic radius decreases with the increase of atomic number, and increases before and after the copper subgroup. The atomic radii increase from top to bottom in each group, but the atomic radii of the fifth and sixth period congeners are very close, with the atomic radii of hafnium (146 pm) almost identical to that of zirconium (146 pm).
The electrons in the d orbital of the same period transition element are not full, the shielding effect of the d electrons is small, the nuclear charge increases successively, and the attraction of the outer shell electrons increases, so the atomic radius decreases successively. Before and after the copper subgroup, the full d orbitals enhance the shielding effect and increase the atomic radius. Due to the lanthanide contraction, the atomic radii of the fifth and sixth period congeners are similar.
The change of ionic radius is similar to that of atomic radius, that is, from left to right in the same period, the ionic radius with the same oxidation state decreases with the increase of nuclear charge. The ionic radius of the highest oxidation state increases with the number of electron shells from top to bottom. The lanthanide contraction effect also affects the ionic radii of the fifth and sixth period congeners.
Comparison of transition elements properties
① Transition elements generally have a small atomic radius, the outermost s electron and the lower outer d electron can participate in the formation of metallic bonds, so that the strength of the bond increases.
② The transition metal is generally silver white or gray (osmium is gray blue), with metallic luster.
③ Except scandium and titanium, which are light metals, the rest are heavy metals.
(4) Most of the transition elements have higher melting and boiling points and greater hardness and density. Tungsten is the hardest of all metals to melt and chromium the hardest of all metals.
Chemical properties of transition elements
① The gold property of transition elements is stronger than that of the elements in the same period p region, but weaker than that in the same period s region.
② The elements of the first transition system are more active than those of the second and third transition systems ----- nuclear charge and atomic radius.
In the same group, the atomic radius from the top to the bottom does not increase much, but the nuclear charge increases a lot. The attraction of the outer shell electrons is enhanced, and the nuclear charge plays a dominant role. The third transitional element has a small increase in atomic radius compared to the second transitional element (the effect of lanthanide shrinkage), so its chemical properties appear less reactive.
The first transition system elements can generally replace hydrogen from dilute acids (hydrochloric acid and sulfuric acid), and the standard electrode potential increases gradually from left to right, which is consistent with the gradual weakening of gold properties.
Manganese has an exception (lower than chromium) : it loses two 4s electrons to form a stable 3d configuration.
Scandium, yttrium and lanthanum are the most active metals among the transition elements. They can be oxidized quickly in air, react with water to release hydrogen, and also dissolve in acid. This is because there is only one electron in their secondary outer d orbital, which is easy to lose.