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The IIB group elements Zn, Cd ans HG are not considered as transition elements but they are present in d-block elements. 

In these elements, the last electron normally enters the d orbital which is the penultimate shell, this is due to the aufbau principle of increasing order of energy level filling of orbitals i.e. the last electron goes to (n-1) d orbital. Hence these elements are named as d-block elements and they are called as transition metals because they form the bridge between the representative elements. These elements have partly filled d-subshells in their elementary form or in their simple ions. The electrons in the outer most orbit are sheilds by inner d-electrons. There fore the size of terminal elements is more than the preceding ones. 

All the transition metals have metallic character. The atoms in the metals are bound by strong metallic bonds with strong inter atomic bonds. Therefore the melting and boiling points are high.

The elements belonging to d-block are metals. The d-block elements are classified into four transition series. These 4 series corresponds the filling of 3d, 4d, 5d and 6d orbitals. 

The series of elements, that are formed by filling the 3d, 4d and 5d shells of electrons, comprise the d-block elements. They are often called as transition elements because their position in the periodic table is between s-block and p-block elements. Their properties are transitional between the highly reactive metallic elements of the s-block, which form ionic compounds and elements of p-block which form covalent compounds. In s and p blocks electrons add to the last shell, in d-block electrons are added to the penultimate shell. Typically the transition elements have an incompletely filled d level. The zinc group has d¹⁰ configuration and compounds of these elements show some differences from other transition elements. The elements make up three complete rows of ten elements and an incomplete fourth row. The position of the incomplete fourth series is discussed along with the f-block elements. 

General Characteristics Of Transition Elements 

(i) Except for mercury, which is a liquid at room temperature all other elements are solid metals exhibiting all the characteristics of a metal. 

(ii) The show variable oxidation states unlike s and p block elements. 

(iii) They, and some of their compounds, show catalytic properties. 

(iv) Their compounds are coloured. 

(v) They have great tendency to form complex compounds 

(i) Except for mercury, which is a liquid at room temperature all other elements are solid metals exhibiting all the characteristics of a metal. 

(ii) The show variable oxidation states unlike s and p block elements. 

(iii) They, and some of their compounds, show catalytic properties. 

 (iv) Their compounds are coloured 

Reactive decreases with increase in atomic number due to decrease in the size and increase in ionization enthalpy. Added to this they form a layer of oxide which makes them passive.

This is because there is space around rhOs atom for four bonded oxygen atoms and not for eight fluorine atoms. 

Transition elements usually exist in several different oxidation states and the oxidation states changes in units of one, e.g. Fe²⁺ and Fe⁺³, Cu⁺¹ and Cu⁺².

In the transition elements, the energies of (n-1)d orbitals and ns orbitals are very close. Hence electrons from both can participate in bonding. Therefore they show a number of oxidation states.

Transition metal form alloys with other transition metals easily due to almost similar atomic radii,they can mutually substitute their positions.

Due to similar size as a result of lanthanoid contraction. Hence they have higher enthalpy of atomization.

Transition Metals

The elements belonging to d-block are metals. The d-block elements are classified into four transition series. These 4 series corresponds the filling of 3d, 4d, 5d and 6d orbitals. 

 The series of elements, that are formed by filling the 3d, 4d and 5d shells of electrons, comprise the d-block elements. They are often called as transition elements because their position in the periodic table is between s-block and p-block elements. Their properties are transitional between the highly reactive metallic elements of the s-block, which form ionic compounds and elements of p-block which form covalent compounds. In s and p blocks electrons add to the last shell, in d-block electrons are added to the penultimate shell. Typically the transition elements have an incompletely filled d level. The zinc group has d¹⁰ configuration and compounds of these elements show some differences from other transition elements. The elements make up three complete rows of ten elements and an incomplete fourth row. The position of the incomplete fourth series is discussed along with the f-block elements.

First transition series

This is also called as 3d series which corresponds the filling of 3d orbital. It starts from scandium whose atomic number is 21 and includes 10 elements till zinc whose atomic number is 30. 

First transition series (or) 3d series