The d-block elements are those which have incomplete d-subshell in its atomic or ionic state. The outermost or valence differentiating electron enters in the d-orbital belonging to the penultimate shell means (n-1)d shell where n is the last shell. They are also known as transition elements because their properties are lies between more electropositive elements (s-block) and less electropositive elements (p-block). Zinc (Zn), Cadmium (Cd), Mercury (Hg) have completely filled d-subshell in their ground state and excited state. So, they can not be incorporated with d-block elements.
The d –block elements are situated in the middle of the periodic table and includes 3 – 12 groups in the modern periodic table.
The general electronic configuration of d-block elements are (n-1)d1-10ns1-2 where (n-1) stands for inner shell which may have 1 to 10 electrons and n stands for outer shell which may have 1 to 2 electrons. The d-block elements incorporates three series in which each series contains ten elements. The three series are 3d series (Sc to Zn), 4d series (Y to Cd) and 5d series (La to Hg). The fourth (6d) series comprising of ten elements from Ac to Uub in which 6d series is still incomplete. The electronic configuration of these series are given below –
First transition series (3d series) – 4th period – Sc (atomic number, 21) to Zn (atomic number, 30)Sc (atomic number, 21) [Ar]3d14s2
Transition metals shows the variable oxidation state due to the presence of electrons in the inner (n-1)d and outer ns orbital, as they can participate in bond formation due to less energy difference between them. It means the no. of oxidation state depends on the number of electrons available in outermost shell.
The valence electrons of the d-block elements are in (n-1) d and ns shell which can easily removed by oxidation. Mainly, there are following factors which favor the higher oxidation state –
As we move from left to right, ionic radii decreases because –
Nuclear charge dominates the effect of added electron and reducing the size.
Atomic radius increases from top to bottom and decreases along the period. For atomic radii, the trend is not uniform. As we move along the period-
In the beginning of the series, there is smaller number of electrons in their outermost d-shell. So, their nuclear charge predominates and decreases the atomic radii. As we going later in the series, the number of electrons in the inner (n-1)d-shell increases. The repulsion between the outer ns electrons increases due to which they slightly pushed away and increases the atomic radii.
Note- But the radii of 2nd and 3rd transition series are nearly same due to lanthanide contraction.
Metallic nature means the reactivity of an element (capability of an atom to lose the electrons). Transition metals shows metallic character because of –
All the transition elements are metal and they are hard. Their property of hardness shows that the presence of strong metallic bond in them. Transition metals form the covalent bond due to the presence of unpaired electrons in the d-orbital. These unpaired electrons may overlap and makes covalent bond. So, their property of good conductivity and their hardness indicates that they have covalent or metallic bonding. Therefore, the transition elements possess metallic character represents both metallic and covalent bonding in them.
As we move from left to right, the number of electrons increases, nuclear force increases and the capability of losing electron decreases. So, the metallic character decreases. Copper (Cu), Molybdenum (Mo), manganese (Mn) has maximum number of unpaired electrons. So, they are very hard. Whereas zinc (Zn), cadmium (Cd), mercury (Hg) do not have any unpaired electrons. So, these metals are not very hard.
The trend of melting and boiling point of the transition metals is not regular across the period. The melting and boiling point of d-block elements increases firstly, reaches maximum and then gradually decreases.
All transition elements are very hard. They have closely packed and held together by the strong metallic bonding due to the presence of unpaired electrons in the outermost shell. The melting point is mainly depends on the strength of bonding and number of unpaired electrons. More is the number of unpaired electrons; more is the strength of metallic bonding and high is the melting and boiling point.
As we move along the period, the first four row elements has higher melting and boiling point are due to the more electrons present in the inner (n-1)d orbital and outer ns-electrons which results in strong metallic bonding. Later in the series, the last five elements have low melting and boiling point because unpaired electrons in the d- orbital get paired up.
It is the minimum amount of the energy which is required to remove the most loosely bound electron of isolated gasesous atom.
Generally, as we move from left to right –
These are the factors which tend to increases the ionization energy. The ionization energy of d –block elements are lies between s and p – block.
• The first ionization energy of 5d series is more than 3d and 4s.
Explanation -– In the 5d series, the electrons are added in the inner 4f orbital which has poor shielding effect. So, outermost electrons experience greater nuclear attraction due to which leads to high ionization energy.
The substances are classified into diamagnetic, paramagnetic and ferromagnetic on the basis of magnetic field.
All the transition elements forms colored compounds because they have vacant d-orbital. Their color variation is depend on the charge on the metal ion and number and type of atoms attached to it. Actually, the transition metals ions are not colored on their own.They become colored only when they become complex ions.
Their complexes are formed when they bonded to the ligands. Without bonded to the ligands, all the d-orbitals are degenerate (all have same energy level).But when they form complexes, their d-orbital interact in such a way that they become non-degenerate. It means orbitals are spilt into different energy level. This can be explained on the basis of crystal field splitting energy (CFSE) in which we can indentify the energy of different d- orbitals. Their splitting is depend on the geometry of complex, nature of metal and oxidation state of the metal ion.
When the electron is excited from lower energy level to the higher energy level, which is called d-d transition in which d orbitals are involved (t2g and eg for octahedral complex and e and t2 for tetrahedral complex). This d-d transition falls in visible region for all transition elements. Then, some amount of energy is absorbed and remaining energy emitted as colored light.
Here, you can see that color of complex ion is due to excitation and de-excitation. The color of the ion is complementary of color absorbing by it.The colors of some transition elements are –
Here, you can see that the chromium in different oxidation state gives different color. This color change help us to identify the end point of redox reaction.
Note – Transition metals having zero (Sc+3, Ti+4) or ten (Zn+2) d electrons will be colorless.
D- Block elements form complexes because of following reasons –
Catalysts are those substances which alter the rate of reaction. Transition metal and their oxides are used as catalyst in many chemical reactions such as platinum, iron, nickel. In the hydrogenation of unsaturated organic compound, nickel is used as catalyst. Iron is used as catalyst in haber process. They show good catalytic properties due to –
When transition metal is used as catalyst in any reaction, they form unstable intermediate with the reactants due to their ability to show the variable oxidation state. This unstable intermediate lower the activation energy of the reaction, increase the rate of reaction and converted into final product. The catalyst used in this reaction provides the large surface area on which reactant molecules move closer to each other and get absorbed on the surface.
All the transition metals forms interstitial compounds. Interstitial compounds are those compounds in which small atoms like C, H or N get trapped into the interstitial sites of their crystal lattice. They form non-stoichiometric material. Some characteristics of interstitial compounds are
An alloy is the combination of metals or metal with another element which conserve the desired properties of metal and enhances the properties of constituent elements. In transition elements, the atomic size of the elements are very similar to each other due to which they are easily replace with each other in the crystal lattice. Therefore, a solid solution is formed which is known as alloy. Alloy is corrosion resistant and having wide range of other applications. For example – brass – alloy of copper and zinc.
Reactivity means the ease to lose the electrons. D-elements are less reactive because they lies in the middle of the modern periodic table. They are transit between s and p block elements which are more reactive than d block elements. As we move from left to right, their reactivity decreases due to –
So, reactivity decreases.
Standard reduction potential means tendency to gain the electrons. The d-block elements are weak reducing agents due to –
Note-greater reduction electrode potential make their oxidation state unstable which means the metal in that oxidized that acts as strong oxidizing agent.
D- block elements includes 3-12 group in the modern periodic table. The d block elements includes titanium, iron, gold, silver, copper, zinc etc. Some applications of d-block elements are given below –