D block elements are the elements which can be found from the third group to the twelfth group of the modern periodic table. The valence electrons of these elements fall under the d orbital. D block elements are also referred to as transition elements or transition metals. The first three rows of the d block elements which correspond to the 3d, 4d, and 5d orbitals respectively are given in the below article.
Table of Content
- D Block as Transition Element
- Electronic Configuration
- Atomic and Ionic Radii
- Oxidation States
- Formation of Coloured Ions
- Alloy Formation
- Important Compounds
What are D Block Elements?
Elements having electrons (1 to 10) present in the d-orbital of the penultimate energy level and in the outer most ‘s’ orbital (1-2) are d block elements. Although electrons do not fill up ‘d’ orbital in the group 12 metals, their chemistry is similar in many ways to that of the preceding groups, and so considered as d block elements.
These elements typically display metallic qualities such as malleability and ductility, high values of electrical conductivity and thermal conductivity, and good tensile strength. There are four series in the d block corresponding to the filling up of 3d, 4d, 5d or 6d orbitals.
- 3d- Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn
- 4d- Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd
- 5d- La, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg
- 6d- incomplete.
There are 10 elements filling up the ‘d’ orbital in each series.
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Position of D Block Elements in Periodic Table
D block elements, occupy columns 3 to 12 and may have atoms of elements with completely filled ‘d’ orbital. IUPAC defines a transition metal as “an element whose atom or its cations has a partially filled d sub-shell.
Why D Block Elements are called Transition Elements?
Transition elements occupy groups 4–11. Scandium and yttrium of group 3, having a partially filled d subshell in the metallic state also are considered as transitions elements. Elements like Zn, CD and Hg of the 12 column of the d block have completely filled d-orbital and hence are not considered as transition elements.
Transition Elements are so named, indicating their positioning and transition of properties between, s and p block elements. So, all the transition metals are d block elements but all d block elements are not transition elements.
Properties of Transition Metals
- Electrons added to the ‘d’ sub-orbitals that lie between their (n+1) s and (n+1) p sub-orbitals.
- Placed between s and p block elements in the periodic table.
- Properties between s and p-block elements.
Electronic Configuration of D Block Elements
D block Elements have a general electronic configuration of (n-1)d 1-10ns 1-2. These elements can find stability in half-filled orbitals and completely filled d orbitals. An example of this would be the electronic configuration of chromium, which has half filled d and s orbitals in its configuration – 3d54s1. The electronic configuration of copper is another such example. Copper displays an electronic configuration of 3d104s1 and not 3d94s2.
This can be attributed to the relative stability of the completely filled d orbital. Zinc, Mercury, Cadmium, and Copernicium exhibit completely filled orbitals in their ground states and in their general oxidation states as well. For this reason, these metals are not considered as transition elements whereas the others are d block elements.
- The electronic configuration for period 4, transition elements is (Ar) 4s 1-2 3d 1-10
- The electronic configuration for period 5, transition elements is (Kr) 5s 1-2 4d 1-10
- The electronic configuration for period 6, transition elements is (Xe) 4s 1-2 3d 1-10
Along the period, from left to right, electrons are added to the 3d subshell as per the Aufbau principle and Hund’s rule of multiplicity.
|1st transition series||Sc||Ti||V||Cr||Mn||Fe||Co||Ni||Cu||Zn|
|2nd transition series||Y||Zr||Nb||Mo||Tc||Ru||Rh||Pd||Ag||Cd|
|3rd transition series||La||Hf||Ta||W||Re||Os||Ir||Pt||Au||Hg|
Anomalies do occur in all the series, which can be explained from the following considerations.
- The energy gap between the ns and (n-1) d orbitals
- Pairing energy for the electrons in s-orbital
- Stability of half-filled orbitals to the partly filled orbitals.
Chromium has 4s13d5 electron configuration rather than the 4s23d4 configuration and copper 4s13d10 rather than 4s23d9. These anomalies in the first transition series can be understood from the stability of half-filled orbitals compared to the partly filled orbitals.
In the second series transition metals, from niobium, electron presence in d orbitals appears to be preferred than being shared in s orbitals. Between the available s and d orbitals, the electron can either go for sharing in s-orbital or excited to d-orbital. Obviously, the choice depends on the repulsive energy it has overcome on sharing and the energy gap between the s and d-orbitals.
In the second series, s and d-orbital have the almost same energy, because of which electrons prefer to occupy the d-orbital. So from niobium, s-orbital has mostly only one electron. Third series transition metals, on the other hand, has more paired s configuration even at the expense of half-filled orbitals (Tungsten- 6s25d4). This series comes after filling up of 4f orbitals and resulting lanthanide contraction.
The reduced size results in high shielding of d orbitals by the ‘f’ electron. This shielding increases the energy gap between the s and 5d orbitals such that pairing energy is less than the excitation. Excitation of the electron does not take place in tungsten, in spite of the stability possible because of half-filled orbitals.
Atomic and Ionic Radii of D Block Elements
Atomic and ionic radii of elements of all three-transition series
- Decreases rapidly, from column 3 to 6
- Remains steady, from column 7 to 10 and
- Start increasing from column 11 to 12.
For example, in the first transition series, atomic radii, the decrease is more from Sc to Cr (group 3 to 6 ), is almost same for Mn, Fe, Co, Ni (group 7,8 9 &10) and increase in cu and Zn.
- The larger decrease in atomic radii, in column 3 to 6 elements is due to the increase in effective nuclear charge but poor shielding because of the smaller number of d-electrons.
- In elements of column 7 to 10 increasing effective nuclear charge is balanced by the repulsion between the shared d electrons so that radii remain the same.
- In the case of 11 and 12 columns elements, the d orbital is full with ten electrons and shield the electrons present in the higher s-orbital. So, group 11 &12 elements like Cu & Zn have bigger size than its earlier elements in the block.
Since electrons occupy a higher orbital, radii of the third series are to be more than the second series elements. But radii of both series are almost the same. In the third series elements, 5d orbitals are filled only after the filling up of 4f orbitals, which increases the effective nuclear charge by 14 units.
This higher nuclear charge leads to the larger shrinkage of radii known as Lanthanide contraction. Increase in radii due to the higher orbital will be effectively neutralized by the increase the nuclear effective charge. So, radii of second and third series elements have same atomic radii. For example, Niobium and hafnium have almost the same atomic radii.
Properties of D Block Elements
Ionization Energy of D Block Elements
Ionization energy is the energy needed to remove the valence electron from the atom/ion and is directly related to the force of attraction on the electron. Hence larger the nuclear charge and smaller the radii of the electron larger will be the ionization energy (IE). Ionization Energy also will be more for half-filled and fully filled orbitals.
Ionization Energy of the d block elements is larger than s-block and smaller than the p-block elements, between which, they are placed. In the first series, except chromium and copper first Ionization Energy involves removal from filled s-orbital. Among them, Ionization Energy of d block elements increases with the increase in atomic number up to Fe.
In Co and Ni, increasing sharing of d-electrons compensate for the atomic number increase resulting in the decrease of Ionization Energy. Copper and zinc show increasing IE, as s -block elements. In the second series, elements from Niobium have single electrons in the s-orbital.
Hence, they show a gradual increase in IE with increasing atomic number. Palladium, on the other hand, has completed d-shell and no electron in s-shell. So, Pd shows the maximum IE. Because of lanthanide contraction, the attraction of electrons by the nuclear charge is much higher and hence IE of 5d elements are much larger than 4d and 3d. In 5d series, all elements except Pt and Au have filled s-shell.
Elements from Hafnium to rhenium have same IE and after IE increases with the number of shared d-electrons such that Iridium and Gold have the maximum IE.
D block elements show typical metallic behaviour of high tensile strength, malleability, ductility, electrical and thermal conductivity, metallic lustre and crystallize in bcc/ccp/hcp structures.
They are very hard and have a high enthalpy of atomization and low volatility except for Copper. Hardness increases with the number of unpaired electrons. Hence Cr, Mo and W are very hard metals among d block elements. The group-12 elements (Zn, Cd and Hg) shows the exception in this regard also.
Oxidation States of D Block Elements
Oxidation state is a hypothetical state, where the atom appears to release or gain electrons more than the usual valency state. It is still useful in explaining the properties of the atom/ion. Transition elements/ions may have electrons in both s and d-orbitals.
Since the energy difference between s and d-orbital are small, both the electrons can involve in ionic and covalent bond formation and hence exhibit multiple(variable) valency states (oxidation states).
Each transition element can hence exhibit a minimum oxidation state corresponding to the number of s-electrons and maximum oxidation state equivalent to the total number of electrons available in both s and d-orbitals. In between oxidation states also become possible.
Trends in the Oxidation States
1. The minimum Oxidation state of 1 is shown by Cr, Cu, Ag, Au and Hg.
2. More stable Oxidation state increases in the order 3d ˂ 4d ˂ 5d. 3d series elements are most stable in +2; 4d series in +2 and +4 and 5d series in +4. Cr6+ and Mn7+ (of 3d) are not stable in their higher OS. Compounds containing them, CrO42- and MnO4– are very reactive and strong oxidizing agents.
While Mo6+ and Tc7+ (of 4d) are stable in their higher OS. Compounds containing them, MoO42- and TcO4– are unreactive and stable. Similarly, W6+ and Re7+ (of 5d) are stable in their higher OS. Compounds containing them, WO42- and ReO4– are unreactive and stable.
Cations of the second and third-row transition metals in lower oxidation states (+2 and +3) are much more easily oxidized than the corresponding ions of the first-row transition metals. For example, the most stable compounds of chromium are those of Cr(III), but the corresponding Mo(III) and W(III) compounds are highly reactive.
In fact, they are often pyrophoric, bursting into flames on contact with atmospheric oxygen. As we shall see, the heavier elements in each group form stable compounds in higher oxidation states that have no analogues with the lightest member of the group.
3. Strongly oxidizing, high oxidation number elements form compounds of oxides and fluorides and not bromides and iodides.
Vanadium form only VO4–, CrO42-, MnO4– , VF5, VCl5, VBr3, VI3 and not VBr5, VI5. V5+ oxidizes Br– and I– to Br2 and I2 but not fluoride because of its high electronegativity and small size.
Similarly, strongly reducing, low oxidation number elements form bromides and iodides and not oxides and fluorides.
4. Maximum oxidation state equal to the s and d-electrons is exhibited by middle-order elements in each series. Thus, manganese in 3d series has +7, Ru in 4d and Os in 5d possess +8 maximum oxidation state.
5. Elements may show all the Oxidation states in between the minimum and maximum.
6. Elements in their lower oxidation states will be ionic and basic (TiO,VO, CrO, MnO, TiCl2 and VCl2) in-between state amphoteric (Ti2O3, V2O3, Mn2O3, CrO3, Cr2O3, TiCl3, VCl3 ) and higher oxidation state covalent and acidic (V2O5, MnO3, Mn2O7, VCl4 and VOCl3 ) .
7. Lower oxidation state may get stabilized by back bonding in complexes. Ni(CO)4, Fe(CO)5, [Ag(CN)2]–, [Ag(NH3)2]+
Lower oxidation states in these metals are stabilised by ligands like CO, which are pi-electron donors, whereas the higher oxidation states are stabilised by electronegative elements like Fluorine(F) and Oxygen(O). Hence the high oxidation compounds of these metals are mainly fluorides and oxides.
8. Relative stabilities of the oxidation states depend on many factors, like, the stability of the resulting orbital, IE, electronegativity, enthalpy of atomization, enthalpy of hydration, etc.
- Ti4+ (3d0) is more stable than Ti3+(3d1). Mn2+ (3d5) is more stable than Mn3+(3d4).
- Ionization energies contribute to the relative stability of transition metal compounds (ions). For example, Ni2+ compounds are thermodynamically more stable than Pt2+, Whereas, Pt4+ compounds are more stable than Ni4+. The relative stabilities can be explained as follows
|Metal||(IE1+IE2) kJmol−1,||(IE3+IE4) kJmol−1,||Etotal, =(=IE1+IE2+IE3+IE4) kJ mol−1|
Thus, the ionization of Ni to Ni2+ requires lesser energy (2490 kJ mol−1) as compared to the energy required for the production of Pt2+ (2660 kJ mol−1). Therefore, Ni2+ compounds are thermodynamically more stable than Pt2+ compounds.
On the other hand, the formation of Pt4+ requires lesser energy (9360 kJ mol1) as compared to that required for the formation of Ni4+ (11290 kJ/mol). Therefore, Pt4+ compounds are more stable than Ni4+ compounds. This is supported by the fact that [PtCl6]2+ complex ion is known, while the corresponding ion for nickel is not known.
9. In p-block, the heavier elements prefer lower oxidation states due to what is called the inert pair effect. But in the case of d block elements, the higher oxidation states are more stable for heavier members in a group.
Electrode Potential in D Block Elements
Relative stabilities of transition metal ions in different oxidation states in the aqueous medium can be predicted from the electrode potential data. The oxidation state of a cation for which ΔH(ΔHsub + lE + ΔHhyd) or E° is more negative (for less positive) will be more stable.
- E° becomes less negative along the series indicating higher stability of the reduced state.
- Compared to first and second group metals, transition elements have low E°.
Physical Properties of D Block Elements
Density: Among the transition series, the trend in density will be reverse of atomic radii, i.e. density increase remains almost the same and then decreases along the period.
Down the column density of 4d series is larger than 3d. Because of lanthanide contraction and a larger decrease in atomic radii and hence the volume density of 5d series transition elements are double than 4d series.
In the 3d series, scandium has the lowest density and copper highest density. Osmium (d=22.57g cm-3) and Iridium (d=22.61g cm-3) of 5d series have the highest density among all d block elements.
Some relative radii of d block elements are Fe ˂ Ni ˂ Cu, Fe ˂ Cu ˂ Au, Fe ˂ Hg ˂ Au.
Why D Block Elements have high Melting and Boiling Point?
Unpaired electrons and the empty or partially filled d-orbitals form covalent bonding in addition to the metallic bonding by s-electrons. Because of such strong bonding, d-block elements have high melting and boiling points than s and p block elements. This trend goes till d5 configuration and then decreases as more electrons get paired in the d-orbital.
- Cr, Mo and W possess the highest melting at boiling point in their series of elements.
- Manganese (Mn) and Technetium (Tc) have half-filled configuration resulting in weak metallic bonding and abnormally low melting and boiling points.
- Group12, Zn, Cd and Hg have no unpaired d-electrons and hence no covalent bonding. Their melting and boiling point will be the lowest in their series.
Mercury – the liquid metal: Mercury is the only metal that exists in its liquid state at room temperature. 6s valence electrons of Mercury are more closely pulled by the nucleus (lanthanide contraction) such that outer s-electrons are less involved in metallic bonding.
What Transition Elements are Considered Noble Metals?
In the three transition series
- The ionization energies of elements increase very slowly across a given row
- From left of 3d series to the right corner 5d transition elements, density, electronegativity, electrical and thermal conductivities increase, while enthalpies of hydration of the metal cations decrease in magnitude.
This indicates that the transition metals become steadily less reactive and more “noble” in character. The relatively high ionization energies, increasing electronegativity, and decreasing low enthalpies of hydration make metals (Pt, Au) in the lower right corner of the d block as unreactive that they are often called the “noble metals.”
Magnetic Properties of D Block Elements
Materials are classified by their interaction with the magnetic field as:
- Diamagnetic: if repelled,
- Paramagnetic: if attracted and
- Ferromagnetic: if it can retain the larger magnetic nature even in the absence of magnetic field.
Paired electrons cause diamagnetism. Unpaired electrons result in para-magnetism and aligned together unpaired electrons produce ferromagnetism. D block elements and their ions exhibit this behaviour depending on the unpaired electrons.
Unpaired electrons contribute to ‘orbital magnetic moment’ and ‘spin magnetic moment’. However, for 3d series, the orbital angular moment is negligible and the approximate spin-only magnetic moment is given by the formula:
µ = √[4s (s + 1)] = √[n (n + 1)] BM
where ‘S’ is the total spin and ‘n’ is the number of unpaired electrons. Its unit is Bohr Magneton (BM). For higher d-series, the actual magnetic moment includes components from the orbital moment in addition to the spin moment. Chromium and molybdenum possess maximum number (6) of unpaired electrons and magnetic moment.
|Ion||Outer configuration||No. of unpaired electrons||Magnetic moment (BM)|
Formation of Coloured Ions by D Block Elements
Compounds of d block elements have a variety of colours. When a frequency of light is absorbed, the light transmitted exhibit a colour complementary to the frequency absorbed. Transition element ions can absorb the frequency in the visible region to use it two ways and produce visible colour.
1. d-d Transition
One way is the excitation of an electron to a higher energy level. In transition element ions, the presence of d-electron and empty d-orbital shall result in colour formation. Valence electron excitation and de-excitation. This is called d-d transition.
D-orbitals are generally degenerate and have the same energy. Presence of ligands that can form coordinate bonds with these ions, remove the degeneracy and split them into two groups namely eg and t2g d-orbitals. The energy difference (∆E) depends on the strength of the incoming ligand.
Electrons in the lower d-orbitals can be excited into the higher d-orbitals by absorbing energy in the visible region (λ=400-700nm) and transmit (give) a colour complementary to it.
For example, [Cu(H2O)6] 2+ ions absorb red radiation and appear complimentary blue-green. Hydrated Co2+ ions absorb radiation in the blue-green region, and therefore, appear red in the sunlight.
Cupric ion is colourless and in the presence of water molecules becomes blue in colour.
a) Colour of the ions varies with its oxidation state. The Cr6+ as in potassium dichromate yellow in colour, whereas Cr3+ and Cr2+ are generally green and blue respectively.
b) Colour of the compound is dependent on the complexing or coordinating group also. For example, Cu2+ shows light blue colour in the presence of water as ligand but the deep blue colour in the presence of ammonia as the ligand.
c) Transition metal ions which have:
- Completely filled d-orbitals having no vacant d-orbitals for excitation of electrons are colourless. Cu+(3d10), Zn2+(3d10), Cd2+(4d10) Hg2+(5d10), and Zn, Cd, Hg are colourless.
- Transition metal ions which have completely empty d-orbitals without d-electrons are also colourless. Sc3++(3d0), and Ti4++(3d0), ions are colorless..
L-M and M-L dπ – pπ bonding
Ligands may donate their p electrons into the empty d orbitals of metal ions. This interaction called as ligand-metal / metal-ligand or dπ – pπ bonding also may give colour to the compounds.
Complex Formation Tendency of D Block Elements
Complex compounds are compounds wherein a number of neutral molecules or anions are bound to a metal. Metals which are a part of the d block elements form many complex compounds owing to their small ionic size, high charge, and relative availability of d orbitals for the formation of bonds.
Transition metal and their ions With their larger nuclear charge and smaller size can attract electrons and Receive lone pair of electrons from anions and neutral molecules into their empty d-orbitals forming coordinate bonding.
Transition elements thus form complex molecules with CO, NO, NH3, H2O, F–, Cl–, CN–. Examples of transition metal complexes are, [Co(NH3) 6] 3+ [Cu(NH3)4] 2+, Y(H2O) 6]2+, [Fe(CN)6]4−, [FeF6] 3−, [Ni(CO)4],
Catalytic Activity of Elements
Catalysts are important for the industrial bulk production of many chemicals. Many d-block elements as metals are in their ionic form are being used as a catalyst on many chemical and biological reactions.
Iron in Haber’s process to make ammonia, vanadium pentoxide in the manufacture of sulphuric acid, titanium chloride as Zigler Natta catalyst in polymerization and Palladium chloride in the conversion of ethylene to acetaldehyde are some very important commercial catalytic processes involving d block metals.
Most transition elements act as good catalyst because of,
- The presence of vacant d-orbitals.
- The tendency to exhibit variable oxidation states.
- The tendency to form reaction intermediates with reactants.
- The presence of defects in their crystal lattices.
They take the reaction through a path of low activation energy by:
- Providing a large surface area for absorption and allowing sufficient time to react,
- May interact with the reactants through their empty orbitals.
- May actively interact by redox reaction through their multiple oxidation states.
Alloy Formation in D Block Elements
Atomic radii of the transition elements in any series are not much different from each other. As a result, they can very easily replace each other in the lattice and form solid solutions over an appreciable composition range. Atoms within 15% of the difference in radii can form alloys.
Such solid solutions are called alloys. Alloys are homogeneous solid solutions of two metals or a metal with a non-metal. The alloys of transition metals are hard and high metals are high melting as compared to the host metal.
Various steels are alloys of iron with metals such as chromium, vanadium, molybdenum, tungsten, manganese etc. Some important alloys are:
Bronze – Cu(75-90%) + Sn (10-25%); Chromium steel – Cr(2-4% of Fe) Stainless steel- Cr(12-14% and Ni(2-4%) of Fe; Solder- Pb +Sn
Interstitial Compounds of D Block Elements
Transition metal has a void in their crystal lattice structure. Small non-metallic atoms and molecules like hydrogen, boron, carbon etc can be trapped in the void during crystal structure formation. These are called interstitial compounds. They are neither ionic nor covalent and non-stoichiometric as in TiH1.7, VH0.56.
Interstitial compounds have the following properties:
- Their melting points are very high.
- They are extremely hard.
- They have similar conductivity properties when compared to other metals
- They are unreactive and tend to be chemically inert.
Examples for the interstitial compounds that are formed with transition metals are TiC, Mn4N, Fe3H, and TiH2.
Transition metal compounds of different oxidation state may sometimes present together. They may be formed by solid structure defect or by the prevalent conditions. But, this mixture behaves like a single compound.
This compound will not have any finite composition, structure. Non-stoichiometric is shown particularly while combining with group16 (O, S, Se, Te) elements. Examples: Fe0.94O, Fe0.84O, VSe0.98, Se1.2
Important Compounds of D Block Elements
The D Block Elements form some compounds of vital industrial importance. Some such compounds include:
K2Cr2O7 (Potassium Dichromate)
This compound is considered very important in the leather industry. It is also used as an oxidant in most azo compound preparing processes.
The structure of the dichromate ion is made up of two tetrahedra that share a single corner with a Chromium-Oxygen-Chromium bond angle of 1260. Potassium dichromate is a strong oxidizing agent. Potassium dichromate is also used as a primary standard in the process of volumetric analysis.
KMnO4 (Potassium Permanganate)
The permanganate ion exhibits diamagnetism due to the absence of unpaired electrons in it. Potassium permanganate is also used as an oxidant in the preparation of various products in organic chemistry. It also finds use in the bleaching of cotton, silk, and wool. It can also be used for the decolourization of oils owing to its strong oxidizing ability.