The d-and f-block elements - Metallic properties,M.P, Atomic size, atomisation enthalpy Part- I

 

d-block elements-

·       those elements in which last electron enters the d-orbital or (n-1) d orbital, are called d-block elements.

·       the elements which lie in- between s and p block elements in the long form of periodic table i.e elements of groups 3-12

·       d-block elements present in 4^th, 5^th 6^th 7^th period in periodic table also known as

1^st transition series-   from Sc (21) to Zn (30)

2^nd transition series – from Y (39) to Cd (48)

3^rd transition series – from La (57) Hf (72) to Hg (80).

4^th transition series – from Ac (89) Rf (104) to Uub (112).

·       General electronic configuration is (n-1) d^1-10 ns^1-2

Exception of 3d 4d 5d elements

3d-series  Sc (21)to Zn (30)]	4d-series [ Y (39) to Cd (48)]	5d-series  [from La (57) Hf (72) to Hg (80)]-
Cr - 3d54s1 Cu - 3d104s1	Nb (41) -4d45s1              Mo (42)- 4d55s1 Ru (44)- 4d75s1    Rh (45)- 4d85s1 Pd (46)- 4d105s1             Ag (47)- 4d105s1	Pt (78)- 5d96s1 Au (79)-5d106s1

·       Transition elements – have partially or incompletely filled d-orbitals in its ground state or in any one of its oxidation states.                              

          but Zn, Cd, Hg are not transition element since they have completely filled d -orbitals as

                     Zn- 3d¹⁰4s²,

                     Cd- 4d¹⁰5s² and

                     Hg- 5d¹⁰6s²

Important characteristics of transition elements

physical properties

1). Metallic properties- all transition elements show typical metallic properties as lustre, high tensile strength, malleability, ductility, good conductor of electricity  and heat

-   due to their small atomic sizes, large nuclear charges and the presence of unpaired d - electrons.

   Lustre nature or shiny surface due to presence of unpaired electron d-orbital

2). M.P of transition elements-In any row the melting points of these metals rise to a maximum at d5 except of Mn and Tc and fall regularly as the atomic number increases. It depends on

   Number of unpaired valence electrons —> increase strong bonding —> higher enthalpy of atomisation —> higher boiling point

3). Atomic size

In a given series, it decreases with increasing atomic number ,this is because the new  electron enters in the d-orbital each time ,the nuclear charge increase by unity

Note Similar behaviour has been observed in the second and third transition series.

·      In a vertical row, the atomic radii is expected to increase from top to bottom. Therefore, the atomic radii of transition metals of second series have larger values than those of the first transition series. However, the transition metals of third series except the first member, lanthanum, have nearly the same radii as metals of second transition series above them. This is due to Lanthanide contraction.

  i.e. Due to inclusion of fourteen lanthanides between lanthanum and hafnium, [there is continuous decrease in size from Ce(58) to Lu(7l)] hafnium size becomes nearly equal to the size of zirconium.

·      The atomic radii of 2^nd  and 3^rd  transition series (4d and 5d series) are greater as compared to 1^st transition series(3dse)

                          Or

          4d and 5d metals are very similar in size.

Ans -this is due to lanthanoid contraction, is a regular decrease in atomic radii by filling of 4f before 5d orbital. Since 4f orbital causes poor shielding effect(screening) than 5d.

Because 4f orbital is highly diffused orbital that’s why it shows poor shielding effect.

4.atomisation enthalpy- The required energy to convert 1 mole solid substance (metal) into atom is called atomisation enthalpy

     M(s) ----------> M(gas) +   Δ_aH =atomisation enthalpy

·      Why do the transition elements exhibit higher enthalpy of atomisation?

Ans

transition elements have large no of unpaired electrons, they have stronger interatomic interaction. Hence stronger metallic bonding between atoms resulting in higher atomisation enthalpy.

Concept-- since atomisation enthalpy depends on number of unpaired electrons which define metallic bond strength.

i.e. metallic bond strength α number of unpaired electrons

·   Zn has low atomisation enthalpy Ans – since it has no unpaired electron in its atom(3d¹⁰4s²)

Click here to see next Trends in ionisation enthalpy of d-block elements part- 2

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