Lanthanide contraction

lanthanide contraction

1. LANTHANIDE CONTRACTION

2. The atomic size or the ionic radii of tri positive lanthanide ions decrease steadily from La to Lu due to increasing nuclear charge and electrons entering inner (n-2) f orbital. This gradual decrease in the size with an increasing atomic number is called   lanthanide contraction The lanthanide contraction is the result of a poor shielding effect of 4f electrons

3. Shielding cause contraction?????

4. Shielding effect T he inner-shell electrons shield the outer- shell electrons so they are not effected by nuclear charge There will be repulsive force between inner and outer electrons which leads to restrict the penetration of the outer shell towards the nucleus and thus shield the nucleus charge for outer shell electrons POOR SHIELDING Positively charge nucleus decreasing the has greater attraction to electrons atomic radius as the Z Eu Gd Tb Dy H o Er Tm La Ce Pr Nd Pm Sm yb SIZE DECREASES

5. + + + + + + + + + 5d 6s 4f nucleus Shielding effect: s > p > d > f 1 2 3 1 Nucleus charge on f-orbitals is more [Attractive force > Repulsive force] 2 Due to unsymmetrical shape of f-orbital , it cannot shield the outer orbitals so, the outer orbitals also feels nuclear charge 3 Nucleus force of attraction on outer electron increases and therefore it inward towards the nucleus and cause reduce in size Outer shell Inner shell

6. CONSEQUENCES

7. SEPERATION OF LANTHANIDES Since all the elements exhibit the +3 oxidation state as common, they have similar properties Thus, the separation of elements in its pure state is difficult. we use Repeated Fractional Crystallization [ Ion Exchange method] Similarities in atomic size of second and third transition series elements : Due to lanthanide contraction, the atomic and ionic radius of the next and before on the lanthanide elements with same group, are about same. For examples, the atomic radius of  Zr [ group — 4 , period — 5] and Hf [ group — 4 , period — 6] are  160 pm and 159 pm  respectively .

8. ELECTRONEGATIVITY Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. The  Pauling scale  is the most commonly used La to Lu Electronegativity IONISATION ENERGY The amount of energy required to remove an electron from an isolated atom or molecule M (g) + IE M + (g) + e Due to lanthanide contraction, the force of attraction on the outer shell electrons will be more which leads to increase the ionization energy. La to Lu Ionization Energy A B   𝛿+ 𝛿-

9. BASIC STRENGTH OF HYDROXIDES As the size decreases, charge to size ratio increase , the ionic character decreases or covalent character increases making hydroxides less and less basic [removal of OH-] More the charge to size ratio, the electron cloud of anion is more polarized, more covalent character [ fajan’s rule] CHARGE COVALENT CHARACTER SIZE

10.  As the size of lanthanides decreases from La to Lu, the covalent character of the hydroxides increases and hence their basic strength decreases. Thus, La (OH)3 is more basic and Lu(OH)3 is the least basic . La +3 Lu +3 3 OH- 3 OH- Basicity decreases

11. The difference of density between the second and third transition metal series : The density of second and third transition metal series are much more higher due to   lanthanide contraction . Because the atomic volume of two different elements of   4d and 5d series   with same group is near about same , but atomic mass of  5d series elements  becomes much higher. Complex formation Lanthanides exhibiting 3+ oxidation state is the larger and hence low charge to radius ratio. This reduces the complex-forming ability of lanthanides compared to d-block elements. Still they, form complexes with strong chelating agents like EDTA, β- diketones , oxime etc. They do not form Pπ -complexes

12. The decrease in  chemical reactivity of next transition elements on the lanthanide : Due to lanthanide contraction, the ionization energy of the next on the lanthanide elements increases ( Ionization energy of 5d elements are much larger than 4d and 3d. In 5d series, all elements except Pt and Au have filled s-shell.) Hence , their chemical  reactivity decreases . For examples, the chemical reactivity of the element which belong to sixth period such as , gold  ( Au ), platinum ( Pt ), mercury ( Hg )  etc are very much less. 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 .

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