Lanthanide Contraction

  • Atomic and ionic size –

We all know that, the atomic size increase with increase in atomic number due to addition of new shell but in lanthanide series, as we move from left to right (cerium to lutetium), the atomic and ionic size decrease with increase in atomic number. The greater than expected decrease in the atomic size is called Lanthanide contraction or Lanthanoid contraction. The main reason of the Lanthanide contraction is the poor shielding effect of f-orbital. Due to poor shielding effect, the f-orbital is not able to counterbalance the effect of increasing nuclear charge than screening effect.

The atomic radii of third transition series are similar to those of corresponding elements of second transition series due to lanthanide contraction.

Second transition series – Yttrium(39) to Cadmium(48) – 4d orbital is gradually filled

Third transition series   – Lanthanum (57) to hafnium (72) to mercury (80) – 5d orbital is gradually filled.                                                                                                                           

Cause of Lanthanide contraction

Each succeeding lanthanide differs from predecessor by one electron in 4f – orbital. As we move from Ce to Lu, the nuclear charge increases with increase in atomic number. When one electron is added to the 4f-orbital every time means proton keeps on increasing, one 4f-electron shields another 4f-electron. Then, the nuclear charge increases irregularly which causes the slight poor shielding effect.

The 4f-orbital shows poor shielding effect because of which there is gradual increase in effective nuclear charge experienced by outer electrons. The attraction of the nucleus for the electrons in the outermost shell increases in the atomic number.

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