Lanthanide contraction is the gradual decrease in atomic and ionic sizes of the lanthanide series elements (from La to Lu) despite having increasing atomic number. This happens because the added electrons enter the 4f orbitals, which poorly shield the increasing nuclear charge, pulling the electrons closer to the nucleus.
Effects on periodic trends:
- Atomic and ionic size: Elements after the lanthanides (like Hf, Ta) are smaller than expected, affecting their chemistry.
- Ionization energy: Slightly higher ionization energies than expected for elements after lanthanides.
- Electronegativity: Slight increase due to smaller atomic size.
- Chemical similarity: Transition metals of the 4d and 5d series (like Zr & Hf, Nb & Ta) have very similar properties, making separation difficult.
In short: Lanthanide contraction shrinks atomic sizes and increases similarities among elements in the later periods, affecting trends like size, ionization energy, and chemical behavior.