Lanthanide contraction describes the phenomenon that lanthanides—part of the third row of transition metals—are not much larger in atomic radii than the second row. This is due to the fact that lanthanides mainly gain f-orbital electrons, which are not effective in shielding because higher orbitals are not highly distributed in radial nodes (and neither are them distributed close to the nucleus). Due to the poor shielding, valence electrons of third-row transition metals are as attracted to the nucleus as the second row.