f – Block Elements
f-Block Elements :
→ Twenty eight elements in periodic table ie, atomic number 58 to 71 (14 elements) and atomic number 90 to 103 (14 elements). which are placed at the bottom of periodic table in two horizontal rows, are known as f-block elements. In these elements the last differentiating electron enters in f-orbitals of (n – 2) shell.
→ These elements are also known as inner-transition elements because the antipenultimate energy shell (next to penultimate) i.e. (n – 2) f-orbitals, lie comparatively deep within the kernel. In these element f-subshell is incompletely filled both in atomic as well as in ionic state. f-block has two series :
- Lanthanide series (fourteen elements after Lanthanum)
- Actinide series (fourteen elements after Actinium)
→ There is quite similarities in the properties of Lanthanum (57La) and fourteen elements of Lanthanides, hence lanthanum is also included with the studies of lanthanides. The general symbol for these elements is Ln.
→ Similarly, the properties of actinium and actinides are so similar that the fifteen element from Ac to Lr can be considered as equivalent to one element. These element are assigned at different positions in order of their increasing atomic numbers, the symmetry of the whole arrangement would be disrupted.
→ Due to this reason, the two series i.e. 4f series and 5f-series are placed at the bottom of the periodic table. These two series 4f-series (Lanthanides) and 5f-series (Actinides) constitute one block of the element i.e. f-block. The general electronic configuration of block elements is
(n – 2)f1-14 (n – 1)d0-1ns2