The number of elements in the lanthanoids of the periodic table is

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UPSC CISF-AC-EXE – 2019

The number of elements in the lanthanoids of the periodic table is 14.

The lanthanoids (or lanthanides) are a series of chemical elements comprising the 15 elements from lanthanum (atomic number 57) through lutetium (atomic number 71). However, the term “lanthanoid series” usually refers to the 14 elements immediately following Lanthanum, from Cerium (atomic number 58) to Lutetium (atomic number 71), as these are the elements where the 4f subshell is progressively filled. Lanthanum itself has the electron configuration [Xe] 5d¹ 6s², not filling the 4f subshell, but it is chemically similar to the other lanthanoids and is often included in the series or considered the first element.

In the context of the f-block of the periodic table, which is typically displayed as two rows below the main body, the lanthanoids constitute the upper row. This row consists of 14 elements, starting after Barium (Z=56) and before Hafnium (Z=72) in the main table. These 14 elements are Cerium (58), Praseodymium (59), Neodymium (60), Promethium (61), Samarium (62), Europium (63), Gadolinium (64), Terbium (65), Dysprosium (66), Holmium (67), Erbium (68), Thulium (69), Ytterbium (70), and Lutetium (71).

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Given the options, 14 is present and is the standard number of elements represented in the f-block lanthanoid series, corresponding to the filling of the 4f orbitals. While the definition can sometimes include Lanthanum (making it 15), 14 is the number that fits the typical arrangement and f-orbital filling.

The actinoids are the second series in the f-block, starting from Actinium (Z=89) or Thorium (Z=90) up to Lawrencium (Z=103), also containing 14 elements where the 5f subshell is filled. The lanthanoids and actinoids are sometimes collectively called the inner transition metals.

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