What are Lanthanoids?

14 elements in the VI period (atomic number = 58 to 71) that are filling the 4f-sublevel

The correct answer is option 1. 14 elements in the VI period (atomic number = 58 to 71) that are filling the 4f-sublevel.

Let us delve into the concept of lanthanoids (or lanthanides) and understand why the correct answer is the 14 elements in the VI period (atomic numbers 58 to 71) that are filling the 4f sublevel.

Lanthanoids (Lanthanides): Periodic Table Placement

Period: Lanthanoids are found in the 6th period (also known as the VI period) of the periodic table.

Atomic Numbers: Their atomic numbers range from 58 to 71.

Electron Configuration

Filling of the 4f Sublevel: The key characteristic of lanthanoids is that they involve the gradual filling of the 4f sublevel. The electron configuration for these elements includes electrons being added to the 4f orbital.

For example:

Cerium (Ce): Atomic number 58, electron configuration \([Xe] 4f^1 5d^1 6s^2\)

Lutetium (Lu): Atomic number 71, electron configuration \([Xe] 4f^14 5d^1 6s^2\)

Lanthanoid Series: Elements: The lanthanoid series includes the following 14 elements:

  1. Cerium (Ce)

  2. Praseodymium (Pr)

  3. Neodymium (Nd)

  4. Promethium (Pm)

  5. Samarium (Sm)

  6. Europium (Eu)

  7. Gadolinium (Gd)

  8. Terbium (Tb)

  9. Dysprosium (Dy)

  10. Holmium (Ho)

  11. Erbium (Er)

  12. Thulium (Tm)

  13. Ytterbium (Yb)

  14. Lutetium (Lu)

These elements are known for their similar chemical properties, which arise due to the filling of the 4f orbitals. The 4f orbitals are buried deep inside the electron cloud, and the electrons in these orbitals are not very effective at shielding the outer electrons. This results in similar radii and a phenomenon known as the "lanthanoid contraction," where the size of the atoms decreases slightly across the series.

Incorrect Options Explained

2. 14 elements in the VII period (atomic number = 50 to 71) that are filling the 4f-sublevel: This is incorrect because the lanthanoids are in the VI period, not the VII period, and atomic numbers 50 to 71 would include elements that are not lanthanoids.

3. 14 elements in the VII period (atomic number = 90 to 103) that are filling the 5f-sublevel: These elements are known as actinoids (or actinides), not lanthanoids. They fill the 5f sublevel and are in the 7th period.

4. 14 elements in the VI period (atomic number = 90 to 103) that are filling the 4f-sublevel: This is incorrect because elements with atomic numbers 90 to 103 are also actinoids, not lanthanoids, and they fill the 5f sublevel.

Conclusion

The lanthanoids are the 14 elements in the VI period (atomic numbers 58 to 71) that involve the filling of the 4f sublevel. This defines their unique properties and places them correctly on the periodic table. Therefore, the correct answer is: 1. 14 elements in the VI period (atomic number = 58 to 71) that are filling the 4f-sublevel.