Lanthanide (Ln)
La: [Xe]5d16s2 Gd: [Xe]4f75d16s2
Ce: [Xe]4f15d16s2 Tb: [Xe]4f96s2
Pr: [Xe]4f36s2 Dy: [Xe]4f106s2
Nd: [Xe]4f46s2 Ho: [Xe]4f116s2
Pm: [Xe]4f56s2 Er: [Xe]4f126s2
Sm: [Xe]4f66s2 Tm: [Xe]4f136s2
Eu: [Xe]4f76s2 Yb: [Xe]4f146s2
Lu: [Xe]4f145d16s2
Exceptions in oxidation states:
- Extra stability to favor 3+ oxidation states Tb4+: [Xe]4f7
La3+: [Xe] Stabilized by strong electronegative ions, CeO2, TbF4.
Gd3+: [Xe]4f7 - 2+ oxidation states, divalent
Lu3+: [Xe]4f14 Eu2+: [Xe]4f7
- 4+ states Yb2+: [Xe]4f14
Ce4+: [Xe]
Properties of 4f orbitals:
- Diffuse orbitals:
It has 3 nodal planes in angular function, and 0 radial node, gives this diffuse orbital made by long and thin lobes.
- “Rare”
They have same charge and similar size, means they are found together in nature, hard to isolate.
- Sensitive to effective nuclear charge
The 4f orbitals are located deep within the electron cloud, shielded by the outer orbitals (5s, 5p, and 6s), causing
the electrons in the 4f orbitals to be significantly influenced by the nucleus. As a result, the nucleus' effective
charge, Zeff, experienced by 4f electrons is relatively high compared to other orbitals in the same shell.
- Pulled fully into the core for 3+ cations
When lanthanides lose electrons to form 3+ cations, they typically lose their valency electrons from the 6s and
sometimes 5d orbitals. This ionization leads to a noble gas configuration with a filled or partially filled 4f subshell.
The 4f electrons in lanthanides are poor at shielding each other and other electrons due to their spatial
distribution. This poor shielding effect means that the Zeff felt by the remaining 4f electrons is higher once the
outer electrons are removed. This increased Zeff pulls the remaining electrons closer to the core.
- Ln contraction
Moving across the series, electrons are filling into 4f subshell. Because 4f orbitals are more diffuse and are located
deeper inside the atom. The 4f electrons has poor shielding effect, thus, increasing nuclear charge is not effectively
neutralized by the additional electron. The Zeff felt by the electrons in the shells increases, pulls the electrons closer
to the nucleus, cause a decrease of the atomic radius across the series, known as the "lanthanide contraction."
For Lanthanide metal: decrease in atomic size except Eu2+ and Yb2+, exhibits a larger atomic radius. Which has
half-filled and filled 4f subshell respectively, and result in a relatively lower binding energy, means electrons are
, stable within their configuration. The low binding energy leads to a weaker metallic bond, as electrons contributes
to less electrostatic interaction with nucleus.
For Lanthanide cation (all 3+): smoothly decrease in atomic size.
Consequence of Ln contraction to transition metals
As move across the Ln series, the atomic and ionic radii gradual decrease. When the 4f orbitals are fully filled by
the time we reach the 3rd-row transition elements, the size of these transition elements are smaller than expected
due to the cumulative effect of the lanthanide contraction. Since third-row TM has greater atomic number, its
greater mass packed into a comparable volume, contributes to their denser behavior, which make them similar to
second-row TMs, in terms of chemical behavior.
Ionization energy:
Ln have a general electron configuration of [Xe]4fn6s2 or [Xe]4fn5d16s2, n varies from 0 to 14 across the series. To
form 3+ state, typically lose two 6s electron and one 4f/5s electron, the removal requires similar amount of energy.
The order of penetration of the orbitals into the inner electron core is 4f>5d>6s. the 4f electrons are the closest
to nucleus and attracted by it the most. As successive ionization increases the net charge on the lanthanide cation,
being closest to the nucleus, 4f electrons are pulled even more closer than 5d and 6s electrons. Thus, for Ln3+, 4f
electrons are just too strongly pulled by the nucleus to be ionized further. So, the jump from 3rd to 4th ionization
energy requires much higher extra energy, making 3+ oxidation state the favored one.
Generally, increasing trend across series, except Eu and Yb, where one has half-filled and fully filled 4f in 2+ state,
so require more energy.
Discovery:
One of the notable properties of lanthanide cations is that they exhibit the same oxidation state, and they are of
similar size, and they all have a similar abundance, where Ce get most, Pm get least. In nature, compounds
containing Ln3+ cations, usually contain them as a mixture. Two common mineral ores: Monazite (LnPO4),
Bastnaesite (LnCO3F).
- Missing element, Pm, by Moseley’s Law in 1913
There was no Periodic Table to act as a reference during early 19 Century.
In 1913, Henry Moseley provides a numerical basis for the arrangement of elements in order by atomic number,
rather than atomic mass. Moseley’s Law relates the frequency of the X-ray emitted by an element to its atomic
number. He discovered a missing element by looking at the gap between Nd, with atomic number 60, and Sm
