Chemistry PERIODIC PROPERTIES OF LANTHANOIDS

Oxidation States :

(i) Oxidation states in brackets are unstable states

(ii) The lanthanides contains two `s` electrons in the outermost shell, they are therefore expected to exhibit a characteristic oxidation state of `+2`. But for the lanthanides, the `+3` oxidation is common.

(iii) This corresponds to the use of two outermost electrons (`6s^2`) alongwith one inner electron. The inner electron used is a `5d` electron (in `La`, `Gd` and `Lu`), or one of the `4f` electron if no `5d` electrons present.

(iv) All the lanthanides attains `+3` oxidation state and only cerium, Praseodymium, and terbium exhibit higher oxidation state (`+4`). Oxidation states `+2` and `+4` occur particularly when they lead to

(a) A noble gas configuration e.g. `Ce^(4+) (f^0)`
(b) A half filled 'f' orbital e.g. `Eu^(2+), Tb^(4+), (f^7)`
(c) A completely filled 'f ' orbital e.g. `Yb^(2+) (f^(14)`

Therefore, in higher oxidation state, they act as oxidising while in lower state as reducing agents.

Magnetic properties

In tripositive lanthanide ions the number of unpaired electrons regularly increases from lanthanum to Gadolinium (`0 to 7`) and then continuously decreases upto lutecium (`7 to 0`). So lanthanum and lutecium ions which are diamagnetic, all other tripositive lanthanide ions are Paramagnetic.

`text(Colour)`-The lanthanide ions have unpaired electrons in their `4f` orbitals. Thus these ions absorbs visible region of light and undergo `f-f` transition and hence exhibit colour. The colour exhibited depends on the number of unpaired electrons in the `4f` orbitals. The ions often with `4f^(n)` configuration have similar colour to those ions having `4f^(14-n)` configuration. Lanthanide ions having `4f^0, 4f^7,4f^(14)` are colourless. Lanthanide ions `4f^1` and `4d^(13)` are also colourless.

Other Properties :

(a) Highly dense metals with high m.pts. (do not show any regular trend).

(b) `text(Ionisation Energies)`- Lanthanides have fairly low ionisation energies comparable to alkaline earth metals.

(c) `text(Electropositive Character)`- High due to low I.P.

(d) `text(Complex formation)`- Do not have much tendency to form complexes due to low charge density because of their large size. `Lu^(+3)` is smallest in size can only form complex.

(e) `text(Reducing Agent)`- They readily lose electrones so are good reducing agent.

(i) In `+3` oxidation states, nitrates, perchlorates and sulphates of lanthanides and actinides are water soluble, while their hydroxides, fluorides and carbonates are water insoluble.

(ii) Alloys of lanthanides with `Fe` are called misch metals.

(iii) `La(OH)_3` is most basic in nature while `Lu(OH)_3` least basic.

(iv) Lanthanides form `MC_2` type carbide with carbon, which on hydrolysis gives `C_2H_2`.