What is the lanthanide contraction?

Answer 1

The decrease in size of elements in the lanthanide series with increase in atomic number due to the poor screening power of the 'f' orbital is known as lanthanide contraction. This effect is also found in transition series where it is due to imperfect sheilding of 'd' orbitals.

Answer 2

The "lanthanide contraction" refers to the fact that elements in column 3 and period 6 of a wide form Periodic Table have atomic radii that are slightly smaller than the elements in column 3 and period 5 and elements in columns 4 through 13 of period 6 have much smaller increases in atomic radii from the elements in the same column in period 5 than period 5 elements have from period 4 elements in the same column. (Period 6 is the first period that contains lanthanide elements.) The reason is that electrons in the lanthanide elements are the lightest ones to occupy f orbitals, and these f orbital have principal quantum numbers less by 2 than the principal quantum numbers of the valence electrons of these elements. As a result, the increase in atomic number, the greater positive charge on the nucleus overcomes the effect of the increase of the principal quantum number for these elements, causing the effective radius of the electrons with principal quantum number 6 in these elements to be smaller than or very close to the effective radius of the electrons with principal quantum number 5 in elements in the same column that do not contain any electrons in f orbitals.

Answer 3

There have been hundreds of ways to present the elements

systematic arrangement known as the periodic table (1, 2).

This paper focuses on the placement of lanthanum (La) and ac-

tinium (Ac) in the d-block versus the f-block, and lutetium (Lu)

and lawrencium (Lr) in the f-block versus the d-block. Some

general chemistry textbooks (for example 3, 4) have adopted

the placement of lanthanum (La) and actinium (Ac) in the f-

block and lutetium (Lu) and lawrencium (Lr) in the d-block.

Other texts including general chemistry and specialized ones

such as Advanced Inorganic Chemistry (5), widely considered

a landmark inorganic textbook for 45 years, place lanthanum

(La) and actinium (Ac) in the d-block and lutetium (Lu) and

lawrencium (Lr) in the f-block.

The placing of lutetium (Lu) and lawrencium (Lr) in the d-

block below yttrium (Y) in group 3 (IIIB) has been justified on

the basis of periodic trends and electron configuration (6-11).

To be more exact, these publications have justified the placement

of lutetium (Lu) in group 3 (IIIB) based on periodic trends and

electron configuration, whereas lawrencium (Lr) is placed in the

same group as a result of its predicted electron configuration.

Overlooking the distinction that the electron configura-

tion of lutetium (Lu) is empirically known whereas that of

lawrencium (Lr) is predicted, based on their electron con-

figurations lutetium (Lu), [Xe]4f 145d16s2, and lawrencium

(Lr), [Rn]5f 146d17s2, could be part of the d-block as they both

have one electron in a d-orbital. Although more recent work

suggests that lawrencium (Lr) may not have an electron in a 6d

orbital (14-17 and references therein)1. However, for lantha-

num (La), [Xe]5d16s2, and actinium (Ac), [Rn]6d17s2, there

is no ambiguity with their empirically known and confirmed

electron configurations. They do have one electron in a d-orbital

and no electrons in f-orbitals outside their inert gas core, [Xe]

and [Rn] respectively.

Another justification for switching the positions of lantha-

num (La) and actinium (Ac) with lutetium (Lu) and lawrencium

(Lr), with the latter below yttrium (Y) in group 3 (IIIB), is that

this arrangement gives similar periodic trends in several physical

properties (atomic radii, sum of the first two ionization poten-

tials, melting point, and electronegativity) when comparing the

first three periods of the d-block (6). In other words this applies

to only lutetium (Lu). That is, the atomic radius, sum of the first

two ionization potentials, melting point, and electronegativity

of lutetium (Lu) are more similar to scandium (Sc) than lan-

thanum (La). On this basis lutetium (Lu) and lawrencium (Lr)

have been placed in the same group as scandium (Sc) (6-8).

With respect to lutetium (Lu), this in itself is reasonable. How-

ever many elements with similar properties or similar trends

in properties are not placed in the same group. For example,

the well known diagonal relationships as found in lithium (Li)

and magnesium (Mg), beryllium (Be) and aluminum (Al), and

boron (B) and silicon (Si), would, applying the same reasoning,

result in lithium (Li) and magnesium (Mg) in the same group

because they have similar physical and chemical properties. For

example their respective atomic radii are 1.57 Å and 1.60 Å and

both react directly with nitrogen to form nitrides. Similarly for

boron (B) and silicon (Si), which are metalloids, with electro-

negativities of 2.0 and 1.9 respectively (Pauling scale). Why

not place them in the same group? In fact why not place all the

metalloids in the same group as they have similar properties?

The answer is a resounding no. As the above examples illustrate,

similarity (or trends) of properties is not the de facto standard

for placing elements in the same group. The placing of elements

in the periodic table is currently accepted as a combination and

balance of factors including the following empirical observa-

tions: atomic number, properties, periodic trends, and atomic

ground-state electron configuration.

However even if we consider the special case of placing

lutetium (Lu) and lawrencium (Lr) in the d-block below yt-

trium (Y) in group 3 (IIIB) because of similar periodic trends

when comparing the first three periods of the d-block, where do

lanthanum (La) and actinium (Ac) end up? In the f-block, and

neither of them have electrons in f-orbitals outside their inert

gas core. The remedy leads to an even worse outcome. The entire

modern basis of the periodic table is the grouping of elements

by occupied outer orbital type giving rise to the s-block (two

outer electrons in an s-orbital and two groups), the p-block (six

outer electrons in three p-orbitals and six groups), the d-block

(ten outer electrons in five d-orbitals and ten groups), and the

f-block (14 outer electrons in seven f-orbitals and 14 groups).2

The placing of lanthanum (La) and actinium (Ac) in the f-block

is not justified and causes even more problems.3 Lanthanum

(La) and actinium (Ac) should be where the IUPAC2 periodic

table (18) and the NIST periodic table (19) places them, below

yttrium (Y) in group 3 (IIIB).

To conclude there are many examples of elements with

similar properties in different groups and many examples of

elements with different properties in the same group. However

placing lanthanum (La) and actinium (Ac) in the f-block is the

only case where a pair4 of elements that belong in the same group

are systematically placed in a group that results in their being

part of a block with no outer electrons in common with that

block. Please leave lanthanum (La) and actinium (Ac) where

they belong, in the d-block.

--Kunal Kishore (kunal.indian@live.com)

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A more short answer:

- a decrease of the ionic radius from cerium (atomic number 58) to lutetium (atomic number 71); from 103 pm to 86 pm.

- increase of hardness, density, melting point from cerium to lutetium

- influence on the chemical properties of lanthanides (ex.: the separation of lanthanides is very difficult)

- a marked influence on the atomic radius of some elements, placed after lanthanides in the periodic table

Answer 4

IN lanthanides differenating electrons enters into 4f sub level. Due to poor shielding effect and peculiar shape off f-orbital,the increased necular charge attracts valency electrons firmly causing causing a steady decrease in size of atom. IT IS CALLED LANTHANIDE CONTRACTION