Editing Atomic radius (section)

===Lanthanide contraction===
{{main|Lanthanide contraction}}
The electrons in the 4f-[[Electron shell|subshell]], which is progressively filled from [[lanthanum]] (''[[Atomic number|Z]]''&nbsp;= 57) to [[ytterbium]] (''Z''&nbsp;= 70), are not particularly effective at shielding the increasing nuclear charge from the sub-shells further out. The elements immediately following the [[lanthanide]]s have atomic radii which are smaller than would be expected and which are almost identical to the atomic radii of the elements immediately above them.<ref name="Jolly_contract">
{{cite book
 |last1=Jolly |first1=W. L.
 |year=1991
 |title=Modern Inorganic Chemistry
 |page=22 |edition=2nd
 |publisher=[[McGraw-Hill]]
 |isbn=978-0-07-112651-9
}}</ref> Hence [[lutetium]] is in fact slightly smaller than [[yttrium]], [[hafnium]] has virtually the same atomic radius (and chemistry) as [[zirconium]], and [[tantalum]] has an atomic radius similar to [[niobium]], and so forth.  The effect of the lanthanide contraction is noticeable up to [[platinum]] (''Z''&nbsp;= 78), after which it is masked by a [[relativistic effect]] known as the [[inert-pair effect]].{{Citation needed|date=February 2023}}

Due to lanthanide contraction, the 5 following observations can be drawn:

# The size of Ln<sup>3+</sup> ions regularly decreases with atomic number. According to [[Fajans' rules]], decrease in size of Ln<sup>3+</sup> ions increases the covalent character and decreases the basic character between Ln<sup>3+</sup> and  OH<sup>−</sup> ions in Ln(OH)<sub>3</sub>, to the point that Yb(OH)<sub>3</sub> and Lu(OH)<sub>3</sub> can dissolve with difficulty in hot concentrated NaOH. Hence the order of size of Ln<sup>3+</sup> is given: <br /> La<sup>3+</sup> > Ce<sup>3+</sup> > ..., ... > Lu<sup>3+</sup>.
# There is a regular decrease in their ionic radii.
# There is a regular decrease in their tendency to act as a reducing agent, with an increase in atomic number.
# The second and third rows of d-block transition elements are quite close in properties.
# Consequently, these elements occur together in natural minerals and are difficult to separate.