Editing Atomic radius (section)

==Explanation of the general trends==
[[File:Atomic number to radius graph.png|thumb|A graph comparing the atomic radius of elements with atomic numbers 1–100. Accuracy of ±5 pm.]]

Electrons in atoms fill [[electron shell]]s from the lowest available energy level. As a consequence of the [[Aufbau principle]], each new [[period (periodic table)|period]] begins with the first two elements filling the next unoccupied [[atomic orbital|s-orbital]]. Because an atom's s-orbital electrons are typically farthest from the nucleus, this results in a significant increase in atomic radius with the first elements of each period.

The atomic radius of each element generally decreases across each period due to an increasing number of protons, since an increase in the number of protons increases the attractive force acting on the atom's electrons. The greater attraction draws the electrons closer to the protons, decreasing the size of the atom. Down each group, the atomic radius of each element typically increases because there are more occupied 
electron [[electron shell|energy levels]] and therefore a greater distance between protons and electrons.

The increasing nuclear charge is partly counterbalanced by the increasing number of electrons—a phenomenon that is known as [[shielding effect|shielding]]—which explains why the size of atoms usually increases down each column despite an increase in attractive force from the nucleus. Electron shielding causes the attraction of an atom's nucleus on its electrons to decrease, so electrons occupying higher energy states farther from the nucleus experience reduced attractive force, increasing the size of the atom. However, elements in the 5d-block ([[lutetium]] to [[mercury (element)|mercury]]) are much smaller than this trend predicts due to the weak shielding of the 4f-subshell. This phenomenon is known as the [[lanthanide contraction]]. A similar phenomenon exists for [[actinoid contraction|actinides]]; however, the general instability of [[transuranic element]]s makes measurements for the remainder of the 5f-block difficult and for transactinides nearly impossible. Finally, for sufficiently heavy elements, the atomic radius may be decreased by [[relativistic quantum chemistry|relativistic effects]].<ref>{{cite journal |last1=Pitzer |first1=Kenneth S. |title=Relativistic effects on chemical properties |journal=Accounts of Chemical Research |date=1 August 1979 |volume=12 |issue=8 |pages=271–276 |doi=10.1021/ar50140a001|url=https://escholarship.org/uc/item/2vb947cs }}</ref> This is a consequence of electrons near the strongly charged nucleus traveling at a sufficient fraction of the speed of light to gain a nontrivial amount of mass.

The following table summarizes the main phenomena that influence the atomic radius of an element:

{| class="wikitable"
|-
! Factor !! Principle !! increase in...  !! ''tend to'' !! effect on radius
|-
| electron shells || quantum mechanics || principal and azimuthal [[quantum numbers]] || increase down each column || increases the atomic radius
|-
| nuclear charge || attractive force acting on electrons by protons in nucleus || atomic number || increase along each period (left to right) || decreases the atomic radius 
|-
| shielding || repulsive force acting on outermost shell electrons by inner electrons || number of electrons in inner shells || reduce the effect of nuclear charge || increases the atomic radius 
|}

===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.

===d-block contraction===
{{main|d-block contraction}}
The d-block contraction is less pronounced than the lanthanide contraction but arises from a similar cause. In this case, it is the poor shielding capacity of the 3d-electrons which affects the atomic radii and chemistries of the elements immediately following the first row of the [[transition metal]]s, from [[gallium]] (''Z''&nbsp;= 31) to [[bromine]] (''Z''&nbsp;= 35).<ref name="Jolly_contract"/>