View source for Construction kit analogy for the periodic table of elements

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[[File:Knex-connexions-base.jpg ‎|193px|thumb|right|Construction kit analogy: A defined number of connection points in defined directions.]]

The '''construction kit analogy for the periodic table of elements''' is about naively treating the periodic table of elements as if it where a construction kit. <br>
That is assuming the atoms of chemical elements act like soft flexible construction kit parts that come with a fixed number of connection points (given by their group in the periodic table). <br>
This has severe limits though. 

To some degree it works well for some combinations of the light nonmetallic elements on the upper right corner of the periodic table (noble gasses have zero "connection points"). <br>
But even there are wild deviations:

= Where the construction kit analogy breaks down =

See main page: [[Limits of construction kit analogy]]

== Electron deficiency allowing for multi center electron bonds ==

One prime example is borane B<sub>2</sub>H<sub>6</sub>. <br>
There the hydrogen atoms can form bridge-bonds instead of the usual (construction kit analogy default) single bonds.

== Deviations within the upper right corner of the periodic table ==

The bigger and the higher period the element, the more promiscuous they are with coordination order. <br>
Sulfur and chlorine are coordination promiscuous than oxygen and fluorine respectively. <br>

Not bond order though. There the reverse is the case. <br>
Carbon forms sp2 double bonds (and even more so sp3 triple bonds) much more readily and stably than silicon. <br>
As an intuitive explanation: This is due to silicon having the inner electrons in the way like a repulsive ball presumably. <br>

=== Examples ===

'''Sulfur and oxygen:''' <br>
* In sulfur trioxide sulfur forms more than just two single bonds to oxygen. <br>(Two double and one single bond in resonance structure or so.)
* In sulfates there are even four oxygens coordinated to a single sulfur atom.

'''Sulfur and halogens:'''<br>
* SH<sub>2</sub> (H-S-H) and SF<sub>2</sub> (F-S-F) and SCl<sub>2</sub> (Cl-S-Cl) all follow the construction kit analogy maximally closely 
* Hydrogen disulfide (H-S-S-H), disulfur dichloride (Cl-S-S-Cl), and disulfur difluoride (F-S-S-F) too but …
* SCl<sub>4</sub>, SF<sub>4</sub> and the (very stable) SF<sub>6</sub> all deviate from the construction kit analogy. <br>[[Sulfur]] should have two bonds given its group ... <br> ... but instead four more fluorines aggressively grab four more electrons though due to fluorines high electronegativity. <br>[[Fluorine]] really really wants to fill that last missing electron to become as noble as [[neon]].

'''Chlorine seems very coordination promiscuous with oxygen:''' <br>
* [https://en.wikipedia.org/wiki/Hypochlorous_acid Hypochlorous acid] H-O-Cl (as expected)
* [https://en.wikipedia.org/wiki/Chlorous_acid Chlorous acid] H-O-Cl=O (deviating a bit)
* [https://en.wikipedia.org/wiki/Chloric_acid Chloric acid] HClO<sub>3</sub> (deviating a lot)
* [https://en.wikipedia.org/wiki/Perchloric_acid Perchloric acid] HClO<sub>4</sub> (WTF?! four oxygens on a single chlorine [[accidentally suggestive|*]])

'''Interhalogen compounds:''' <br>
* Chlorine monofluoride (expected), chlorine trifluoride and chlorine pentafluoride (deviating)

== Unstable polymers chains ==

'''Oxygen:''' <br>
Despite oxygen often forming bridges with two single bonds, <br>
oxygen chains like such …-O-O-O-O-… with increasing length quickly get extremely unstable. <br>
* Neutral dihydrogen polyoxides: https://en.wikipedia.org/wiki/Hydrogen_polyoxide
* [https://en.wikipedia.org/wiki/Hydrogen_peroxide Wikipedia: Hydrogen peroxide] – still quite stable
* [https://en.wikipedia.org/wiki/Trioxidane Wikipedia: Tridioxane]] – decomposes in water after 16 minutes
Only the shortest ones being (with synthetic physical chemistry trickery) synthesizable thermodynamically in small and/or diluted quantities. <br>
Longer ones in larger more concentrated quantities may likely become mechanosynthesizable via piezomechanosyntehsis at very low temperatures. <br>
But these will explosively decompose if warmed too much or perhaps mechanical shock. <br>
A high energy density explosive with low activation energy. Dangerous, risky and typically highly undesirable.

'''Nitrogen:''' <br>
Unlike oxygen nitrogen can form stable solids at room temperature. <br>
But it's still an extremely high energy explosive due to nitrogen having the option of forming much more stable N<sub>2</sub> tripple bonds.

'''Getting it stable:''' <br>
Adding carbon one gets to [[beta carbon nitride]], which (hopefully) is no longer an explosive. <br>
But it most likely will boost fires much more potently than desirable. To mitigate that one could:
* include sufficient water pockets to keep fires from fast runaway expansion
* replacing some nitrogens with phosphors and carbons with silicons (slack forming non volatile elements)
The latter option comes at the cost of requiring lithospheric resource elements that can only be obtained by mining solid material. <br>
{{wikitodo|Maybe move details here to [[beta carbon nitride]] page?}}

== Metallic bonds – promiscuous coordination if any covalently enforced coordination ==

With an delocalized electron gas serving as valence electrons holding atoms together angular bond constraints are mostly gone.
Rather dense packing of spheres determines the nearest neighbor geometry.
This give just two options
* ABAB stacking, hexagonal, hexagonal close packed HCP
* ABCABC stacking, cubic, face centered cubic FCC
* (No, body centered cubic BCC does not lead to a close packing) 
Beyond layer order choices there are no other options for metastable structures as there are plenty with covalent directed bonds.

Alloys with atoms of different radii complicate things a bit. <br>
If there is a low stoichometric ratio and the metals are dissimilar enough then <br>
intermetallic compounds can be formed.

Intermetallic compounds are a bit of an exception.  <br>
Unlike metals '''intermetallic compounds often have quite strong covalent character of bonds'''.

= Where the construction kit analogy it holds for metals =

== Oxidic gemstones ==

While transition metals are typically more promiscuous with number and geometry of bonds, but <br>
in conjunction with oxygen as bridging gap-filler at least these bonds are clearly covalent. <br>
Given the situation at hand one often can take educated guesses about which <br>
of the preferred geometries of the chosen transition metal element will be most stable. <br>
or vice-versa chose chelating geometries for a deliberately stable or deliberately less stable configuration.

=== Iron oxides and sulfides ===

Thinsg like pyrite and hematite. while these are have enough free electrons to be electrically conductive and optically reflective in the huuman visible spectrum, the valence electrons are still highly covalent giving these compounds nonmetallic mechanical character.

= Perhaps interesting side-notes =

== The stishovite-rutile-analogy ==

While pure titanium as a transition metal with metallic bonds (HCP structue, no construction kit character here)
Oxidized titanium likes to form rutile (beside andatase and brookite)

It is possible to structure-preservingly replace titanium with silicon.
Interestingly the silicon then has 8 oxygens as nearest neighbouring atoms rather than four.
An untypical deviation 

Substituting titanium with silicon (and vice versa) can be motivated by <br>
both having 4 electrons in the outermost shell (Ti: [Ar] 3d2 4s2). <br>
That is: The titanium group can be seen as alternate extension to the carbon group, <br>
splitting up to geranium and titanium below silicon.

== Grey tin ==

Tin normally behaves as transition metal element. BCC white (β). <br>
But under 13.2°C pure tin transforms to it's non-metallic form. FCC gray (α). <br>
* β form being BCC is already odd for a metal since it's not a dense packing of spheres. <br>https://som.web.cmu.edu/structures/S018-beta-Sn.html
* α form: Sn is tetrahedrally coordinated just like its lighter group members germanium, silicon, and carbon (construction kit like behavior)
Due to it's limited stability both thermally and mechanically nonmetallic α-tin seems limited in usefulness though. <br>
Just a quite interesting [[oddball compound]].

= Related =

* For details see: [[Limits of construction kit analogy]]
* [[Molecular dynamics simulation]]

= External links =

Wikipedia:
* [https://en.wikipedia.org/wiki/Coordination_number Coordination number]
* [https://en.wikipedia.org/wiki/Bond_order Bond order]