Main Group Elements and
The Main Group Elemental Hydrides
chemogenesis story starts with the periodic table,
simplified to the first 36 main group (s & p-block) elements, hydrogen
to barium. The main group elements are then normalised to the corresponding
main group elemental hydrides, a set that includes such well known species
as: hydrogen, water, ammonia, methane, lithium hydride, xenon and hydrogen
chloride. Patterns in structure and reaction behaviour are noted.
36 Main Group Elements: Hydrogen to Barium
Initially we shall limit our
initial discussions to s and p-block elements, to first 36 main group
elements, hydrogen to barium:
However, as simple substances
in their standard state (1atm pressure and 25°C) the main group elements
present as a diverse and complicated collection of substances:
As discussed elsewhere in this
web book see the pages on the Classification
of Matter and the Tetrahedron of Structure, Bonding & Material Type
pages elemental and binary substances exhibit four extreme material types
[metallic, ionic, molecular & network], however the elements present
as just: metallic, molecular & network:
When this analysis is applied
to the set of the first 36 main group elements we find:
elements are molecular, 15 are metallic, 5 is network covalent (carbon) and none are ionic.
However, this is a gross simplification because several elements have metallic and non-metallic allotropes that are intermediate between metallic and network, and are metalloid
or semi-metallic in nature: C, Si, Ge, As, Sn, Sb & Te.
As a set, the first 36 main
group elements exhibit complexity.
Data normalisation is a common
procedure in science, here.
- It is convenient
to normalise length data inch, foot, millimetre, cubit and light
year with respect to the metre.
- Chemists routinely
normalise the "amount of substance" data with respect
to the mole.
The question is:
- Can the diversity
associated with the elements in their standard state be removed?
there a proxy for the elemental basic substance?
- Is it possible to normalise the first 36 main group elements
so that they can be meaningfully compared and contrasted with each other?
One way to normalise the elements
is to examine the gas phase mono-atomic species, here.
While this tells us much about the physics of atoms and atomic structure,
it tells us little about the reaction chemistry of chemical substances
The approach taken by the chemogenesis
analysis is to saturation bond each of the first 36 main group elements
with a common bonding partner, and then to explore the chemistry of the
resulting binary compounds. There are several candidate bonding partner
to give the corresponding:
hydrides, lithides, fluorides and oxides
far the most interesting bonding partner is hydrogen. This gives rise
the corresponding set of 36 main group elemental hydrides.
Group Elemental Hydrides
The 36 of the main group elemental
hydrides, H2 to BaH2, are
all well known species with non-controversial structure and reaction chemistry.
Indeed, the set includes such common chemical species as water, H2O,
methane, CH4, ammonia, NH3,
hydrogen sulfide, H2S, hydrogen, H2,
and neon, Ne.
Of more importance and
crucial to the normalisation argument is that all the main group
elemental hydrides can be found in the gas phase where they are molecular.
This "gas phase
and molecular" point is crucial for two reasons:
- Most reaction
chemistry involves molecules and/or molecular environments (the gas
phase or in solution).
- Gas phase
and molecular chemical species are simple to model, both on paper and in silico. This makes it possible to carry out computer aided virtual reaction
- The Group
1 and 2 saline hydrides lithium hydride to barium hydride, LiH
to BaH2 are known to exist as ionic solids rather than as discrete
molecules. However, the saline hydrides and other ionic materials can
be studied in the gas phase molecular entities by laser
ablation of the crystal surface.
- Molecular lithium hydride, LiH,
is a very common species for theoretical study because it is the simplest
species with a heteronuclear bond, a bond between two dissimilar elements.
- The Group
13 hydrides such as borane, BH3, dimerise to B2H6.
- These seeming
exceptions to the molecular and in the gas phase argument are
actually core components of the chemogenesis argument, as should become
- An additional
reason for restricting the current discussion to s and p-block elemental
hydrides, as discussed above, is that the d and f-block elements form
non-stoichiometric hydrides which can vary in composition. These interesting
materials are being studied for the storage of hydrogen.
Patterns In Reaction Chemistry Space
The main group elemental hydrides
show patterns of reaction chemistry behaviour which correspond with the
s-block and p-block construction of the periodic table in that they show real
and rich periodicity.
- The main group elemental
hydride species are shown below in coloured blocks that correspond to
classification as Lewis acids, Lewis bases and Lewis acid/base complexes.
- While these classifications
may seem a little odd and arbitrary, they will be shown to be an intrinsic
part of the self-consistent chemogenesis classification system:
Group Elemental Hydrides as: Lewis acids, Lewis bases and Lewis acid/base
[For a revision
of the difference between the "Lewis" and "Brønsted"
definitions of acids and bases, look here,
and for a discussion of frontier molecular orbitals, look here.]
In the original
language of chemistry:
Lewis base is "a species with an electron pair".
Lewis acid is " an electron pair acceptor".
Lewis acid interact with a Lewis base to give a complex with a polar
covalent, two electron chemical bond.
In the 1960s, frontier molecular
orbital (FMO) theory said:
- A Lewis base interacts with a Lewis acid via
its highest occupied molecular orbital or HOMO.
- A Lewis acid interacts with a Lewis base via its lowest
unoccupied MO or LUMO.
- HOMO + LUMO interactions are new bonding.
In the analysis presented here:
Lewis acid's LUMO interacts with a Lewis base's HOMO to give a Lewis acid/base
complex with a net bonding molecular orbital:
LUMO + HOMO
There is a self-consistent
colour scheme running through chemogenesis:
- Lewis acids are RED
- Lewis bases are BLUE
- Lewis acid/base complexes are
in 500 Seconds
Five Hydrogen Probe Experiments
© Mark R. Leach 1999-
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