Lecture 5b. Structure of the elements (non-metals, B, C)
The structure of non-metals
A non-metal can be classified as an element that mostly lacks metallic
attributes. Physically, non-metals tend to be highly volatile (easily
vaporised), have low elasticity, and are good insulators of heat and
electricity; chemically, they tend to have high ionization energy and
electronegativity values, and gain or share electrons when they react
with other elements or compounds. Seventeen elements are generally
classified as nonmetals; most are gases (hydrogen, helium, nitrogen, oxygen,
fluorine, neon, chlorine, argon, krypton, xenon and radon);
one is a liquid (bromine); and a few are solids (carbon, phosphorus,
sulfur, selenium, and iodine).
On moving across the periodic table, nonmetals are seen to adopt structures
with progressively fewer nearest neighbours. Polyatomic nonmetals have
structures with either three nearest neighbours, as is the case (for example)
of carbon (in its standard state of graphite), or two nearest neighbours
(for example) in the case of sulfur. Diatomic non-metals, such as hydrogen, have
one nearest neighbour, and the monatomic noble gases, such as helium, have none.
This gradual fall in the number of nearest neighbours is associated with
a reduction in metallic character and an increase in nonmetallic character.
Allotropes are different structural forms of the same element
in which changes in the connectivity of the covalent bonding between atoms
results in substances with quite different chemical and/or physical properties.
The change between allotropic forms is triggered by factors such as pressure,
light, and temperature. Therefore the stability of a particular allotrope
depends on particular conditions.
If covalent connectivity is the same but packing is different then you have
polymorphs (eg. Monoclinic and Rhombic sulfur (S8) are
polymorphs not different allotropes. S8 and S12 are different allotropes of S.
Many nonmetals have allotropes (that are less stable than their standard form)
with either nonmetallic or metallic properties. Graphite, the standard state
of carbon, has a lustrous appearance and is a fairly good electrical conductor.
The diamond allotrope of carbon is nonmetallic, being translucent and having
relatively poor electrical conductivity.
Catenation is the ability to form element-element bonded
Boron can be prepared in several crystalline and amorphous forms.
The best known crystalline forms are α-rhombohedral, β-rhombohedral,
and β-tetragonal. Under special circumstances, boron can form
α-tetragonal, and γ-orthorhombic allotropes.
Two amorphous forms, one a finely divided powder and the other a glassy solid,
are also known and a further 14 allotropes have been reported.
Carbon is capable of forming many allotropes in addition to the well known
diamond and graphite forms.
Allotropes of Boron
high pressure form
The physical properties of carbon vary widely with the allotropic form.
For example, diamond is highly transparent, but graphite is opaque and
black. Diamond is the hardest naturally-occurring material known,
while graphite is soft enough to form a streak on paper
(hence its name, from the Greek word
"γρáφω" which means "to write").
Diamond has a very low electrical conductivity, while graphite is a
very good conductor. Under normal conditions, diamond, carbon nanotubes,
and graphene have the highest thermal conductivities of all known materials.
All carbon allotropes are solids under normal conditions, with graphite
being the most thermodynamically stable form. They are chemically resistant
and require high temperature to react even with oxygen.
Allotropes of Carbon
The system of carbon allotropes spans a range of extremes:
Return to the
or move on to Lecture 5: Structure of the elements
and Phosphorus, Sulfur.
Much of the information in these course notes has been sourced from Wikipedia under
the Creative Commons License.
'Inorganic Chemistry' - C. Housecroft and A.G. Sharpe, Prentice
Hall, 4th Ed., 2012, ISBN13: 978-0273742753, pps 24-27, 43-50,
172-176, 552-558, 299-301, 207-212
'Basic Inorganic Chemistry' - F.A. Cotton, G. Wilkinson and P.L.
Gaus, John Wiley and Sons, Inc. 3rd Ed., 1994.
'Introduction to Modern Inorganic Chemistry' - K.M. Mackay, R.A.
Mackay and W. Henderson, International Textbook Company, 5th Ed.,
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