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 molecular networks.

Allotropes of boron

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.

Allotropes of Boron

B12
icosahedral unit
α-rhombohedral
β-rhombohedral
γ-orthorhombic
high pressure form

Allotropes of carbon

Carbon is capable of forming many allotropes in addition to the well known diamond and graphite forms.

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

graphite
diamond
C60-fullerene
"BuckyBall"

The system of carbon allotropes spans a range of extremes:

Synthetic nanocrystalline diamond is the hardest material known. Graphite is one of the softest materials known.
Diamond is the ultimate abrasive. Graphite is a very good lubricant, displaying superlubricity.
Diamond is an excellent electrical insulator, and has the highest breakdown electric field of any known material. Graphite is a conductor of electricity.
Diamond is the best known naturally occurring thermal conductor Some forms of graphite are used for thermal insulation (i.e. firebreaks and heat shields), but some other forms are good thermal conductors.
Diamond is highly transparent. Graphite is opaque.
Diamond crystallizes in the cubic system. Graphite crystallizes in the hexagonal system.
Amorphous carbon is completely isotropic. Carbon nanotubes are among the most anisotropic materials ever produced.

Return to the course outline or move on to Lecture 5: Structure of the elements and Phosphorus, Sulfur.

References

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., 1996.
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Created November 2014. Links checked and/or last modified 19th February 2015.
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