Introduction to macromolecules

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Introduction to macromolecules

Macromolecules | Biology | Science | Khan Academy

G raphite is another form of carbon 2nd allotrope of carbon to be discussed The carbon atoms form joined hexagonal rings forming layers 1 atom thick in graphite.

Each carbon atom is strongly covalently bonded to three other carbon atoms. So every carbon atom in a layer is strongly bonded to three other carbon atoms. A crystal of graphite contains millions of layers of these sheets of carbon atoms.

Although graphite is almost black and opaque unlike diamondit does look a bit shiny and smooth. There are three strong covalent bonds per carbon atom in graphite 3 C—C bonds in a planar arrangement from 3 of its 4 outer electrons. So three of the electrons are tightly held in three directed covalent bonds, BUT, the fourth outer electron is 'delocalised', that is one electron per carbon atom is shared between the carbon atoms to form the equivalent of a 4th bond per carbon atom.

This 4th electron is free to move around 'delocalised'hence graphite's ability to conduct electricity, giving graphite a similarity with metals. This electrical conductivity of carbon in the form of its graphite allotrope is quite unusual for giant covalent structures.

This situation requires advanced level concepts to fully explain the structure of graphite, and this bonding situation also occurs in fullerenes described below, and in aromatic compounds you deal with only at advanced level.

The layers are only held together by weak intermolecular forces shown by the dotted lines NOT by strong covalent bonds, so graphite, for a giant covalent structure, is unusually weak physically. There are no strong covalent bonds between carbon atoms of adjacent layers. Like diamond and silica above the large molecules of the layer ensure graphite has typically very high melting point because of the strong 2D bonding network note: NOT a 3D network.

It takes a lot of energy to break overcome the carbon-carbon bonds within individual layers of graphite, hence its very high melting point. Graphite will not dissolve in solvents because of the strong bonding in the layers.

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BUT there are two crucial differences compared to diamond Electrons, from the 'shared bond', can move freely through each layer, so graphite is a conductor like a metal. Diamond is an electrical insulator and a poor heat conductor.

Introduction to macromolecules

For a non-metal, graphite is a relatively good conductor of heat and electricity, which gives it some similarity with metals. Graphite is used in electrical contacts e. The weak forces enable the layers to slip over each other so where as diamond is hard material graphite is a 'soft' crystal, it feels slippery.

This enables graphite to be used as a lubricant in its own right, but it is also an important additive that is sometimes added to other oil based lubricants to enhance their performance.

Carbon in the form of graphite is the only non—metal that is a significant electrical conductor. Graphite is used in pencils often wrongly called lead pencils!

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Graphite can act as a lubricant for the same reason, the slipperiness of the layers! These two different characteristics of graphite described above are put to a common use with the electrical contacts in electric motors and dynamos.

These contacts called brushes are made of graphite sprung onto the spinning brass contacts of the armature.

The graphite brushes provide good electrical contact and are self—lubricating as the carbon layers can slide over each other on the rotating metal contacts. You should be able to explain the properties of graphene in terms of its structure and bonding. Know that fullerenes are molecules of carbon atoms with hollow shapes.

The structure of fullerenes is often based on hexagonal rings of carbon atoms but fullerenes may also contain rings with five or seven carbon atoms. The first fullerene to be discovered was Buckminsterfullerene a C60 molecule of hexagonal and pentagonal rings which has a spherical shape.

Carbon nanotubes are long cylindrical fullerenes with very high length to diameter ratios. Again, their properties make them useful for nanotechnology, electronics and materials.

You should be able to recognise graphene and fullerenes from diagrams and descriptions of their bonding and structure and give examples of the uses of fullerenes, including carbon nanotubes.Buy An Introduction to Macromolecules (Heidelberg Science Library) on barnweddingvt.com FREE SHIPPING on qualified ordersAuthor: L.

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See all Macromolecules ACS Editors'' Choice articles. View one new peer-reviewed research article from any ACS journal, selected daily, and made open access based on recommendations by ACS journal scientific editors from around the world.

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CH Chapter 11 - Introduction to the Major Macromolecules - Chemistry