Physical Properties of Giant Covalent Substances

8/2/2019 Physical Properties of Giant Covalent Substances

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Physical Properties in Giant

Covalent Substances


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Structure: Giant covalent macromolecule Bonding: Each carbon atom has 4 bonding pairs of

electrons and 0 lone pairs of electrons. According to VSEPRtheory in order to minimize the repulsion the 4 pair of

bonding electrons repel each other equally. Bond angle: 109.5o Shape: tetrahedral No intermolecular forces of attraction, only

intramolecular attractions between the carbon atoms.

Diamond


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Physical Properties of Diamond

Hardness

Hardest substance known Each carbon atom is held in the lattice by 4 strong covalent

bonds, making diamond a strong rigid structure.

The carbon carbon bonds can only be broken up by

another diamond. Because of their hardness diamonds have many applications

in industry. Parts of cars, planes, and other machines areshaped by diamonds.

Diamond saw


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Solubility in water The covalent bonds between the carbon

atoms are much stronger than the forceof attraction exerted by polar watermolecules. Diamond is not soluble inwater.

Melting Point

To break the strong intramolecularcovalent bonds requires a large amount ofenergy, therefore they have very high

temperature. M.pt of diamond 4000oC


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Conductivity of electricity

All four outer valence electrons in in each C atom areinvolved in forming covalent bonds with four other Catoms. Therefore the valence electrons are not free tomove making diamond a non conductor of electricity.


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Graphite

Structure: Giant Covalent macromolecule Bonding: Layers of hexagonal rings of carbon atoms.

Within each layer intramolecular covalent bonds attacheach carbon atom to three other carbon atoms. The 4thbonding electron is delocalized (mobile) and movesbetween the layers. These electrons form Dispersionintermolecular forces that hold the layers together.


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The Physical Properties of Graphite

Conducts ElectricityThe one electron not used in bonding is free / mobile /delocalized and can carry a current.

Lubricant

The weak Dispersion forces between the layers allow them toslide over one another making graphite an excellent lubricant.

Insoluble in water

The intramolecular covalent bonds between the carbon atoms

are too strong to interact with water. Low density

Compared to diamond. This is because of the relatively largeamount of space between the layers.


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Soft and slippery

The layers of carbon atoms slide over each other due toweak Dispersion forces between them. When you use apencil, the layers of carbon atoms are rubbed off andstick to the paper. Graphite is one of the softestsubstances.

High melting point and boiling point

Mpt = 3652C - 3697C : b.pt = 4200C.

Similar to diamond. To melt graphite both theintermolecular Dispersion forces and intramolecularcovalent bonds need to be broken.

Volatility

Not volatile


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C60 Fullerenes

Structure: Giant Covalent macromolecule

Bonding: 60 carbon atoms covalently bonded in a apolyhedron with 20 hexagonal (6-angled) surfaces and 12pentagonal (5-angled) surfaces.


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Physical Properties of fullerenes

Reactivity with other compoundsNot very reactive due to the stability of the carbon-carbonbonds. Chemists have been able to increase thereactivity by attaching active groups to the surface.

Insoluble in many solvents Partial conductor of electricity

Due to the one delocalized electron per carbon atom.

Soft

C60 molecules can slide over one another making themsofter than diamond but not as soft as graphite

Volatility

Not volatile


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Silicon, Si

Si - semimetal or metalloid element.

Structure:

Giant Covalent macromoleculeBonding: Tetrahedral structure. Each silicon atom has 4 bonding

pairs of electrons and 0 lone pairs of electrons.

According to VSEPR theory in order to minimize therepulsion between the 4 pairs of bonding electrons theyrepel each other equally. Bond angle: 109.5o.

No intermolecular forces of attraction, onlyintramolecular attractions between the silicon atoms.


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The Physical Properties of Silicon

Does not conduct electricity

No delocalized valence elections to carry a current because they are allinvolved in bonding. Called an insulator.

Hard

Each silicon atom is held in the lattice by 4 strong covalent bonds,giving it a strong rigid structure.

Insoluble in water

The intramolecular covalent bonds between the silicon atoms are toostrong to interact with water.

Melting point

Lower than diamond. Si-Si bond is longer than C-C bond in diamond,because Si has a larger radius. As the bond length increases, theamount of energy needed to break the attractive force between the pairof electrons in the covalent bond and the protons in the nucleusdecreases.


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Silicon as a semiconductor Si can be made into a partial of conductor of electricity.

Si semiconductors are found in the microprocessor chips of all electronicdevices

Silicon can be made into a conductor (but not as good as a metal), bydoping adding B, P, Ga or As atoms are added to the lattice.

P and As have five valence electrons. The fifth electron does not covalently

bond with the Si and remains delocalized and able to conduct a current.

B and Ga have three outer electrons. When they bond with Si they form ahole (the 4th valence electron that is not involved in bonding). Theabsence of an electron on each Si atoms creates the effect of a positivecharge which can conduct a current. Alternatively the non bonded electrons

can move from hole to hole carrying a current.
http://computer.howstuffworks.com/microprocessor.htmhttp://computer.howstuffworks.com/microprocessor.htm


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Silicon dioxide, SiO2

Naturally occurring in the earthscrust.

Each Si atom is linked to four Oatoms, and each O to two Si atoms

in a 1:2 ratio. Bent shape.Physical Properties:

High melting point (1650 - 1730 C)and boiling point

insoluble in water not volatile

does not conduct electricity


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Bibliography

http://chemcases.com/silicon/sil15one.htm

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Physical Properties of Giant Covalent Substances