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4.5 Physical Properties in Giant Covalent Substances. Diamond. Structure: Giant covalent macromolecule Bonding: Each carbon atom has 4 bonding pairs of electrons and 0 lone pairs of electrons. According to VSEPR

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diamond

Diamond

Structure: Giant covalent macromolecule

Bonding: Each carbon atom has 4 bonding pairs of

electrons and 0 lone pairs of electrons. According to VSEPR

theory 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.

physical properties of diamond

Diamond saw

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 are shaped by diamonds.
slide4
Solubility in water
  • The covalent bonds between the carbon atoms are much stronger than the force of attraction exerted by polar water molecules. Diamond is not soluble in water.

Melting Point

  • To break the strong intramolecular covalent bonds requires a large amount of energy, therefore they have very high temperature.
  • M.pt of diamond  4000oC
slide5
Conductivity of electricity
  • All four outer valence electrons in in each C atom are involved in forming covalent bonds with four other C atoms. Therefore the valence electrons are not free to move making diamond a non conductor of electricity.
graphite
Graphite
  • Structure: Giant Covalent macromolecule
  • Bonding: Layers of hexagonal rings of carbon atoms. Within each layer intramolecular covalent bonds attach each carbon atom to three other carbon atom. The 4th bonding electron is delocalized (mobile) and moves between the layers. These electrons form Van der Waal’s intermolecular forces that hold the layers together.
the physical properties of graphite
The Physical Properties of Graphite
  • Conducts Electricity

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

  • Lubricant

The weak van der Waals forces between the layers allow them to slide 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 large amount of space between the layers.

slide8
Soft and slippery

The layers of carbon atoms slide over each other due to weak Van der Waal’s forces between them. When you use a pencil, the layers of carbon atoms are rubbed off and stick to the paper. Graphite is one of the softest substances.

  • High melting point and boiling point

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

Similar to diamond. To melt graphite both the intermolecular van der Waals forces and intramolecular covalent bonds need to be broken.

  • Volatility

Not volatile

c60 fullerenes
C60 Fullerenes
  • Structure: Giant Covalent macromolecule
  • Bonding: 60 carbon atoms covalently bonded in a a polyhedron with 20 hexagonal (6-angled) surfaces and 12 pentagonal (5-angled) surfaces.
physical properties of fullerenes
Physical Properties of fullerenes
  • Reactivity with other compounds

Not very reactive due to the stability of the carbon-carbon bonds. Chemists have been able to increase the reactivity 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 them softer than diamond but not as soft as graphite

  • Volatility

Not volatile

silicon si
Silicon, Si
  • Si - semimetal or metalloid element.

Structure:

  • Giant Covalent macromolecule

Bonding:

  • 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 the repulsion between the 4 pairs of bonding electrons they repel each other equally. Bond angle: 109.5o.
  • No intermolecular forces of attraction, only intramolecular attractions between the silicon atoms.
the physical properties of silicon
The Physical Properties of Silicon
  • Does not conduct electricity

No delocalized valence elections to carry a current because they are all involved 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 too strong 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, the amount of energy needed to break the attractive force between the pair of electrons in the covalent bond and the protons in the nucleus decreases.

silicon as a semiconductor
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 electronic devices
  • Silicon can be made into a conductor (but not as good as a metal), by doping – 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 a “hole” (the 4th valence electron that is not involved in bonding). The absence of an electron on each Si atoms creates the effect of a positive charge which can conduct a current. Alternatively the non bonded electrons can move from “hole” to “hole” carrying a current.
silicon dioxide sio 2
Silicon dioxide, SiO2
  • Naturally occurring in in the earths crust.
  • Each Si atom is linked to four O atoms, 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
silicone bake ware
Silicone bake ware
  • Synthetically manufactured rubber like polymer containing silicon and oxygen.

Properties that make it useful for baking:

  • durable and long lasting
  • Soft and flexible
  • Doesn’t break like glass, or ceramic (other silicon based substances)
  • Does not dent or rust like metal
  • Lightweight
  • Does not react with food
  • Can withstand high temp
silicone breast implants
Silicone Breast Implants
  • Synthetically manufactured rubber sac filled with silicon gel like polymer containing silicon and oxygen atoms.

Properties that make is useful for breast

implants:

  • Soft, flexible and tactile
  • Stable at all temperatures
  • Long lasting
  • Won’t react with other chemicals in the body
  • Despite silicones chemical and physical stability some women’s bodies have rejected the silicone implants resulting in many law suits in the US and concerns about their safety.
bibliography
Bibliography
  • http://chemcases.com/silicon/sil15one.htm