C2.2 Structure and properties

Ionic compounds have HIGH melting points and boiling points because of the many strong electrostatic forces of ATTRACTION between OPPOSITELY charged ions in the giant LATTICE. Ionic compounds will CONDUCT electricity when MOLTEN or in SOLUTION because the ions are able to MOVE freely around the liquid. 

E.G. Sea water is a better conductor of electricty as sodium chloride( ionic compound) is dissolved so the eletcrons can move freely around the lattice, and carry charge. 

ATTRACTION between the IONS and the ELECTRONS keep all the ions in place properly, so they remain in LATTICE shape. 

IONIC SOLID: Ions are fixed in the lattice.DO NOT conduct electricity as cannot move. 

MOLTEN IONIC COMPOUND: High temperature breaks up the ions and electrons attraction, so they can move about freely in teh molten compound. So they can CARRY CHARGE. 

DISSOLVED IONIC COMPOUND: water molecules separate the iosn from the lattice, so the ions can MOVE. 

• Covalently BONDED
• WEAK INTERMOLECULAR FORCES- so there are low forces between molecules, so LOW boiling and melting points. Less ENERGY required to break them down. 
• Simple molecules have relatively simple formulas like: CH4, CO2, H2O
• Usually GAS at room temperature, but SOME SOLIDS which have low b.p and m.p. 
• Do NOT CONDUCT electricity- as they have no ions to carry around charge around the material. 
• NO OVERALL CHARGE

Non metals react together to form MOLECULES which are held together by COVALENT bonds. These hold the atoms together very STRONGLY. If these substances are made of simple molecules, tehy have low MELTING points and low BOILING points. So at room temperature tehy often exist as gases and LIQUIDS or as SOLIDS which melt relatively easily. 

The postively charged ENERGY in metals are held together by DELOCALISED electrons. These also allow the layers to SLIDE over each other so taht the metal's SHAPE can be changed. they also allow the metal to conduct HEAT and ELECTRICITY. 

SHAPE MEMORY ALLOYS: 

• They can be bent into differnet SHAPES, but when you HEAT them they RETURN to their ORIGINAL shape. 
• They seem to 'remember ' their original shape.

USES: 

• Badly broken bon can be used to hold the bones in place while they heal. Alloy is COOLED before wrapped around bone, and HEATED to remove. 
• Also braces are used by Dentists in a similar fashion. Easier to use as tehy jsut need to be heated and cooled, also very trustworthy as will not corrode in mouth and are safe. 

The polymer chain in a thermosoftening polymer form a TANGLED web. Teh polymer softens at a relatively low temperature. Othe rpolymers have strong COVALENT bonds between tehir chains which form CROSS LINKS. We call these THERMOSETTING polymers. 

• MONOMERS affect the properties of the POLYMERS they form. 
• Also CONDITIONS which polymerisation is carried out AFFECTS the properties of the polymer. 

THERMOSOFTENING & THERMOSETTING POLYMERS:

Thermosoftening: 

• INDIVIDUAL polymer chains- TANGLED together
• Can be RESET when heated
• WEAK covalent bonds between the polymer chains- so tehya re broken easily and remade,s o are remoulded

Thermosetting:

• Strong co-valent bonds- FORMS CROSS LINKS- as soon as heated

LDPE/ HDPE

LOW DENSITY POLY(ETHENE):

• HIGH pressure 
• TRACE of oxygen 
• Polymer chains are BRANCHED- so not packed closely together
• Lower softening temp- WEAKER

HIGH DENSITY POLY (ETHENE):

• CATALYST at 50oC
• RAISED PRESSURE
• Polymer chains are NOT branched together, so tehy pack in together CLOSELY. 
• Higher softening temp- STRONGER

• 1 nanometre: 1 billionth of a metre
• A material's NANOPARTICLE behaves DIFFERENTLY to larger atoms
• Large Surface area- good for reacting

AT WORK:

• Coal dust in air causes an explosion.

  • However lumps of coal does not explode.

  • The dust has a larger surface area, so more of it is exposed to the air.

  • Nanoparticles are smaller than coal dust.

  • As the name suggests that are a small number of nanometres in size.

  • They are smallerr than the smallest cellular lifeforms.

  They can be used form many things:

  • Opaque substances turned into transparent ones.

  • Solids can be turned into their liquid state at room temperature.

• Stable materials can be made combustible.

• Alternately materials which do not normally react ( are inert ) are turned into

catalysts.

• Insulators can become conductors.

• Nanoparticles can be used in cosmetics (make-up).

• The skin is a barrier to normal chemicals, but nonparticles can be absorbed by the

skin and so they penetrate.

• These particles are smaller than the wavelength of light, and so are too small to see.

• Titanium oxide is used in sun creams because they are absorbed by the skin and

provide better protection from the sun rays. It is also invisible.

• However despite the benefits that arise from using nanoparticles, there are

problems.

• Toxicologists however are concerned that the skin is a barrier, and the use of

nanoparticles is invading this barrier.

• They feel that this may lead to nanoparticles in the blood and brain. Nobody knows

what sort of damage they could do.

• Although cosmetic companies claim these particles do not penetrate into the skin,

futher research is required.