- Created by: Holly Bennett
- Created on: 21-05-12 18:38
Atomic Number: Number of Protons
Mass Number: Total number of protons and neutrons.
Electron very small
Compound are chemically bonded: formed when atoms of 2+ elements combine chemically.It is difficult to seperate the 2 original elements.
Isotopes:diffent atomic forms of the same element which have same number of protons but differnent number of neutrons.
Ionic Bonding- Transferring Electrons
Atoms lose or gain electrons to form ions which are strongly attracted to each other and chemical bond together.
On the periodic table atoms 1,2,3 loose electrons on their outer shells to other elements but elements 6,7 gain electrons from other elements to have full outer shells. After this they will have electronic structure of noble gas.
Example: Sodium + Chlorine
Sodium gives up electron from outer shell to become Na+. The chlorine atoms then picks up electron and become Cl- ion.
REMEBER: If you give something away you become GOOD (posotive)
If you take something then you are bad (negative)
- Giant Ionic Lattices ALWAYS, which are very closesly packed regular lattices.
- Very strong electrostatic forces of attraction between oppositley charged ions.
- Have high melting and boiling points due to these forces which require large amounts of energy to overcome.
- When substances melt, ions are free to move and carry electric current.
- They dissolve easily, ions seperate and are free to move in solution - carry electric current.
Exam Tip: When drawing diagram for ionic bonding include brackets, charges and symbol.
Covalent Bonding: Sharing Electrons
Atoms share electrons in highest energy levels/outer shells.
This way both have full outer shell and have structure of noble gas.
Example: Hyrogen + Chlorine
Hydrogen (H2) have ust one electron so need 1 more. Chlorine has 7 on outer shell so needs one more. So they form single covalent bond to achieve this.
Exam Tip: In diagram only have to draw the outer shell of electrons.
Covalent Substances, 2 KINDS.
- Simple Molecular Substances: atoms form very strong covalent bonds to form several small molecules of several atoms.
- The forces between molecules are very weak.
- Intermolecular Forces require little energy so low melting and boiling point. When substance melt or boil it is the forces that break not the covalent bonds.
- Most gases and liquids but can be solids.
- Don't conduct electricity as no ions.
Giant Covalent Structures
Structures are macromolecules.
- VERY similar to giant ionic but are no charged ions.
- This means have very high melting and boiling points.
- They DON'T conduct electricity NEVER. (execpt graphite)
Main Examples: Diamond: Each carbon atom has four covalent bonds, this makes diamond the hardest natural substance, it is also good for presentation.
Graphite: Each carbon atoms only forms 3 covalent bonds. Creates layers which are free to slide over each other, so graphite is soft and slippery. Weak intermolecular forces between layers. Graphite is only non-metal which is good at conducting heat and electricity. Each carbon atoms has one delocalised electron and it is these that conduct heat and electricity.
Metal Properties due to Sea of Free Electrons(Delocalised)
Metals have giant structure. The metallic bonds involve delocalised electrons which produce all properties of metals. These electrons are free to move through whole structure so metals good conductors of heat and electricity.
These electrons hold atoms together in regualr structure. There are strong forces of electrostatic attraction between + ions and - electrons. So high melting point as large amount of energy needed to overcome these forces.
- Behave differntly depending on conditions.
- 'Shape memory alloy', it's a metal alloy but when cool you can bend it and shape it. If heat at certain temperature goes back to remebered shape.
- Handy for glasses frames as return to original shape.
- Nitinol used for braces as mouth warms brace and trys to return to rembered shape.
Really tiny particles: 1-100nm across.
- They contain a few hundred atoms
- Include the fullerene which are molecules of carbon
- Have differnent properties from the main chemical made from,
- Fullernes can be joined to form nanotubes which are hollow carbon tubes: have strong covalent bonds which used in tennis rackets.
Uses of Nanoparticles.
Uses of Nanoparticles:
- They have huge surface are to volume ratio so could make catalysts.
- Can use as sensors to detect one type of molecule, already used to test water purity.
- Could be stronger and lighter building material.
- New cosmetics: sun tan and deodrant (don't leave white marks).
- Nanomedicine: fullerenes absorbed by body so could deliver drugs to right cell.
- Lubricants from fullerenes. (ball bearings).
- Can conduct electricity so used in tiny electric circuits.
Thermosetting and Thermosoftening.
Thermosetting: Strong forces between molecules. (Stromg intermolecular forces) between polymer chains called cross links. These crosslinks hold polymer in solid structure. Therefore does not sofren when heated. They are: strong,hard and rigid.
Thermosoftening: Weak forces between molecules. The indivdual chains held together by weak forces which can slide over each other. They do not have cross-linking chains. Forces easy to overcome --- melt plastic. When it cools the polymer hardens into new shape. You can remould them all the time.
How it is made affects properties:
Starting material and reaction conditions:
Eg : Low density polythene made by heating at 200c under high pressure makes it flexible.
High density and lower temp and pressure (with catalyst) makes more rigid.