Chemistry GCSE (Unit 2) AQA (C2)

Unit 2 (additional chemistry) AQA Dual Core Science (C2)

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Pre- test: Structures and properties

1. Why are ionic compounds always solid at room temperature?

2. When can ionic compounds conduct electricity?

3. Why are many covalent substances gases or liquids at room temperature?

4. Why do covalent compounds not conduct electricity?

5. Why can some covalently bonded substances form giant structures?

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Pre- test: Structures and properties (cont.)

6. Why are diamond and graphite so different?

7. What happends to the atoms when a metal bends?

8. Do all metals conduct heat and electricity?

9. What is nanoscience?

10. What use can we make of nanoparticles?

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IONIC COMPOUNDS- key points:

1. Ionic compounds have high melting and boiling points.

2. Ionic compounds conduct electricity when molten or in solution.

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Ionic Compounds-

  • Ionic compounds have giant structures in which many strong electrostatic forces hold the ions together.
  • They are solids at room temp.
  • Lots of energy is needed to overcome the ionic bonds to melt the solids.
  • Therefore  ionic compounds have high melting and boiling points. 
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Ionic Compounds- (cont.)

  • HOWEVER when they have been melted the ions are free to move.
  • This allows them to carry an electrical charge.
  • SO the liquids conduct electricity.
  • Some ionic substances dissolve in water because the water molcules can split up the lattice.
  • The ions are free to move in the solutions and so they also conduct electricity. 

    (N.B.- solid ionic compounds cannot conduct electricity)
     
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Ionic Compounds- (cont.)

The attractive forces between the oppositely charged ions in an ionic compound are very strong.

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_51.gif) ---- Strong ionic bonds.

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Ionic bonding- (cont.)

  • Metal atoms lose the electron, or electrons, in their highest energy level and become positively charged ions.
  • Non-metal atoms gain an electron, or electrons, from another atom to become negatively charged ions.
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Ionic bonding- (cont.)

Lithium, Li

Diagrams of a lithium atom (2,1) with two electrons in its inner shell and one electron in its highest energy level, and a lithium ion (2)+ with two electrons in its highest energy level (http://www.bbc.co.uk/schools/gcsebitesize/science/images/diag_lithium.gif)

Lithium is in Group 1. It has one electron in its highest energy level. When this electron is lost, a lithium ion Li+ is formed.

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Ionic bonding- (cont.)

Sodium, Na

Diagrams of a sodium atom (2,8,1) and a sodium ion (2,8)+ (http://www.bbc.co.uk/schools/gcsebitesize/science/images/diag_sodium.gif)

Sodium is also in Group 1. It has one electron in its highest energy level. When this electron is lost, a sodium ion Na+ is formed.

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Ionic bonding- (cont.)

 

Structure of a neon atom. A black dot represents the nucleus. The small circle around this has two red dots on it, representing the first energy level with two electrons. A larger outer circle has eight red dots on it, representing the second energy level with eight electrons (http://www.bbc.co.uk/schools/gcsebitesize/science/images/atom_neon.gif)

Neon atom

Note that a sodium ion has the same electronic structure as a neon atom (Ne).

But be careful - a sodium ion is not a neon atom. This is because the nucleus of a sodium ion is the nucleus of a sodium atom and has 11 protons - but the nucleus of a neon atom has only 10.

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Ionic bonding- (cont.)

Magnesium, Mg

Diagrams of a magnesium atom (2,8,8,2) and a magnesium ion (2,8)2+ (http://www.bbc.co.uk/schools/gcsebitesize/science/images/diag_magnesium.gif)

Magnesium is in Group 2. It has two electrons in its highest energy level. When these electrons are lost, a magnesium ion Mg2+ is formed.

A magnesium ion has the same electronic structure as a neon atom (Ne).

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Ionic bonding (cont.)- Non-metal ions

Fluorine, F

Diagrams of a fluorine atom (2,7) and a fluoride ion (2,8)- (http://www.bbc.co.uk/schools/gcsebitesize/science/images/diag_fluorine.gif)

Fluorine is in Group 7. It has seven electrons in its highest energy level. It gains an electron from another atom in reactions, forming a fluoride ion, F-.

Note that the atom is called fluorine, but the ion is called fluoride.

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Ionic bonding (cont.)- Non-metal ions

Once again, a fluoride ion is not a neon atom, because the nucleus of a fluoride ion is the nucleus of a fluorine atom, with 9 protons, and not of a neon atom, with 10.

Structure of a neon atom. A black dot represents the nucleus. The small circle around this has two red dots on it, representing the first energy level with two electrons. A larger outer circle has eight red dots on it, representing the second energy level with eight electrons (http://www.bbc.co.uk/schools/gcsebitesize/science/images/atom_neon.gif)

Neon atom

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Ionic bonding (cont.)- Non-metal ions

Chlorine, Cl

Diagrams of a chlorine atom (2,8,7) and a chloride ion (2,8,8)- (http://www.bbc.co.uk/schools/gcsebitesize/science/images/diag_chlorine.gif)

Chlorine is in Group 7. It has seven electrons in its highest energy level. It gains an electron from another atom in reactions, forming a chloride ion, Cl-.

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Ionic bonding (cont.)-

 

  • sodium + chlorine - sodium chloride
  • magnesium + oxygen - magnesium oxide
  • calcium + chlorine - calcium chloride 

In each of these reactions, the metal atoms give electrons to the non-metal atoms. The metal atoms become positive ions and the non-metal atoms become negative ions.

There is a strong electrostatic force of attraction between these oppositely charged ions, called an ionic bond.

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Ionic bonding- (cont.): Dot and cross diagrams

Diagram of bonding in calcium chloride. A calcium ion (2,8,8)2+ gives one electron to a chloride ion (2,8,8)- and another electron to another chloride ion (2,8,8)-. All three ions have full highest energy levels (http://www.bbc.co.uk/schools/gcsebitesize/science/images/diag_calcium_chloride.gif)Calcium ions have the formula Ca2+. Chloride ions have the formula Cl-.

You need to show two chloride ions, because two chloride ions are needed to balance the charge on a calcium ion.

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Ionic bonding- (cont.)

Properties of ionic compounds
Ionic compound Properties
Sodium chloride, NaCl, 
High melting point: 800ºC, Non-conductive in its solid state, but when dissolved in water or molten NaCl will conduct electricity.

Magnesium oxide, MgO, Higher melting point than sodium chloride: around 2,800ºC. This is because its Mg2+ and O2- ions have a greater number of charges, so they form stronger ionic bonds than the Na+ and Cl- ions in sodium chloride. 

  • Because magnesium oxide stays solid at such high temperatures, it remains non-conductive. It is used for high-temperature electrical insulation.

KEY WORDS: giant structures, ionic bonds, conduct, dissolve.

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Ionic compounds- CHECK YOURSELF QUESTIONS:

1. Why do ionic solids have high melting points?

2. Why can some ionic solids dissolve in water?

3. Why can molten ionic substances conduct electricity?

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SIMPLE MOLECULES- key points:

1. The forces between simple molecules are weak so many of these substances are gases or liquids at room temperature.

2. Simple molecules do not have a charge and so cannot conduct electricity.

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Simple molecules-

  • The atoms in molecules are held together by strong covalent bonds.
  • These bonds act only between the atoms within the molecule, and so simple molecules have little attraction for each other.
  • Substances made of simple molecules have relatively low melting and boiling points.
  • The forces of attraction between molecules, called 'intermolecular forces', are weak.
  • These forces are overcome when a molecular substance melts or boils. 
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Simple molecules- (cont.)

  • This means that substances made of small molecules have low melting and boiling points.
  • Those with the smallest molecules, like H2, Cl2, and CH4, have the weakest intermolecular forces and are gases at room temperature.

  • Larger molcules have stronger attraction and may be liquids at room temp.- E.G. Br2 & C6H14

  • OR solids wth low melting points, like I2.

  • Molecules have no overall charge and cannot carry an electric current, so these substances do not conduct electricity. 

    KEY WORDS: intermolecular forces. 

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Simple molecules- CHECK YOURSELF QUESTIONS:

1. Why is Oxygen, O2, a gas at room temperature?

2. Why does petrol not conduct electricity?

3. What type of forces act between molecules?

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GIANT COVALENT SUBSTANCES- key points:

1. Some covalently bonded substances form giant covalent strtures.

2. These substances have very high melting points.

3. Diamond and graphte are both forms of carbon but have many different properties.

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Giant covalent substances-

  • A covalent bond is a strong bond between two non-metal atoms. It consists of a shared pair of electrons. A covalent bond can be represented by a straight line or dot-and-cross diagram.
  • Hydrogen and chlorine can each form one covalent bond, oxygen two bonds, nitrogen three, while carbon can form four bonds.

  • Covalent bonds are strong - a lot of energy is needed to break them. Substances with covalent bonds often form molecules with low melting and boiling points, such as hydrogen and water.
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Giant covalent structures- (cont.)

  • Atoms of some elements can form several covalent bonds.

  • These atoms can join together in giant covalent structures (sometimes called 'macromolecules').

  • Every atom in the structure is joined to several other atoms by strong covalent bonds.

  • It takes vast amounts of energy to break down the lattice- and so these substances have very high melting points. 
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Giant covalent structures- (cont.)

  • Diamond (a form of carbon), and silica (silicon dioxide) have regular 3D giant structures.

  • They are very hard and transparent. 

  • Silica- contains silicon and oxygen atoms, instead of carbon atoms.(The fact that it is a semi-conductor makes it immensely useful in the electronics industry: most transistors are made of silica.)
  • Graphite (another form of carbon) is a substance in which the atoms join in flat 2D layers. There only weak forces between the layers- so they slide over each other. Graphite is slippery and grey. Graphite has delocalised electrons (as in a metal structure) along its layers and so conducts electricity.
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Giant covalent structures- (cont.)

The graphic shows the molecular structure of diamond and graphite: two allotrope of carbon, and of silica (silicon dioxide).

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_55.jpg)

From left to right - graphite, diamond, silica

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Giant covalent structures- CHECK YOURSELF QUESTION

1. What is a macromolecule?

2. Why are diamond and silica transparent and hard?

3. Why is graphite slippery?

4. Why can graphite conduct electricity?

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GIANT METALLIC STRUCTURES- key points:

1. Metals can be bent and shaped because their layers of atoms can slide over each other.

2. Delocalised electrons move throughout metals and can carry heat and electricity.

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Giant metallic structures-

  • Metal atoms are arranged in layers.
  • When a force is applied the layers of atoms can slide over each other.
  • They can move into a new position without breaking apart- the metal bends or stretches into a new shape.
  • THEREFORE metals are useful for making wires, rods and sheet materials. 
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Giants metallic structures- (cont.)

Metals form giant structures in which electrons in the outer shells of the metal atoms are free to move. The metallic bond is the force of attraction between these free electrons and metal ions. Metallic bonds are strong, so metals can maintain a regular structure and usually have high melting and boiling points.showing free electrons from the outer electron shells mingled with positively charged metal ions (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_60.gif)

        Atomic structure of a metal

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Giants metallic structures- (cont.)

  • Delocalised electrons hold the atoms in place. The delocalised electrons are free to move throughout the metal structure. 
  • Metals are good conductors of electricity and heat, because the free electrons carry a charge or heat energy through the metal.

  • The free electrons allow metal atoms to slide over each other, so metals are malleable and ductile.

    KEY WORDS: delocalised electrons 
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Giant metallic structures- CHECK YOURSELF QUESTION

1. Why can we change the shaped of metals?

2. How do metals conduct electricity?

3. What allows metals to conduct heat?

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NANOSCIENCE AND NANOTECHNOLOGY- key points:

1. Nanoscience is about structures that are a few nanometres in size.

2. Nanoparticles behave differently to the same materials in bulk. 

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Nanoscience and nanotechnology-

  • When atoms are arranged into very small particles they behave differently to ordinary materials made of the same atoms.

  • A nanometre is one billionth of a metre.

  • Nanoparticles are a few nanometres in size.

  • They contain a few hundred atoms arranged in a particular way.

  • Their structures and very small sizes give them new properties that can make them very useful. 
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Nanoscience and nanotechnology- (cont.)

  • Nanoparticles have large surface areas.
  • This exposes many more atoms at their surface than normal materials. 

  • Electrons can move through them more easily than ordinary materials.

  • They can be sensitive to light, heat, pH, electricity and magnetism.

  • Nanotechnology uses nanoparticles as very selective sensors, highly efficient catalysts, new coatings and sonstruction materials with spec. properties- also to make drugs more effective. 
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Nanoscience and nanotechnology- (cont.)

Nanoparticles range in size from about 100 nm down to about 1 nm. They are typically the size of small molecules, and far too small to see with a microscope.

Working with nanoparticles is called nanotechnology.

KEY WORDS: nanometre, nanoparticles, nanotechnology.

Nanotubes like this could be used to make tiny mechanical devices, molecular computers or extremely strong materials.(http://www.bbc.co.uk/schools/gcsebitesize/science/images/spl_nanotube.jpg)

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Nanoscience and nanotechnology- CHECK YOURSELF QUE

1. About how many atoms are there in a typical nanoparticle?

2. Why do nanoparticles have different properties to ordinary materials?

3. Suggest three ways in which nanotechnology is being used.

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C2 2- END OF CHAPTER QUESTIONS:

1. Why does it take a lot of energy to melt ionic compounds?

2. Why can solutions of ionic compounds conduct electricity?

3. What are 'intermolecular forces'?

4. What happens to the molecules when water boils?

5. Why does diamond have a high melting point?

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C2 2- END OF CHAPTER QUESTIONS:

6. How are carbon atoms bonded in graphite?

7. Why can we pull metals into wires/

8. Why do metals stay the same when they conduct electricity?

9. How are nanoparticles different to ordinary materials?

10. What is 'nanotechnology'?

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halima rashid

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