GCSE Chemistry Unit 2: Structure & Properties

For AQA GCSE chemsitry.

I used the CGP 'GCSE Chemisty Revision guide' and the Nelson Thornes 'AQA Science chemistry' to help with my revision. 

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  • Created by: Rhys Owen
  • Created on: 31-03-13 15:47

Ionic Compounds

IONIC COMPOUNDS are formed through IONIC bonding - bonding between a METAL and a NON-METAL

IONIC COMPOUNDS have GIANT IONIC LATTICES.

This means that the ions form a TIGHTLY PACKED regular LATTICE structure.

They have HIGH MELTING POINTS and HIGH BOILING POINTS due to the very strong ELECTROSTATIC FORCES OF ATTRACTION between the OPPOSITELY CHARGED ions. It takes a lot of ENERGY to overcome this attraction.

When they IONIC COMPOUNDS do MELT, the IONS are FREE to move and they can carry ELECTRIC CURRENT. The same thing happens when they are DISSOLVED in WATER.

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Ionic Bonding

Atoms 'want' to have a FULL outer shell of electrons so they bond together to achieve this.

IONIC BONDING is ALWAYS between a METAL and a NON-METAL.

In IONIC BONDING, the METAL atom GIVES an electron to the NON-METAL so both atoms have a FULL OUTER SHELL of electrons.

Electrons are NEGATIVELY charged.

Because each atom has either gained or lost an electron, the overall charge of the atom is no longer neutral as the number of positive protons is not the same as the number of negative electrons.

This means that the atoms become CHARGED PARTICLES called IONS.

Therefore as the METAL LOSES an electron, it's ion is POSITIVELY charged. The NON-METAL GAINS an electron so, it's ion is NEGATIVELY charged. 

Eg. When sodium (Na) reacts with Chlorine (Cl) to form sodium chloride, the Na atom becomes a NA+ ion. The Cl atom becomes a Cl- ion. 

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Isotopes

ISOTOPES are different atomic forms of the same element, which have the SAME number of PROTONS but a DIFFERENT number of NEUTRONS.

They have the SAME ATOMIC NUMBER but a DIFFERENT MASS NUMBER.

Eg. Carbon-12 has 6 protons, 6 electrons and 6 neutrons.

      Carbon-14 has 6 protons, 6 electrons and 8 neutrons.

The mass number is the number of PROTONS & NEUTRONS

The atomic number is the number of PROTONS.

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Covalent Bonding

Atoms 'want' to have a FULL outer shell of electrons so they BOND together to achieve this.

COVALENT BONDING is ALWAYS between a two NON-METALS.

In COVALENT BONDING, the atoms SHARE electrons so both atoms have a FULL OUTER SHELL of electrons.

They only SHARE electrons in their OUTER SHELL

Each covalent bond provides ONE EXTRA SHARED ELECTRON for each atom.

So a COVALENT BOND is a SHARED PAIR of electrons.

Eg. Hydrogen choride is formed through covalent bonding. 

This is shown well through dot and cross diagrams.


 

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Covalent substances: Simple Molecules

Chlorine gas (C2) is a  SIMPLE MOLECULE

Chlorine atoms form VERY STRONG covalent bonds to form SMALL molecules- C2. So the atoms within each C2 molecule are held VERY TIGHTLY together.

However, the forces of attraction BETWEEN the individual molecules are VERY WEAK. This is true for ALL simple molecules. 

Because of these WEAK INTERMOLECULAR FORCES, the MELTING and BOILING points of simple molecules is very LOW, as the molecules are easily separated from each other.

Remember it's the WEAK INTERMOLECULAR FORCES that get BROKEN when simple molecules melt or boil, NOT the strong covalent bonds.

  • Simple molecules have LOW BOILING POINTS
  • Simple molecules have LOW MELTING POINTS
  • Simple molecules CAN'T CONDUCT ELECTRICITY, as there are no ions.  


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Covalent Substances: Giant Covalent Structures

Giant covalent structures are MACROMOLECULES. Eg. Diamond and Silicon dioxide.

In macromolecules, ALL the atoms are BONDED to each other by STRONG covalent bonds. This means they have HIGH melting and boiling points as the covalent bonds require LOTS of ENERGY to brake. 

  • Macromolecules are VERY HARD
  • Macromolecules have very HIGH MELTING & BOILING POINTS
  • Macromolecules CAN'T CONDUCT ELECTRICITY. (Apart from Graphite)

GRAPHITE is different. Its carbon atoms only bond to THREE other carbon atoms, which form hexagons arranged in giant LAYERS. These layers can SLIDE over each other as there are no covalent bonds between them. This makes graphite SOFT and SLIPPERY.

Remember, in graphite the carbon atoms only form THREE bonds. But carbon has FOUR electrons its outer shell for bonding leaving ONE SPARE electron on each carbon atom.  

This electron is FREE to move around the layers and is called a DELOCALISED ELECTRON. These delcoalised electrons allow graphite to conduct ELECTRICITY and HEAT

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Metallic Structures

Metals consist of a GIANT STRUCTURE.

Metal atoms are arranged in LAYERS which can easily SLIDE over each other. This is what allows us to BEND and SHAPE metals so easily. 

Metals contain DELCOALISED ELECTRONS. These are FREE to move throughout the whole structure and PASS on  ELECTRIC current and HEAT

The delocalised electrons HOLD the atoms together in a regular structure because the metal ions are POSITIVELY CHARGED.

We say that there are strong ELECTROSTATIC FORCES of attraction between the POSITIVE METAL IONS and the NEGATIVE ELECTRONS which hold the ions in place. 

  • Metals can be BENT & SHAPED
  • Metals are GOOD conductors of HEAT & ELECTRICITY.

ALLOYS are MIXTURES of metals. This means that there are different size atoms in alloy. These different size atoms disrupt the regular layered structured, so it is more DIFFICULT for the layers to slide over each other. This makes alloys HARDER than pure metals. 

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