STRUCTURE And bonding

PURE substances have different STRUCTURES depending on the type of BONDING they have METALLIC eg copper IONIC eg sodium chloride (salt) GIANT MOLECULAR eg diamond, graphite SIMPLE MOLECULAR eg carbon dioxide, water COVALENT The structure of a substance decides what its PHYSICAL PROPERTIES will be.

IONIC Ionic substances are compounds of metals and non-metals (eg sodium chloride, copper oxide, magnesium sulphide etc) They are made of IONS: atoms which have lost or gained electrons giving them a positive or negative CHARGE The + ions and – ions STRONGLY ATTRACT each other to make a regular crystal structure Positive sodium ion Na + Negative chloride ion Cl -

Because of the very STRONG BONDS between the IONS, ionic compounds have HIGH MELTING & BOILING POINTS Strong ionic bond Sodium chloride melts at over 800°C

Ionic compound Melting point (°C) Iron chloride677 Potassium chloride770 Sodium chloride801 Copper oxide1446 Calcium oxide2707

As ionic compounds are made of CHARGED IONS, they can CONDUCT ELECTRICITY but ONLY if the ions can MOVE MELT DISSOLVE H2OH2O 800°C20°C If it is MOLTEN the ions can move If it is DISSOLVED the ions can move

MOLTEN IONIC COMPOUND MOLTEN ionic compounds CONDUCT ELECTRICITY

When salt is put in water, H 2 O molecules pull the ions apart to make a solution. This lets the ions move around. H 2 0 molecule Ions free to move around

DISSOLVED ionic compounds also CONDUCT ELECTRICITY PURE WATERSOLID SALTSALT SOLUTION

SIMPLE MOLECULAR SUBSTANCES These are substances like carbon dioxide CO 2, water H 2 O and methane CH 4 which are always made of simple molecules whether they are SOLIDS, LIQUIDS OR GASES H atom O atom Whole thing = H 2 O molecule

VERY STRONG bonds BETWEEN ATOMS (so molecule is very hard to break up) WEAK bonds BETWEEN MOLECULES (so molecules are easy to separate) MOLECULES ONLY WEAKLY ATTRACT EACH OTHER

SOLID LIQUID GAS Simple molecular substances can only be a liquid or a solid when the temperature is LOW enough for the WEAK BONDS to pull the molecules together

This means simple molecular substances have LOW melting and boiling points Compound Mpt (°C) Bpt (°C) State at room temp Water H 2 O0100Liquid Butane C 4 H Gas Methane CH Gas Carbon dioxide CO Gas Oxygen O Gas Hydrogen H Gas Liquid oxygen boiling at -183°C Solid oxygen at -240°C

As the bonds between the molecules are weak, simple molecular substances are weak and soft when solid. As the molecules are NOT CHARGED simple molecular substances DON’T CONDUCT ELECTRICITY when solids, liquids or gases.

GIANT MOLECULAR SUBSTANCES In these materials strong covalent bonds join atoms together with other atoms of the same type to make giant structures, rather than little groups. DIAMOND Only STRONG bonds Carbon atom (this is only part of the structure - the same pattern carries on in every direction) Every C atom joined to 4 others

SILICA (Silicon dioxide SiO 2 ) has a similar structure to diamond Silica is the main substance in ROCKS. Pure silica is called QUARTZ Every Si atom joined to 4 O atoms

Because all the atoms in Giant Structures are joined by STRONG BONDS they: Have HIGH melting / boiling points Are usually HARD and STRONG Because all the atoms in Giant Structures are UNCHARGED, they will not conduct electricity.

GRAPHITE – a special case Common form of carbon found in soot, charcoal, pencil leads etc Carbon atoms each joined to 3 others with STRONG bonds to make hexagonal sheets of atoms The sheets of atoms are joined to other sheets by WEAK bonds STRONG BONDS WEAK BONDS

As the bonds between the layers of atoms are weak, they can easily slide over each other As the C atoms are only bonded to 3 others, the extra electrons form clouds of ‘free electrons’ between the layers

GRAPHITE - Properties The STRONG BONDS between the ATOMS mean it has HIGH MELTING and BOILING POINTS The WEAK BONDS between the LAYERS mean it is SOFT and SLIPPERY as the layers SLIDE over each other easily (used in pencils and as a solid lubricant) The FREE ELECTRONS between the layers mean that graphite CONDUCTS ELECTRICITY (used as sliding contacts in electric motors)

METALS In a metal the atoms are held together by strong bonds in regular structures. This means most metals have high melting and boiling points and are hard and strong

In a metal the atoms LOSE SEVERAL OF THEIR OUTER ELECTRONS which drift around between the metal ions as FREE ELECTRONS. As they have LOST a few electrons, the atoms become POSITIVE IONS Free (“delocalised”) electrons e.g. Na  Na + + e - Mg  Mg e - etc

The large number of free electrons makes all metals are GOOD CONDUCTORS of electricity AND heat. The regular structure means the layers of atoms can fairly easily slide over each other without breaking the bonds (though not as easily as graphite) and so metals are MALLEABLE (bend rather than snap)

SUMMARY - Descriptions IONIC Crystals Dissolve in water eg sodium chloride (salt) METALLIC Strong malleable solids Don’t dissolve eg copper SIMPLE MOLECULAR Usually Gases eg CO 2, H 2 O GIANT MOLECULAR Hard strong solids Don’t dissolve eg diamond (graphite – special case) IONS ONLY IONS + FREE ELECTRONS MOLECULES ONLY ATOMS joined into GIANT MOLECULE S

SUMMARY - PROPERTIES StructurePropertyReason IonicHIGH mpt/bpt CONDUCT: Solid NO Molten YES Dissolved YES Strong bonds between IONS Ions can’t move Ions can move to carry current Covalent – Simple molecular LOW mpt/ bpt (often gas at room temp). Soft when solid CONDUCT: Never Bonds between MOLECULES very weak. Molecules aren’t charged Covalent – giant molecular HIGH mpt/bpt. Hard & strong CONDUCT: Never Strong bonds between all ATOMS No free charges/electrons Covalent - graphite HIGH mpt/bpt Soft & slippery CONDUCT: YES (fairly well) Strong bonds between ATOMS Weak bonds between LAYERS Free electrons between layers MetallicHIGH mpt/bpt. Hard & strong Malleable CONDUCT: YES (very well) Strong bonds between IONS Regular structure, layers slide Free electrons between ions