structures module c2
DESCRIPTION
STRUCTURES Module C2. IONIC. COVALENT. eg sodium chloride (salt). PURE substances have different STRUCTURES depending on the type of BONDING they have. METALLIC eg copper. SIMPLE MOLECULAR eg carbon dioxide, water. GIANT MOLECULAR eg diamond, graphite. - PowerPoint PPT PresentationTRANSCRIPT
STRUCTURES
Module C2
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.
IONICIonic 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 chloride 677
Potassium chloride 770
Sodium chloride 801
Copper oxide 1446
Calcium oxide 2707
As ionic compounds are made of CHARGED IONS, they can CONDUCT ELECTRICITY but ONLY if the ions can MOVE.
-+ +- + -
-+ +
- + -
+
-
+-+
- +
-+-
+-
+ +
-
MELT
+ --
++ --+
- +
DISSOLVE
H2O800°C 20°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, H2O molecules pull the ions apart to make a solution. This lets the ions move around.
H20 molecule
Ions free to move around
DISSOLVED ionic compounds also CONDUCT ELECTRICITY
PURE WATER SOLID SALT SALT SOLUTION
SIMPLE MOLECULAR SUBSTANCES
These are substances like carbon dioxide CO2, water H2O and methane CH4 which are always made of simple molecules whether they are SOLIDS, LIQUIDS OR GASES
H atom
O atom
Whole thing = H2O 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
CompoundMpt (°C)
Bpt (°C)
State at room temp
Water H2O 0 100 Liquid
Butane C4H10 -138 -0.5 Gas
Methane CH4 -182 -164 Gas
Carbon dioxide CO2 - -78 Gas
Oxygen O2 -218 -183 Gas
Hydrogen H2 -259 -252 Gas
Liquid oxygen boiling at -183°CSolid 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 SiO2) 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
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 - DescriptionsIONIC
CrystalsDissolve in water
eg sodium chloride (salt)
METALLICStrong malleable solids
Don’t dissolveeg copper
SIMPLE MOLECULARUsually Gases
eg CO2, H2O
GIANT MOLECULARHard strong solids
Don’t dissolveeg diamond
(graphite – special case)
IONS ONLY IONS +
FREE ELECTRONS
MOLECULES ONLY
ATOMS joined into
GIANT MOLECULE
S
SUMMARY - PROPERTIESStructure Property Reason
Ionic HIGH 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
Metallic HIGH mpt/bpt. Hard & strong
Malleable
CONDUCT: YES (very well)
Strong bonds between IONS
Regular structure, layers slide
Free electrons between ions