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