Starter Activity

• State the 3 main properties of metals.

• Explain in terms of metallic structure how these properties arise.

Structures of Ionic and Covalent Compounds

• Describe and explain the physical properties of a giant ionic lattice.

• Describe and explain the physical properties of simple covalent and giant covalent lattices.

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 influences what its PHYSICAL PROPERTIES will be.

Ionic Compounds 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-

Strong ionic bond

Sodium chloride melts at over 800°C

Because of the very STRONG BONDS between the IONS, ionic compounds have high melting & boiling points. All ionic compounds are solids at room temperature.

Ionic compound

Melting point (°C)

Iron chloride 677

Potassium chloride 770

Sodium chloride 801

Copper oxide 1446

Calcium oxide 2707

The greater the charge the stronger the electrostatic forces between ions.

Therefore more energy is needed to break up the lattice.

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

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

Solubility

Ionic lattices can dissolve in polar solvents, such as water.

Polar molecules break down the lattice by surrounding each ion to form a solution. When NaCl dissolved in water the giant ionic lattice breaks down.•Water molecules attract the Na+ and Cl- ions.•The ionic lattice breaks down as it dissolves. Water molecules surround the ions•Na+ attracts the δ- on the O in the water molecules•Cl- attracts the δ+ on the H in the water molecules

Questions

1. Why do ionic compounds have high melting and boiling points?

2. Explain the different electrical conductivities of an ionic compound in different states.

3. Explain why water is a good solvent for ionic compounds

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 -57 Gas

Oxygen O2 -218 -183 Gas

Hydrogen H2 -259 -252 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.

Solubility –

simple molecular structures are soluble in non-polar solvents, such as hexane. Because van der Waals’ forces form between the solvent and the molecule.

The formation of the van der Waals’ forces weakens the lattice structure.

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 etcCarbon 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)

Solubility –

Giant covalent structures are not soluble in both polar and non-polar solvents.

Because the covalent bonds in the lattice are too strong to be broken by either polar or non-polar solvents.

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

Write a summary of the physical properties of different structures with reasons.

Structure

Property Reason

Ionic

Covalent – Simple molecular

Covalent – giant molecular

Covalent - graphite

Metallic

SUMMARY - PROPERTIES

Structure

Property Reason

Ionic HIGH mpt/bptCONDUCT: Solid NOMolten YES Dissolved YES

Strong bonds between IONSIons can’t moveIons can move to carry current

Covalent – Simple molecular

LOW mpt/ bpt (often gas at room temp). Soft when solidCONDUCT: No

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/bptSoft & slipperyCONDUCT: YES (fairly well)

Strong bonds between ATOMS Weak bonds between LAYERSFree electrons between layers

Metallic HIGH mpt/bpt. Hard & strongMalleableCONDUCT: YES (very well)

Strong bonds between IONS and e-

Regular structure, layers slideFree electrons between ions

Questions

1. For each of the following substancesi) NaCl ii) SiO2 (sand) iii) Br2 iv)

C2H5OHPredict the; a) structure b) melting pointc)electrical conductivity d) solubility

2. Explain the different properties of diamond and graphite in terms of their structure.