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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.

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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.


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

The structure of a substance influences what its PHYSICAL PROPERTIES will be.


Ionic Compounds

Positive sodium ion Na+

Negative chloride ion Cl-

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


Strong ionic bond

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

Sodium chloride melts at over 800°C


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

Therefore more energy is needed to break up the lattice.


H between ions. 2O

MELT

DISSOLVE

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As ionic compounds are made of CHARGED IONS, they can conduct electricity but ONLY if the ions can MOVE.

If it is MOLTEN the ions can move

If it is DISSOLVED the ions can move

800°C

20°C


- between ions.

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MOLTEN IONIC COMPOUND

MOLTEN ionic compounds CONDUCT ELECTRICITY


H between ions. 20 molecule

Ions free to move around

When salt is put in water, H2O molecules pull the ions apart to make a solution. This lets the ions move around.


PURE WATER between ions.

SOLID SALT

SALT SOLUTION

DISSOLVED ionic compounds also CONDUCT ELECTRICITY


H between ions. 20 molecule

Ions free to move around

When salt is put in water, H2O molecules pull the ions apart to make a solution. This lets the ions move around.


Solubility between ions.

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 between ions.

Why do ionic compounds have high melting and boiling points?

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

Explain why water is a good solvent for ionic compounds


H atom between ions.

O atom

Whole thing

= H2O molecule

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


MOLECULES ONLY between ions. WEAKLY ATTRACT EACH OTHER

VERY STRONG bonds BETWEEN ATOMS

(so molecule is very hard to break up)

WEAK bonds BETWEEN MOLECULES

(so molecules are easy to separate)


LIQUID between ions.

SOLID

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

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 – molecular substances are weak and soft when solid.

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.


Carbon atom molecular substances are weak and soft when solid.

Only STRONG bonds

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.

Every C atom joined to 4 others

DIAMOND

(this is only part of the structure - the same pattern carries on in every direction)


SILICA (Silicon dioxide SiO molecular substances are weak and soft when solid.2) has a similar structure to diamond

Every Si atom joined to 4 O atoms

Silica is the main substance in ROCKS. Pure silica is called QUARTZ


Because all the atoms in Giant Structures are UNCHARGED, they will not conduct electricity.


GRAPHITE – a special case STRONG BONDS they:

WEAK BONDS

STRONG BONDS

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


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 easily slide over each other

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 – easily slide over each other

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 - Descriptions easily slide over each other

METALLIC

Strong malleable solids

Don’t dissolve

eg copper

IONS ONLY

IONIC

Crystals

Dissolve in water

eg sodium chloride (salt)

IONS +

FREE ELECTRONS

SIMPLE MOLECULAR

Usually Gases

eg CO2, H2O

ATOMS joined into GIANT MOLECULES

GIANT MOLECULAR

Hard strong solids

Don’t dissolve

eg diamond

(graphite – special case)

MOLECULES ONLY



SUMMARY - PROPERTIES structures with reasons.


Questions structures with reasons.

  • For each of the following substances

  • NaCl ii) SiO2 (sand) iii) Br2 iv) C2H5OH

  • Predict the;

  • structure b) melting point

  • c)electrical conductivity d) solubility

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


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