Transcript Chapter 10

Chapter 10
Structure of Substances
Website:
http://www.chemguide.co.uk/atoms/structsmenu.html
Structure of substances
Exist as either molecular structures or giant structures
Construct a concept map for different structures of
substances.
Concept Map
substances
molecular
structures
Giant
structures
giant metallic
structures
examples
simple
molecular
structures
macromolecules
examples
examples
giant ionic
structures
giant covalent
structures
Elements
e.g., copper
examples
examples
examples
Elements
e.g., oxygen,
etc.
compounds
e.g., carbon
dioxide
plasrtic
e.g., polyethene
examples
compounds
e.g., NacCl
Elements
e.g., diaomond
Compounds
e.g., Silicon
dioxide
Simple Molecular Structures
Most non-metals and covalent compounds
Composed of simple discrete covalent molecules
e.g., hydrogen, chlorine, and water
Structure of iodine
I2 molecules packed in a regular pattern.
Held by weak intermolecular forces (van der
Crystal Structure of
Waals’ forces).
Iodine
Structure of dry ice (solid carbon dioxide)
Carbon dioxide molecules paced regularly.
Held together by weak intermolecular forces.
Crystal Structure of
Carbon Dioxide
=C
=O
Electronic diagram of ammonia ?
xx
H x
o
N
xo
H
x H
o
Properties of simple molecular structures
Low melting points and boiling points.
Soft
Usually insoluble in water, but soluble in nonaqueous solvents.
Non-conductors of electricity, whenever as solids,
liquids, or in aqueous solutions.
Except: aqueous solutions of carbon dioxide,
ammonia & sulphur dioxide
As an electrolyte when dissolved in water
Why ???
A few covalent compounds ionize when
dissolved in water.
Mobile ions help to conduct electricity.
SO2(g) + H2O(l)  H2SO3(aq)
H2SO3(aq)  2H+(aq) + SO32-(aq)
Why are covalent compounds volatile?
The covalent molecules are held together by
weak intermolecular forces (van der Waals’
forces)
Little heat energy is needed to separate the
molecules.
Why are they soft?
Intermolecular forces between molecules are
weak.
It is wasy to separate the molecules from each
other.
Why are they insoluble in water?
‘Like dissolves like’ Principle.
The intermolecular forces between covalent
molecules are much weaker than that between
water (solvent) molecules.
Why are they non-conductors of electricity?
They are neutral molecules.
They neither have mobile ions nor mobile
electrons for conducting electricity.
Macromolecules
Containing thousands of atoms which are
bonded together by covalent binds.
Examples: carbohydrates (polysaccharides)
Proteins – made of amino acids.
Plastics - e.g., polyethene, polystyrene, etc.
Giant structures
Giant ionic structures – made up of oppositely
charged ions. (e.g., cations and anions)
Giant covalent structures – made up of atoms
which are bonded together by covalent bonds in
the form of a network.
Giant metallic structures – consists of regularly
packed positive metal ions bathed in a sea of
delocalized electrons.
Giant ionic structures
How an ionic compound is formed?
Usually formed by combining a metal with a non-metal.
Ionic crystals consist of negative and positive ions held
together by strong non-directional electrostatic
attractions (ionic bonds)
Ions are regularly packed in a 3-dimensional structure.
e.g., sodium hydroxide, copper(II) sulphate-5-water
Sodium chloride
Electronic diagram of magnesium chloride
oo
o
o
Cl
oo
xx
o
x
x
x
Mg
xx
x
x
-
oo
2+
o
x
Cl
oo
o
o
Properties of giant ionic structures
Solids at room temperature
High melting points and boiling points
Usually soluble in water, but insoluble in nonaqueous solvents.
Conducts electricity when molten or in aqueous
solution.
Why are they high melting solids?
The oppositely charged ions (caions & anions)
are held together by strong electrostatic
attractions (ionic bonds).,
A lot of heat energy is needed to overcome the
strong ionic bonds between ions.
Electrical Conductors
Definition ???
An electrical conductor conduct electricity
without any chemical change.
e.g., copper, iron, liquid mercury, (metals) and
graphite.
Electrolytes
Definition ???
An electrolyte conducts electricity when molten
or in aqueous solutions. It decomposes at the
electrodes while conducting electricity.
e.g., sodium chloride, sodium sulphate
Aqueous solutions of a few covalent compounds,
such as carbonic acid, and ammonia solution,
are weak electrolytes.
Solid Ionic compounds : Non-conductors of
electricity
Why ???
In solid ionic compounds, ions are held by strong
electrostatic forces and are not mobile.
Hot molten ionic compounds or its aqueous
solutions
Why ???
Ions become mobile when dissolved in water or
in hot molten state.
Can be electrolyzed / decomposed when
conducting electricity.
Ionic Compounds –usually soluble in water
Why ?
Is water a covalent compound?
Why water is an universal solvent?
Water – a polar solvent
Benzene, chloroform, absolute alcohol, etc –
non-aqueous solvent.
Water – a polar solvent
A polar covalent compound ???
Is chorine molecule polar? ?
Why ?
In chlorine molecule, 2 chlorine atoms link
together by a non-polar covalent bond.
The 2 chlorine atoms (identical atoms) share the
2 bonding electrons equally. (Each with 50 %
share.)
Water – a polar solvent
Is the OH bond of the H2O molecule non-polar?
Are the 2 atoms identical?
The bonding electrons are not equally shared by
the two atoms. (oxygen & hydrogen)
The oxygen atom has a greater share of the
bonding electrons.
Water – a polar solvent
Oxygen atom thus has a partial negative charge ( -).
Hydrogen atom has a partial positive charge (+).

O
+
H
H
+
How sodium chloride dissolves in water?
How sodium chloride dissolves in water?
When the attractive forces between ions and water
molecules are greater than that between oppositely
charged ions, the ionic compound dissolves in water.
However, some ionic compounds are insoluble in water.
(i.e., Attractive forces between ions and water are
smaller than that between oppositely charged ions.)
Examples ???
Limestone (calcium carbonate), copper (II) oxide
Giant covalent structures
Millions of non-metal atoms link together by
extensive strong covalent bonds.
Link together by a network of covalent bonds.
e.g., diamond, silicon(IV) oxide, etc.
2 different crystals of carbon: diamond &
graphite.
Diamond
carbon atom
Each carbon atom
links to 4 other
carbon
atoms in a
tetrahedral
arrangement.
covalent bonds
Silicon(IV) oxide
Each silicon
atom
joins to 4
oxygen
atoms
Each oxygen
atom
joins to 2
silicon atoms.
Key:
represents an oxygen atom
represents a silicon atom
Properties of giant covalent structures
Solids with very high melting points and boiling
points.
Hard (except graphite)
Insoluble in any solvent.
Non-conductors of electricity (except graphite)
Why are they high melting solids?
Atoms are bonded extensively by a network of
strong covalent bonds. A lot of heat energy is
needed to to break the extensive covalent bonds.
Why are they non-conductors of
electricity?
Giant covalent structures have neither mobile
ions nor mobile electrons. They are nonconductors of electricity.
Giant metallic structures
Metal atoms are packed closely together in a
regular pattern to form giant metallic structure.
Metallic Bond ??
What is the electronic structure of sodium?
2,8,1
The outermost shell electrons of sodium atoms –
loosely held by the nucleus.
Can be lost (given up) easily.
Move freely and randomly in all directions.
Known as ‘mobile’ / ‘delocalized’ electrons.
Metallic Bond
A piece of metal consists of a giant lattice of
positive metallic ions surrounded by a ‘sea’ of
mobile electrons.
Definition ?
The electrostatic attraction between a sea of
mobile electrons and positive metal ions is
known as the metallic bond.
Non-directional.
Metallic Bond
Metallic structure
Properties of Metals
Good conductors of heat and electricity
Solids with high melting and boiling points
With high densities
Malleable and ductile ???
Malleable – rolled into sheets
Ductile – drawn into thin wires.
High tensile strength
Why are they good conductors of heat & electricity?
There are mobile electrons in giant metallic
structures. The mobile electrons are responsible for
the conduction of electricity. When a piece of metal
is connected to battery, the mobile electrons will
flow in one direction only (from the negative pole of
the battery to the positive pole). Thus, metals are
good conductors of electricity. Metals are also good
conductors of heat. When one end of the metal is
heated, the delocalized electrons there become
more energetic. They move faster, collide more
frequently with neighbouring electrons. Heat is thus
transferred from one end to the other. Heat is also
conducted along the metal by the vibration of metal
ions.
Electrical Conduxtion
Electrical Conduction
Why metals are high melting solids?
In giant metallic structures, atoms are
regularly packed and held strongly by
metallic bonds. A lot of energy is needed to
overcome the strong metallic bonds.
Therefore, metals usually have high melting
points.
Why they have high densities?
Metals have high densities as their atoms are
closely packed in giant metallic structures
Why metals are malleable and ductile?
When a force is applied to a piece of metal,
the layers of atoms can slip over one another.
There is no breaking of the metallic bond.
The metal atoms are still held together by
metallic bond. Metals are malleable and
ductile.