The document describes different structures of pure substances and how their structure determines physical properties. Ionic substances form crystal structures of positively and negatively charged ions with strong bonds, giving them high melting points and ability to conduct electricity when molten or dissolved. Metallic substances have strong atomic bonds and free electrons, making them conductors. Giant molecular and simple molecular substances have covalent bonds that result in high and low melting points respectively. Graphite has strong in-plane bonds but weak between-plane bonds, making it soft and able to conduct. Structure determines properties like melting point, strength, and conductivity.
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2. 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.
3. IONIC
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-
4. 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
6. 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
8. 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
10. 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
11. 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
12. 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
13. This means simple molecular substances have LOW melting
and boiling points
Compound
Mpt
(°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
14. 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.
15. 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
16. 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
17. 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.
18. 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
19. 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
20. 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)
21. 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
22. 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
23. 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)
24. SUMMARY - Descriptions
IONIC
Crystals
Dissolve in water
eg sodium chloride (salt)
METALLIC
Strong malleable solids
Don’t dissolve
eg copper
SIMPLE MOLECULAR
Usually Gases
eg CO2, H2O
GIANT MOLECULAR
Hard strong solids
Don’t dissolve
eg diamond
(graphite – special case)
IONS
ONLY IONS +
FREE
ELECTRONS
MOLECULES
ONLY
ATOMS
joined into
GIANT
MOLECULE
S
25. SUMMARY - PROPERTIES
Structure 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
