Observations Not all ionic compounds have high melting points.

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INTRODUCTION
Observations Not all ionic compounds have high
melting points. Some covalently bonded
compounds have higher than expected boiling
points due to dipoles in their structure Reason
in many substances the bonding is not 100
ionic or covalent Ideal ionic compound complete
ly separate, spherical ions electron densities
are apart from each other However, if the
positive ion has a high charge density it can
distort the negative ion by attracting the outer
shell electrons to give an area of electron
density between the two species ... a bit like a
covalent bond
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INTRODUCTION
The feasibility of having some covalent character
can be predicted using Fajans Rules. A compound
is more likely to be covalent if the ... CATION
SMALL SIZE it is highly polarising and
attracts electrons in the anion HIGH
CHARGE ANION LARGE SIZE it is highly
polarisable and will be easily distorted HIGH
CHARGE N.B. Just because a substance is
less likely to be covalent according to Fajans
Rules doesnt mean it will be ionic it will
remain covalent but have some ionic character
(or vice versa).
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EXTREMES OF CHEMICAL BONDING
IONIC BONDING 3-DIMENSIONAL GIANT IONIC
LATTICE ALTERNATE POSITIVE AND NEGATIVE
IONS HIGH MELTING POINT SOLUBLE IN
WATER MOLTEN STATE CONDUCTS ELECTRICITY
ELECTRON DENSITY IS SEPARATED AND AROUND EACH
SPECIES
The ideal ionic compound has completely separate,
spherical ions and the electron densities are
apart from each other.
CONTENTS
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EXTREMES OF CHEMICAL BONDING
COVALENT BONDING MOLECULAR (SIMPLE OR MACRO)
SIMPLE MOLECULES HAVE LOW MELTING PTS - WEAK
INTERMOLECULAR FORCES USUALLY INSOLUBLE IN
WATER BUT SOME ARE HYDROLYSED MOLECULES DONT
CONDUCT ELECTRICITY IN THE MOLTEN STATE
ELECTRON DENSITY IS BETWEEN EACH SPECIES
H H
The ideal covalent compound has the electron
density exactly in between the species
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BLACK AND WHITE OR SHADES OF GRAY?
POLAR COVALENT BONDS IN MANY MOLECULES THERE
ARE POLAR COVALENT BONDS MOLECULES TEND TO
HAVE HIGHER MELTING/BOILING POINTS FOR THEIR
MASS DIPOLE-DIPOLE INTERACTION OR HYDROGEN
BONDING ARE PRESENT
d d -
H Cl
In some covalent compounds, the electron density
isnt exactly in the centre between the species
HYDROGEN CHLORIDE HAS SOME IONIC CHARACTER
- CHARGE SEPARATION HYDROGEN CHLORIDE REACTS
WITH WATER
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BLACK AND WHITE OR SHADES OF GRAY?
IONIC COMPOUNDS WHICH MISBEHAVE LITHIUM
CHLORIDE SHOULD BEHAVE LIKE A TYPICAL GROUP I
CHLORIDE IT IS HYDROLYSED BY WATER AND HAS A
LOW MELTING POINT
THE POSITIVE ION ATTRACTS THE OUTER SHELL
ELECTRONS OF THE NEGATIVE ION AND DISTORTS THE
SPHERICAL IONIC SHAPE... THERE IS NOW SOME
ELECTRON DENSITY BETWEEN THE SPECIES
SMALL CATION LARGE ANION HIGH CHARGE
DENSITY ELECTRONS FAR FROM NUCLEUS HIGHLY
POLARISING HIGHLY POLARISABLE
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FAJANS RULES
A COMPOUND IS MORE LIKELY TO HAVE SOME COVALENT
CHARACTER IF... THE CATION IS SMALL AND/OR
HAS A HIGH CHARGE - HIGHLY POLARISING THE
ANION IS LARGE AND/OR HAS A HIGH CHARGE - HIGHLY
POLARISABLE
MORE COVALENT CHARACTER
MORE COVALENT CHARACTER
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FAJANS RULES
PROOF Chlorides can be used to demonstrate
changes in bond type as the positive charge
density increases due to higher charge (across
Period 3) or larger size (down Group 1)
charge ionic rad. m.pt./C
solubility bonding Period 3 NaCl 1 0.095nm
808 soluble ionic MgCl2 2 0.065nm
714 soluble ionic AlCl3 3 0.050nm
180 hydrolysed covalent SiCl4 4 0.041n
m -70 hydrolysed covalent Group
1 LiCl 1 0.060nm soluble
cov. character NaCl 1 0.095nm
soluble ionic KCl 1 0.133nm
soluble ionic RbCl 1 0.148nm
soluble ionic
GREATER POSITIVE CHARGE DENSITY
GREATER POSITIVE CHARGE DENSITY