AN INTRODUCTION TO
FAJAN’S RULES
A guide for A level students
KNOCKHARDY PUBLISHING
2015
SPECIFICATIONS
KNOCKHARDY PUBLISHING
FAJAN’S RULES
INTRODUCTION
This Powerpoint show is one of several produced to help students understand
selected topics at AS and A2 level Chemistry. It is based on the requirements
of the AQA and OCR specifications but is suitable for other examination
boards.
Individual students may use the material at home for revision purposes or it
may be used for classroom teaching if an interactive white board is available.
Accompanying notes on this, and the full range of AS and A2 topics, are
available from the KNOCKHARDY SCIENCE WEBSITE at...
www.knockhardy.org.uk/sci.htm
Navigation is achieved by...
either
or
clicking on the grey arrows at the foot of each page
using the left and right arrow keys on the keyboard
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 bonding is not 100% ionic or covalent
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
Ideal ionic
compound
in many substances bonding is not 100% ionic or covalent
completely 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
INTRODUCTION
The feasibility of having some covalent character can be predicted using Fajan’s Rules.
A compound is more likely to be covalent if the ...
CATION
SMALL SIZE
HIGH CHARGE
it is “highly polarising” and attracts electrons in the anion
ANION
LARGE SIZE
HIGH CHARGE
it is “highly polarisable” and will be easily distorted
INTRODUCTION
The feasibility of having some covalent character can be predicted using Fajan’s Rules.
A compound is more likely to be covalent if the ...
CATION
SMALL SIZE
HIGH CHARGE
it is “highly polarising” and attracts electrons in the anion
ANION
LARGE SIZE
HIGH CHARGE
it is “highly polarisable” and will be easily distorted
N.B.
Just because a substance is less likely to be covalent according to
Fajan’s Rules doesn’t mean it will be ionic; it will remain covalent
but have some ionic character (or vice versa).
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
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.
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 DON’T CONDUCT ELECTRICITY IN THE MOLTEN STATE
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 DON’T CONDUCT ELECTRICITY IN THE MOLTEN STATE
ELECTRON DENSITY IS BETWEEN EACH SPECIES
H
H
H:H
The ideal covalent compound has the electron
density exactly in between the species
BLACK AND WHITE OR SHADES OF GREY?
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
H : Cl
d+
H
Cl
d-
In some covalent compounds, the electron density
isn’t exactly in the centre between the species
• HYDROGEN CHLORIDE HAS SOME ‘IONIC CHARACTER’ - CHARGE SEPARATION
• HYDROGEN CHLORIDE REACTS WITH WATER
BLACK AND WHITE OR SHADES OF GREY?
IONIC COMPOUNDS WHICH ‘MISBEHAVE’
• LITHIUM CHLORIDE SHOULD BEHAVE LIKE A TYPICAL GROUP I CHLORIDE
• BUT… IT IS HYDROLYSED BY WATER AND HAS A ‘LOW’ MELTING POINT
BLACK AND WHITE OR SHADES OF GREY?
IONIC COMPOUNDS WHICH ‘MISBEHAVE’
• LITHIUM CHLORIDE SHOULD BEHAVE LIKE A TYPICAL GROUP I CHLORIDE
• BUT… 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
BLACK AND WHITE OR SHADES OF GREY?
IONIC COMPOUNDS WHICH ‘MISBEHAVE’
• LITHIUM CHLORIDE SHOULD BEHAVE LIKE A TYPICAL GROUP I CHLORIDE
• BUT… 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
FAJAN’S 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
FAJAN’S RULES
A COMPOUND IS MORE LIKELY TO HAVE
SOME COVALENT CHARACTER IF...
• THE CATION IS SMALL AND/OR HAS A HIGH CHARGE - HIGHLY POLARISING
MORE COVALENT
CHARACTER
• THE ANION IS LARGE AND/OR HAS A HIGH CHARGE - HIGHLY POLARISABLE
FAJAN’S RULES
A COMPOUND IS MORE LIKELY TO HAVE
SOME COVALENT CHARACTER IF...
• THE CATION IS SMALL AND/OR HAS A HIGH CHARGE - HIGHLY POLARISING
MORE COVALENT
CHARACTER
MORE COVALENT
CHARACTER
• THE ANION IS LARGE AND/OR HAS A HIGH CHARGE - HIGHLY POLARISABLE
FAJAN’S 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)
Period 3 NaCl
MgCl2
AlCl3
SiCl4
1+
2+
3+
4+
0.095nm
0.065nm
0.050nm
0.041nm
Group 1 LiCl
NaCl
KCl
RbCl
1+
1+
1+
1+
0.060nm
0.095nm
0.133nm
0.148nm
m.pt./°C
GREATER POSITIVE
CHARGE DENSITY
ionic rad.
GREATER POSITIVE
CHARGE DENSITY
‘charge’
808
714
180
-70
solubility
bonding
soluble
soluble
hydrolysed
hydrolysed
ionic
ionic
covalent
covalent
soluble
soluble
soluble
soluble
cov. character
ionic
ionic
ionic
AN INTRODUCTION TO
FAJAN’S RULES
THE END
©2015 JONATHAN HOPTON & KNOCKHARDY PUBLISHING