1. A guide for A level students KNOCKHARDY PUBLISHING
AN INTRODUCTION TO FAJAN’S RULES
A guide for A level students
KNOCKHARDY PUBLISHING

2. KNOCKHARDY PUBLISHING
FAJAN’S RULES
INTRODUCTION
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3. 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

4. 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
Ideal ionic
compound 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

5. 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 it is “highly polarising” and attracts electrons in the anion
HIGH CHARGE
ANION LARGE SIZE it is “highly polarisable” and will be easily distorted

6. 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 it is “highly polarising” and attracts electrons in the anion
HIGH CHARGE
ANION LARGE SIZE 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).

7. 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

8. IONIC BONDING EXTREMES OF CHEMICAL 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.

9. 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

10. EXTREMES OF CHEMICAL BONDING ELECTRON DENSITY IS BETWEEN EACH SPECIES
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

11. 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
d d -
H : Cl
H Cl
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

12. 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

13. 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

14. 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

15. A COMPOUND IS MORE LIKELY TO HAVE SOME COVALENT CHARACTER IF...
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

16. 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
MORE COVALENT CHARACTER

17. 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
MORE COVALENT CHARACTER
MORE COVALENT CHARACTER

18. 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)
‘charge’ ionic rad m.pt./°C solubility bonding
Period 3 NaCl nm soluble ionic
MgCl nm soluble ionic
AlCl nm hydrolysed covalent
SiCl nm hydrolysed covalent
Group 1 LiCl nm soluble cov. character
NaCl nm soluble ionic
KCl nm soluble ionic
RbCl nm soluble ionic
GREATER POSITIVE
CHARGE DENSITY
GREATER POSITIVE
CHARGE DENSITY

19. ©2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING
AN INTRODUCTION TO FAJAN’S RULES
THE END
©2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING