Observations Not all ionic compounds have high melting points.

Some covalently bonded compounds have higher thanexpected boiling points due to dipoles in their structure

Reason in many substances the bonding is not 100% ionic or covalent

Ideal ioniccompound 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

INTRODUCTIONINTRODUCTION

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 HIGH CHARGE

N.B. Just because a substance is less likely to be covalent according toFajan’s Rules doesn’t mean it will be ionic; it will remain covalent buthave some ionic character (or vice versa).

INTRODUCTIONINTRODUCTION

CONTENTSCONTENTS

EXTREMES OF CHEMICAL BONDINGEXTREMES 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 BONDINGEXTREMES 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 GRAY?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

H ClH : Cl+-

• HYDROGEN CHLORIDE HAS SOME ‘IONIC CHARACTER’ - CHARGE SEPARATION• HYDROGEN CHLORIDE REACTS WITH WATER

In some covalent compounds, the electron density isn’t exactly in the centre between the species

BLACK AND WHITE OR SHADES OF GRAY?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

SMALL CATION LARGE ANION

HIGH CHARGE DENSITY ELECTRONS FAR FROM NUCLEUS

HIGHLY POLARISING HIGHLY POLARISABLE

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

FAJAN’S RULESFAJAN’S RULES

A COMPOUND IS MORE LIKELY TO HAVESOME 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

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FAJAN’S RULESFAJAN’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 1+ 0.095nm 808 soluble ionicMgCl2 2+ 0.065nm 714 soluble ionic

AlCl3 3+ 0.050nm 180 hydrolysed covalent

SiCl4 4+ 0.041nm -70 hydrolysed covalent

Group 1 LiCl 1+ 0.060nm soluble cov. characterNaCl 1+ 0.095nm soluble ionicKCl 1+ 0.133nm soluble ionicRbCl 1+ 0.148nm soluble ionic

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