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 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 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).

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. CONTENTS

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

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 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

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

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

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 1+ 0.095nm 808 soluble ionic MgCl2 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. 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