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An ionic bond is the electrostatic attraction between a positive cation and a negative anion

A covalent bond is the attraction of two nuclei for a pair of shared electrons

A dative covalent bond is a covalent bond formed when one of the overlapping orbitals contained two electrons and the other none

A metallic bond results from the attraction of the sea of delocalised electrons and the positive ions

The lattice energy is the energy change when one mole of an ionic solid is formed from its gaseous ions under standard conditions

THINGS TO UNDERSTAND...

An ionic crystal consists of a giant lattice of alternating cations and anions. The force between the ions depends on the sizes and the charges of the ions...

The larger the charge, the stronger the force of attraction

The smaller the sum of the ionic radii, the stronger the force of attraction

The ionic radius increases down a group of the periodic table as the number of electron shells increases - Li+

The ions N3-, O2-, F-, Na+, Mg2+, Al3+ are isoelectronic, that is they all have the same electron structure. However, negative ions are larger than positive ions and increase in size as the charge increases. Positive ions decrease in radius as the charge increases. So the radius decreases in the order N3-, O2-, F-, NA+, MG2+. AL3+

EVIDENCE FOR IONS

Molten/aqueous solutions conduct electricity by the movement of ions. if some green copper chromate is placed on a strip of wet filter paper and the ends of the paper are connected to a power supply, the blue copper ions move towards the negative terminal and the yellow chromate ions move towards the positive terminal.

COVALENCE IN IONIC SOLIDS

If the experimental value for lattice energy differs significantly from the theoretical value, the solid has some covalent character. The greater the difference, the greater is the extent of covalency. The extent of covalency is determined by the extent to which the cation polarises the anion. Cations with a small radius and/or a high charge are very polarising. Anions with a large radius and/or a high charge is very polarisable, the outer electrons in the anion are pulled towards the cation and the bond has some covalent character.

FORMULAE OF METAL IONS

NaCl or NaCl2? In order to form NaCl2 a second electron would have to be removed from the sodium. This electron would have to come from an inner shell and so a huge amount of energy would be required. Even though the lattice energy for NaCl2 would be more exothermic

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