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Chemical Bonding (A level) - H2 #2 Cheat Sheet (DRAFT) by

Ionic Bonding and Compounds

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Ionic Bonding

Electr­ostatic attraction between cations and anions
Formed due to large difference (>2) in electr­one­gat­ivity
Ionic bonding is not always present between nonmetal and metal (eg. AlCl3 is covalent molecule)
Transfer of electrons from a metal atom to a non metal atom
Metal atom gives away electrons and becomes a cation
Non- metal atom accepts electrons and becomes anion

Giant Ionic Structure

In solid state, anions and cation are held in fixed alternate positions in a giant ionic crystal lattice
Cations attract anions in every direction around them
Ionic bonds exist extens­ively throughout the structure

Strength of ionic bond

Indicated by lattice energy (LE)
LE is the amount of heat energy evolved when 1 mole of solid ionic compound was formed from its gaseous ions
Directly translates to the amount of heat energy required to break the ionic bond
Magnitude of LE : Product of respective charges of the ions/ sum of the respective radius of both ions
More LE -> stronger electr­ostatic attraction between cation and anion --> more stronger ionic bond
Greater numerator -> higher charge -> greater LE required -> stronger ionic bond
Smaller denomi­nator -> lower radius -> greater LE required -> stronger ionic bond
Explain answers in terms of value of (q+ x q-) and (r+ + r-) and then link to the LE formula
 

Physical properties of Giant ionic struct­ureHi

High MP/BP : large amount of energy required to overcome strong electr­ostatic forces of attraction between cations and anions
Discussing difference in MP/BP: structure -> compare (q+ x q-) and (r+ + r-) -> compare LE -> strength of esf -> energy required to overcome esf -> link
Electrical Conduc­tivity: ionic compounds are good electrical conductors in molten­/aq­ueous state as the ions are free from their fixed , alternate positions in the giant crystal ionic lattice -> presence of mobile ions able to carry charges throughout the compound
Hardness: Very hard ( large amount of energy to overcome strong esf) but they are brittle
When a force is applied along a particular plane -> layers of ions slide -> ions of same charge meet and repel one another -> shatters crystal along fault line

Solubility

Ionic compounds are soluble in water (not non-polar solutions)
when ionic compound is added to water -> ion-dipole attraction establ­ished between oppositely charged ions and the polar water molecules (ions are completely solvated) -> when ion-dipole attraction releases sufficient energy -> enough to overcome the strong esf between the cations and anions -> breakdown of the solid ionic crystal lattice -> solid dissolved

Covalent character in ionic bond

not all ionic bonds are pure -> some possess a certain degree covalent character
in these cases, cations will polarise the anion (attract the electron cloud towards itself -> part of the electron cloud of O2- gets drawn to the region in between both ions -> electron cloud ends up being shared between ions
factors affecting extent of covalent character -> increased polarising power of cation (increased charge density -> small size and higher charge) and larger anions (easier to polarise) -> size of anions decreases down the group