Ionic structure of Na + Cl - (s) This represents the maximum number of chloride ions that you can fit around a central sodium ion before the chloride ions start touching each other. If they start touching, you introduce repulsions into the crystal which makes it less stable. It is 6:6 co-ordinated.

Ionic structures have high melting points because much energy is needed to break up the lattice of ions. The more charge on the ions, the stronger the attraction and higher m.p. Ionic compounds conduct electricity when molten or dissolved in water (aqueous), but not when solid as the ions are not free to move to complete an electric circuit. Negatively charge ions (anions) attracted to anode (+) Positively charged ions (cations) attracted to cathode (-) Ionic compounds tend to be brittle and shatter easily Many ionic solids are soluble in water - although not all. It depends on whether there are big enough attractions between the water molecules and the ions to overcome the attractions between the ions themselves.

Ionic structures are usually brittle and shift the ionic layers, so it collapses

Covalent bonding Covalent bonds form between non-metal atoms The atoms share some of their outer electrons so each atom has a full outer shell of electrons A covalent bond is a shared pair of electrons Energy is released during their formation (exothermic)

Most molecules share electrons to have noble gas structures:

How many bonds can boron form (consider number of electrons in outer shell from atomic number)?

Electronic structure of Boron: 1S 2 2S 2 2P 1 : only 3 outer shell electrons available for bonding; so can only form 3 bonds

Double covalent bonding: O 2 CO 2 :

PCl 3 has noble gas structure (8 shared electrons)

But PCl 3 reacts with Cl 2 to form PCl 5 : not a noble gas structure (10 shared electrons)! The phosphorous atom promotes a 3S electron to a higher energy 3d orbital to be used in bonding orbitals

Co-ordinate (Dative) bonding

representing co-ordinate bonds

Aluminium trichloride (AlCl 3 ) It is actually found to be Al 2 Cl 6

Shapes of molecules: Electron pair repulsion theory: draw outer shell bonding electrons and arrange bonds to avoid repulsion Greatest repulsion lone pair - lone pair lone pair - bond pair Least repulsion bond pair - bond pair lineartrigonal tetrahedral pyrimidal

Non-linear octahedral

Bonding in hydrocarbon – methane, ethane Tetrahedral in shape

In methane, all four C-H bonds are identical, so the electrons in the S and P orbitals rearrange themselves in a process called hybridization hybridization The SP 3 orbitals are tetrahedral in shape to avoid electron repulsion between orbitals and this defines the shape of CH 4 where it can bond with 4 hydrogen atoms. Bond angle 109 o

In ethane, C 2 H 6 there are 4 SP 3 orbitals for each carbon atom available for bonding; Three between C and H and the remaining one between C and C forming a sigma (s) bond:

Bonding in ethane (hydrocarbon)