Macromolecules (polymers) Large number of small repeated units called mers High molecular weight compounds Natural and synthetic Linear or branched Thermoplastic or thermosetting Melting range and not melting points No boiling points

Linear Molecules Linear polymeric chains such as polyvinyl chloride (PVC) n = degree of polymerization: the number of mers per molecule

Secondary bonds Covalent bonds are very strong and called primary bonds Intermolecular forces are called secondary bonds it exists because of the local electric fields within and around uncharged atoms. Three types are known: a) Polarization due to electron oscillations in symm molecules b) dipole-dipole interactions in asymmetric molecules c) Hydrogen bridge

Network structure Phenol formaldehyde reaction Polyfunctional monomers  network structure, e.g Phenolformaldehyde resin (Bakalite) Heating: thermoplastic  thermosetting polymer

Three dimensional bonding

Silicate glasses Fused silica (SiO 2 ), each Si atom is linked to four adjacent oxygen atoms which inturn bridge between two silicons  VERY STABLE network

Ionic Bonding -Electrostatic attraction between cations and anions -Unlimited numbers of ions can be bonded to produce solid materials -Attraction and repulsion forces: F c F c = K 0 (Z 1 q)(Z 2 q)/x 2 q is electron charge = 0.16 x A.s. Z is the valence

NaCl

coordination numbers 1- The strength of materials exhibit under applied stresses is related to the type of bonds that held the atoms together. 2- Interatomic attractions are caused by the electronic structure of atoms. Inert gases such as He, Ne, Ar, etc. have a very stable arrangement of eight electrons (2 for He) in either outer electron orbitals. As a result they have no electrical charge. 3- Most other elements achieve the stable configuration of having 8 electrons in their outer orbital by: -Receiving extra electrons to form anions, or -Releasing electrons to form cations or -Sharing electrons.

coordination numbers Naturally, ions of unlike charges are attracted to one another by electrostatic forces, while electrons sharing requires intimate contact between atoms. Thus, in both instances strong bonding is established between neighbouring atoms. 4- In addition to ionic and covalent bonds a third type of primary attractive mechanics is offered by delocalized electrons or an electron cloud able to move throughout the metal structure. This gives rise to the formation of the so- called metallic bond. 5- It is interesting to note that certain materials exhibit mixed bonding characteristics.

coordination numbers 6- Molecules on the other hand, may be defined as groups of atoms strongly bonded together. 7- Most engineering materials possess coordinated groups of many atoms. Therefore, when analyzing the bonding of atoms within materials, we speak of a coordination number. 8- The Coordination Number (CN) can be simlly defined as number of first nearest neighbors surrounding an atom within a given material. Let us consider, for example, the case of methane, CH 4

coordination numbers As it is already seen, the coordination number for carbon is 4 whereas the hydrogen atoms have only one nearest neighbour. 9- The coordination number of an atom is controlled by 2 factors: A- the number of valence electrons of the atom B- efficiency of atomic packing. 10- the halides which are situated in group VII of the periodic table and have 7 valence electron each, form only one bond and hence have one coordination number when bonded covalently.

coordination numbers Likewise, members of the oxygen group (VI) have a maximum coordination number of 2. (Note this is in the gaseous state) In solids, efficient atomic packing is concerned, since energy is released as ions of opposite charges approach each other, ionic compounds have generally higher coordination numbers, without introducing the strong mutual repulsion forces between ions of the charges. This may be illustrated with MgO. Mg 2+ ions are surrounded by O 2- ions. The Mg 2+ ions has a radius r = 0.66 Ă, which is large enough to allow 6 O 2- ions with R = 1.40 Ă to surround it without direct of negative ions with one another.

CN Ionic Bonding (3-D) a) A maximum of 6 O 2- surrounding Mg2+ b) CN of Si 4+ among O 2- is only 4 (r/R < 0.4)

Ionic coordination numbers Covalent solids are loosely packed and posses a large free space Ionic solids are colsely packed and contains less free space. This due to the columbic attractions are omnidirectional. +ve ions are smaller than –ve ions

R = radius of anion, r = radius of cations Ionic Coordination (2-Dimensional). a)Coordination with r/R > The smaller cation is coordinated with 4 anions (CN = 4 for 2-dimnsions). b) Coordination with r/R < 0.4. The (+ve) ion does not have max. conact with all 4 neighbouring (-ve) ions. There is repulsion between the contacting ions c) When r/R < 0.4, then CN =3 is favoured (2-D). R = radius of anion, r = radius of cations

Example Show the origin of 0.41 as the minimum ratio for a coordination number of 6. Procedure The minimum ratio of possible sizes to permit a CN = 6 is sketched below: Note that the 5 th & 6 th ions sit above and below the central ion

Coordination Calculations: Minimum r/R for 6-fold coordination a)Minimum r/R for 4-fold coordination