LECTURE 5.1

1. LECTURE 5.1

2. LECTURE OUTLINE • Weekly Deadlines. • Molecules, Monomers, Crystals Etc. (Part I)

4. MOLECULES,MONOMERS, CRYSTALS ETC.(PART I) Definitions and Atomic Models

5. MOLECULES,MONOMERS, CRYSTALS ETC. • The Hierarchical Levels of Structure • Definition of a Molecule • Definition of a Monomer • Definition of a Crystal • Definition of a Glass • Study Questions

12. DEFINITION OF A MOLECULE • Molecule. A molecule is a stable, electrically neutral, discrete group of atoms, within which the atoms are covalently bonded. Molecules should, at least potentially, be able to exist as separate entities in the gaseous state. • Molecular Material. A substance may be defined as molecular, if it comprises small, discrete groups of atoms, which are covalently bonded to each other, but to all other atoms, by secondary bonds only.

13. EXAMPLES OF MOLECULES

14. MOLECULES,MONOMERS, CRYSTALS ETC. • The molecule of water consists of one oxygen atom which is covalently bonded to two oxygen atoms. The angle between the two bonds is about 105˚. The molecule and the monomer are one and the same

15. MOLECULES,MONOMERS, CRYSTALS ETC. • A molecule of methane, consists of a single atom of carbon which is tetrahedrally bonded to four hydrogen atoms. The molecule and monomer are equivalent. • In general, carbon forms four bonds, and with a characteristic tetrahedral geometry. (A notable exception is graphite)

17. DEFINITION OF A MONOMER • A monomer is the basic building block of any material. A small group, or assembly of atoms, ions, etc., which is representative of the entire structure of the material. • The monomer must have the property that it can create the entire material through the application of a simple set of rules, called a lattice or translational form.

18. BUILDING A WALL;EGYPTIAN STYLE

19. TETRAHEDRAL MOMOMERS • Ball and stick representations of two tetrahedral monomers. (b) and (c), both of which are created by decorating a geometric form (the tetrahedron of part a)) with atoms. a) A tetrahedral, geometric form. The centroid of the tetrahedron is indicated by the open circle. b) A monomer of the diamond form of carbon; a central carbon atom is tetrahedrally coordinated by four other carbon atoms. Note that the monomer of Figure 11b is not stable; each carbon atom needs to bond covalently to four other carbon atoms. c) A monomer of the silica-based (SiO2) materials. The central silicon atom is tetrahedrally coordinated by four oxygen atoms to yield a structural unit, that if ionically bonded, has the formula (SiO4)4-, i.e., it is not electrically neutral.

20. THE FERRITIC FORM OF IRON

21. THE SILICA MONOMER

22. MOLECULES, MONOMERS, CRYSTALS ETC. • The monomer of diamond consists of a single atom of carbon, tetrahedrally bonded to four other carbon atoms. • THE MONOMER IS NOT A MOLECULE. • Each carbon atom must bond to four other carbon atoms: the monomer is our notional building brick, and it need not be stable!

23. MOLECULES, MONOMERS, CRYSTALS ETC. • A “dimer” of diamond is created when a second carbon atom is tetrahedrally coordinated. The crystal structure of diamond can be built up by creating a “trimer”, then a “tetramer”….

24. MOLECULES, MONOMERS, CRYSTALS ETC. • The (SiO4) monomer which is characteristic of the various polymorphs of silica (e.g., quartz), silica glass and all the silicates. • The central silicon atom is tetrahedrally bonded to four oxygen atoms • This monomer is not a molecule: it does not even have the correct composition!

25. MOLECULES, MONOMERS, CRYSTALS ETC. • A “dimer” of e.g., quartz. Note that each silicon atom is bonded to four oxygen atoms, and each oxygen atom is bonded to two silicon atoms. This yields the correct composition: SiO2. • In crystalline materials which are based on the silica tetrahedron, the monomers are periodically arranged. • In silica-based glasses, the monomers are arranged randomly

26. MOLECULES, MONOMERS, CRYSTALS ETC. • The monomer of polyethylene consists of two carbon atoms and four hydrogen atoms. The carbon atoms are tetrahedrally bonded to two carbon atoms and two hydrogen atoms. • The result of adding monomers to each other to produce a dimer, a trimer, a tetramer…, is a macromolecule.

28. MOLECULES, MONOMERS, CRYSTALS ETC. • A small fragment of a macromolecule of polyethylene ( a “pentamer”). A “real” macromolecule of PE might contain 106 monomers. • Packing of these linear macromolecules can produce a crystalline polymer, called high-density polyethylene (HDPE). • Note that for “polymers” the monomer is much smaller than the (macro)molecule.

29. THE MACROMOLECULE, AND THE MONOMER OF POLYETHYLENE • a) is a small segment of a macromolecule of polyethylene • A typical macromolecule might contain 50,000 carbon atoms • b) shows a monomer of polyethylene: it consists of two carbon atoms and four hydrogen atoms • c) is a structural representation of the monomer

30. MOLECULES, MONOMERS, CRYSTALS ETC. • However, the macromolecules need not be straight, they may be curved, whilst still retaining the tetrahedral geometry of the monomers. • It is now impossible to pack the macromolecules to create a crystalline polymer. • The amorphous low-density polyethylene (LDPE) results