STRUCTURE OF MATTER

1. بسم الله الرحمن الرحيم STRUCTURE OF MATTER

2. All materials are built up from atoms and molecules There is a close relationship between the atomic basis of a material and its properties. Generally, the physical, mechanical and chemical properties of any material depend mainly on: 1) Atomic structure. 2) Inter-atomic bonding. 3) Arrangement of atoms in space.

3. I. Atomic structure The atom is the basic building unit for all elemental matter It is composed of:- 1- Central positive nucleus [positively charged protons and uncharged neutrons]. 2- Revolving electrons around the nucleus in definite orbits [negatively charged particles] (state of energy levels or shells). N.B; Electrical state of the atom: Neutral. Atomic number: Number of electrons. Atomic weight: Protons + Neutrons Valence electrons: Electrons in the outermost shell, They determine the chemical reactivity of the element.

4. II. Interatomic Bonding Atoms achieve a stable state by having eight electrons in their outer shell (as in inert gases). This can be obtained by: 1) Receiving extra electrons to complete the outer shell electrons (and the atom becomes negative ion). 2) Releasing electrons so that the outer shell has eight electrons (and the atom becomes positive ion). 3) Sharing of electrons so that the outer shells of two or more atoms are complete.

5. Interatomic Bonding Forces in the Solid State The formation of bondsinvolves only the outer most valence elec A-Primary atomic bonds. a- Ionic (electron transfer). b-Covalent (electron sharing). c- Metallic (electron release). B- Secondary force

6. 1-Ionic (Electron Transfer). It indicates electron transfer, attraction of positive and negative ions. The classic example is sodium chloride (Na+ C1-), because the sodium atom contains one valence electron in its outer shell and the chlorine atom has seven electrons in its outer shell, the transfer of the sodium valence electron to the chlorine atom results in the stable compound Na+ Cl-. Characteristics of Ionic Bond * Heat resistant. * High strength and hardness. * Insoluble in organic solvents. * Easily dissolved in ionized solvents such as water, acids and alkalis. * Dissociate into their constituent ions in solutions which in their turn can conduct an electric current.

7. i- Ionic Bond

8. 2) Covalent Bond Sharing of electrons Two valence electrons are shared by adjacent atoms. Hydrogen molecule, H2 → single valence electron in each hydrogen atom is shared with that of the other combining atom, and the valence shells become stable. Covalent bonding occurs in many organic compounds, such as hydrocarbons (CH4) and acrylic resin.

9. Characteristics of covalent bonds: * Water in soluble, but dissolved in organic solvents. * Very strong and hard. * Insulators and can with stand high temperature. * Directional: Since the electrons will spend most of their time in the region, where the orbits over lap.

10. 3) Metallic Bond It is the attraction between +ve cores and free electrons or electron cloud. It occurs in metals, because they easily give up the electrons in their valence shells giving positive cores. The electrons move freely through the metal from atom to atom and form electron cloud. There is attraction between free electrons and the positive charged cores. Characteristics of metallic bond: The free mobility of electrons contributes to the following properties of metals: * High thermal and electrical conductivity. * Opacity (de to absorption of light by free electrons). * Metallic luster (free electrons re-emit light). * High ductility and malleability.

11. II. Secondary Forces (Van Der Waal Forces): These forces are physical, weak, less heat resistant and arise from the polarization of molecules i.e. formation of electrical dipoles. Characteristics of secondary bonds: 1. Low strength and hardness. 2. Low thermal resistance. δ- δ+ δ+ δ-

12. A- Fluctuating Dipole: Instantaneous location of more electrons on one side of the nucleus than the other → asymmetry in their electron distribution. Very weak bonds can develop between atoms because these atoms attain a dipole character. This leads to weak attractive forces between the fluctuating dipoles in adjoining atoms

13. b) Permanent Dipole: The hydrogen bond is an important example. In H2O there is a covalent bond because oxygen and hydrogen atoms share electrons. electrons around oxygen nucleus are more than those around the hydrogen nucleus → hydrogen portion of the water molecule is positive in relation to the oxygen portion. "attraction will take place between the positive hydrogen portion of one water molecule and the negative oxygen portion of another water molecule".

14. III-Atomic Arrangement and Crystal Structure Solid substances are classified according to the regularity of the atoms or molecules in the three spatial directions, into:- 1- Crystalline 2-Non-crystalline (Amorphous)

15. 1- Crystalline Solid dental materials are termed crystalline when their atoms are regularly arranged in a space lattice. A space lattice is the regular arrangement of atoms in the space so that every atom is situated similarly to every other atom.

16. Types of Space Lattices There are about 14 different types of space lattice but only few are of dental interest. The simplest way to study these types, is to consider a unit cell which is the smallest repeating unit in the space lattice.

17. 1- Crystalline Structure Unit cells are classified according to: 1) The length of their axes (a,b,c). 2) The interfacial angles (,,). z c α α β y a b x SCS

18. 1) The Cubic System: - The length of the axes a,b,c are equal. - The interfacial angles=  =  = 90° • The are three types of the cubic system. a) Simple Cubic Space Lattice (S.C.): The unit cell has one atom at each corner. Each atom is surrounded with eight unit cells. Therefore each atom has 1/8 of its volume in each of these eight cells, so S.C. contains 8 x 1/8 = one atom.

20. b) Body Centered Cubic (B.C.C.): Each corner is occupied by 1/8 of an atom with one atom at the center of the cube. Therefore, the number of atoms in a B.C.C. unit cell is (8 X 1/8 = 1 + 1 in the center) = 2 atoms c) Face Centered Cubic (F.C.C.): Each corner is occupied with an atom and one in the center of each of the six faces. Atoms at each face shared by two adjacent unit cells. So F.C.C. contains (8 X 1/8 + 6 X 1/2) = 4 atoms.

21. 2) The Hexagonal System: The axis a = b but # c. The angle  =  = 90° but the angle  equals 120°. a) The simple hexagonal system (S.H.) contains: 6 x 1/6 (at corners of top surface) + 6 x 1/6 (at corners of bottom surface) + 2 X 1/2 (at top and bottom surfaces) = 3 atoms. b) The hexagonal closed packed system (HCP) 12 X 1/6 (at corners) + 2 X 1/2 (at top and bottom surfaces) + 3 at the center = 6 atoms, so this structure has the densest packing of atoms.

22. ii- Hexagonal System

23. This Atomic Packing Factor (APF) Definition:It is the fraction of the space of the structure unit occupied by the atoms and is calculated by: APF= vol. of atoms in the unit cell/ vol of unit cell Simple Cubic = 0.54 indicates that nearly 50% of the space is free so that other atoms can occupy this free space without causing too much disruption to the crystalline structure. B.C.C. = 0.68 and F.C.C. = 0.74 With these larger atomic packing factors it is of course more difficult for smaller atoms to occupy the free space without disrupting the structure. Higher APF  * higher stability, * higher densities and *higher strength properties.

24. 2-Amorphous Amorphous means without shape. Gases and liquids are amorphous substances. Some solids like glass and some polymers are amorphous because of the random arrangement of their atoms, yet their atoms may form a short localized range of order lattice with a considerable number of disordered units in between. Since such an arrangement may be considered typical of the liquid structure, these solids are sometimes called "Super cooled liquids".

25. Structure of Solids 2-Amorphous structure 1-Crystalline structure - Regular repetition arrangement of unit cell - No regularity in the arrangement of atoms - Low energy - Higher energy - Definite melting point - Melting range temperature

26. Crystalline Imperfections The calculated theoretical strength of crystalline materials was found to be much higher than the actual strength. Why? Their nature is not perfect = They contain defects or imperfections Types of Crystalline Imperfections *Point defects *Line defects (dislocation) *Planer defects (area defects)

27. Point defects 1) Vacancy: missing atom within the crystal * imperfect packing during crystallization * thermal vibrations→ individual atoms may jump 2) Impurities: an extra atom lodged within the crystal a) interstitial b) substitutional

28. Line defects (dislocation) The most common type of line defect is called dislocation = displacement of a raw of atoms from their normal positions in the lattice → plastic deformation in metals due to movement of dislocations

29. Planer defects (area defect) They are present at grain boundaries in metals.

30. Polymorphism (Allotropy): Polymorphic materials are these that can exist with more than one crystal structure by changing the surrounding physical condition. Polymorphic forms have same chemical composition but different physical properties. Silica (SiO2) It is an important example for allotropy in dentistry. It exist in nature in four different allotropic forms, which are; Quartz, Tridymite, Cryslobalite and Fused quartz. Each form have different physical properties but all are chemically SiO2.

31. Silica (SiO2) It is an important example for allotropy in dentistry. It exist in nature in four different allotropic forms, which are; Quartz, Tridymite, Cryslobalite and Fused quartz.

32. With the application of heat to silica, two types of transformations can take place:

33. Correlation between atomic structure and materials properties The properties of materials depend basically on the type of bonds which dominate in the structure, the space lattice and the atomic packing. 1)Density is controlled by atomic weight, atomic radius, and the atomic packing factor. 2) Melting and boiling temperatures can be correlated with the strength of the bond. Increased temperatures raise the energy until the atoms are able to separate themselves one from the other. Stronger bonds need higher temperature to impart the necessary energy for melting.

34. 3)Thermal expansionsof materials with comparable atomic packing factors vary inversely with their melting temperature. i.e. The higher the melting temperature, the less the coefficient of thermal expansion. 4)Strengthgoverned by the type of bond, although the arrangement of atoms controls the deformation and resistance to stresses. 5) Crystallinestructures have lower energy level whileamorphous structures have higher energy due to irregular arrangement or short order arrangement of their atoms. Therefore the amorphous structures do not have definite melting temperature but rather softening temperature. i.e. They soften before melting.

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36. Regarding Bonding Forces 1)Distinction should be made between atomicsolids(e.g. diamond) and molecular solids(e.g.polymers) where the covalently bonded molecules are held by Vander Waal forces which controls the mechanical props. 2)Some substances can be bonded in more than one method according to its state, e.g. HCl when it is in the gaseous state will be written as H:Cl: and when in aqueous solution will be written as H+:Cl-: .

37. Inter-atomic Distance (I.A.D.) Spacing of solid atoms is influenced by two factors : *Forces of attraction resulting from different types of bonding. *Forces of repulsionoccur between the electrons or positively charged nuclei of neighboring atoms. These forces increase rapidly with decreasing inter-atomic distance. * *When two atoms are spaced such that these forces (repulsive & attraction) are equal and opposite, the atoms are in their equilibrium position. * * The sum of these attraction and repulsion forces provides the basis for bonding energy and equilibrium interatomic distance.

38. Factors Affecting Inter-atomic Distance 1)Number of adjacent atoms:more adjacent atoms, there is less attraction to any specific neighboring atom, and the inter-atomic distance is increased. 2)Effect of temperature:increase temperature will increase the amplitude of atomic or molecular vibration, causing increase in the inter-atomic spacing. 3)Type of the bond: The more the number of electrons shared, the stronger is the bond and the less the inter-atomic distance; e.g. covalent bonding. 4)External force application: Application of an external force to a solid can displace atoms from equilibrium position and change the I.A.D., e.g. tensile stress will increase it.