Structure of Matter Lecture - 8/10/2023 - King Salman International University

Dentistry Program Lecture : structure of matter Dr : Reem Ashraf Date : 8/10/2023 STRUCTURE OF MATTER  In this chapter you will learn 1. State of matter 2. Structure of Atoms 3. Interatomic Attraction 4. Arrangement of Atoms 5. Correlat...

Dentistry Program Lecture : structure of matter Dr : Reem Ashraf Date : 8/10/2023 STRUCTURE OF MATTER  In this chapter you will learn 1. State of matter 2. Structure of Atoms 3. Interatomic Attraction 4. Arrangement of Atoms 5. Correlation between atomic and material properties Matter: any substance that has mass and occupies space Atom: smallest building unit……unbreakable. Molecule: electrically neutral atoms held together by bond, lacking electrical charge Ion: charged atom…total no. of electrons is not equal to protons  1. State of matter  2. Structure of Atoms  3. Interatomic Attraction 1- Primary ionic bond  It is the attraction between positive & negative ions. Where atom transfer valence electrons from metal to nonmetals e.g: NaCl, gypsum structures and phosphate-based cements  Characteristics of ionic bond: 1. Strong, nondirectional and chemical 2. Heat resistant. 3. Easily dissolve in ionized solvents e.g. water, acids and alkalis 4. Dissociate into their constituents in a solution and conduct electric current 5. Brittle 2- Primary covalent bond It is sharing of electrons between two atoms where atoms share valence electrons e.g: O2 ,CH4,H2O  Characteristics of covalent bond : 1. Strong, directional and chemical 2. Water insoluble. 3. Withstand high temperature. 4. Do not conduct electricity. 3- Metallic bond It is the electrostatic attraction between +ve cores and electron cloud. It occurs between metals as Na and Ag  Characteristics of metallic bonds: 1- High thermal and electrical conductivity. 2- Opacity (due to absorption of light by free electrons). 3- Metallic luster (free electrons re-emit light). 4- Relatively ductile and nondirectional  Combination of Ionic & Metallic Bonds:  Example: Amalgam Hard & Brittle ↑ Conductive Prop. Ionic Metallic Primary Bonds Are Strong In Contrast With The Secondary Bonds That Depends On Charge Variations That Induce Polar Forces that Attracts Molecules Secondary bonds Natural oscillation of atoms leads to momentary break down of charge symmetry can generates dipoles  Fluctuating Induced Dipole Bonds Temporary dipole : It is the attraction between positive pole of one atom & the negative poles of another atom. Polarization of molecules  Polar Molecule–Induced Dipole Bonds 2- Permanent dipole (Hydrogen bond): One side of water molecules is negatively charged because the oxygen atom keeps the shared electrons longer than the hydrogen atoms. As a result, the oxygen side is negatively O charged, and the hydrogen side of water is positively charged. E.G : ICE Characteristics of secondary bonds : 1- Weak & physical. 2- Low melting temperature. 3- High thermal expansion. e.g. wax  4. Arrangement of Atoms  What is the difference between Molecular Structures Atomic structures (mesomorphous) Strong structures with no ⬗ Weak structures with ⬗ Secondary forces Secondary forces hold Example: Metals-Diamond ⬗ molecules Example: Polymers Types of solids: 1- Amorphous solids(formed by super cooling). 2- Crystalline solids.  Crystalline solids Is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure , forming a crystal lattice that extends in all directions.  Solid dental materials are termed crystalline when their atoms are regularly arranged in a space lattice. The simplest repeating unit in the crystal lattice is called the unit cell. Crystal lattices are classified according to: 1-The relative length of their axes (a, b, c). 2-Interfacial angles between the crystal planes (α , ,γ).  Types of space lattices: 1.There are about 14 different types of space lattice but only few are of dental interest. 1- Cubic system: a) Simple cubic system: b) Body centered cubic system: e.g Iron c) Face centered cubic system: e.g gold, silver, copper, palladium, platinum c) Face centered cubic system:  2- Hexagonal System a. Simple hexagonal system (S.H.) 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. Closed packed hexagonal system contains: e.g: Mg , Zn 12 X 1/6 (at corners) + 2 X 1/2 (at top and bottom surfaces) + 3 at the center = 6 atoms.  AMORPHOUS SOLIDS Some solids like glass, waxes and some polymers are amorphous because of the random arrangement of their atoms, yet they may form short range arrangement surrounded by disordered units in between. Since such an arrangement may be considered typical of the liquid structure, these solids are sometimes called "Super cooled liquids ‘’ Amorphous solids have no definite melting temperature, but they gradually soften on heating and gradually harden on cooling. -The temperature at which they gradually soften is known as the glass transition temperature (GTT). -They have higher internal energy than crystalline solids  Imperfections in Crystalline Solids: 1) Vacancy: missing atom within the crystal imperfect packing 1- Point Defect: during crystallization thermal vibrations→ individual atoms may jump. 2) Impurities: an extra atom lodged within the crystal a) Interstitial b) Substitutional  plastic deformation in metals due to movement of dislocations 2- Line Defect:  3- Plane Defect: e.g. Grain Boundaries  Atomic Packing Factor: IT IS THE % OF VOLUME IN A UNIT CELL THAT IS OCCUPIED BY CONSTITUENT PARTICLES 57  Atomic Packing Factor (APF): Volume of atoms inside the unit cell APF = Volume of the unit cell ↑ APF → ↑ strength ↑ melting temp. ↑ Density.  Crystalline solids Amorphous solids 1. Have regular unit cell 1. No regular unit cell but with repetition. may have a short range of regularity but no repetition. 2. Have definite melting 2. No definite melting temperature. temperature (gradually soften on heating and gradually harden on cooling).  Compare between crystalline and amorphous solids  Isomerism and Isomorphism ISOMERISM: Is the existence of same elements or compounds in more than one crystalline form. ISOMORPHISM: Same crystal, different element.  Isomerism: a-Allotropy: It is the existence of an element in more than two different crystalline forms. An example of an allotropic element is iron. Iron exists in two stable forms: FCC > 910 OC BCC < 910 OC b-Polymorphism: -It is the existence of a chemical compound in more than two different crystalline forms. -Compounds are more likely to have structural reorientations than elements. -An example is the Different polymorphic forms of silica.  * Polymorphism: e.g. Silica “ It is the existence of the material in nature in different physical forms, while having same chemical structure.” For Inorganic materials → Allotropy. For Organic materials → Isomerism.  Displacive transformation Reconstructive transformation ▪ Transformation of α form to β ▪ Transformation of 1 physical form or Vice versa. form to another. ▪ Atomic displacement without ▪ Breakage of interatomic bonds breakage of interatomic bonds. and orientation of atoms with formation of new crystalline forms. ▪ Occurs at specific temp. ▪ Occurs at high temp. ▪ Accompanied by expansion. ▪ Reversible ▪ No expansion, ▪ Irreversible.  Can you correlate between what you learned about atomic structure and materials properties? ▹ ↑ Atomic weight, ↑ Atomic radius & ↑ Atomic Packing Factor → ↑ Density. ▹ Melting point → Type & strength of Bond. ▹ Stronger interatomic forces → stronger bond ↑→Melting point → ↑Hardness. ▹ ↓ Coefficient of Thermal exp. & contr.→ ↑ The melting temp. ▹ Electrical & Thermal conductivity → Nature of bond ( Metallic. Ionic in solution). ▹ Strength → type of bond & arrangement of atoms. ▹ Internal energy / stability → arrangement of atoms: Crystalline or Amorphous Atomic Solids stronger than Molecular Solids.