Structure of Matter Summer PDF

‫بسم اللة الرحمن الرحيم‬ Structure of Matter Presented by: Dr:Amr Sharawy Associate professor of dental materials  Introduction -The physical, mechanical and chemical properties of any material depend main...

‫بسم اللة الرحمن الرحيم‬ Structure of Matter Presented by: Dr:Amr Sharawy Associate professor of dental materials  Introduction -The physical, mechanical and chemical properties of any material depend mainly on: 1-Type of bond between atoms and molecules. 2-Inter-atomic distance. 3-Mannar of arrangement of atoms. 4-atomic packing.  Atomic Structure The atom is the basic unit of the internal structure of any material The atom consists of: 1- Central positive nucleus [positively charged protons and uncharged neutrons]. 2- Negatively charged particles [electrons] revolving around the nucleus in definite orbits (state of energy levels or shells). N.B; 1-Electrical state of atom: Neutral. 2-Atomic number: Number of electrons around the nucleus or number of protons inside the nucleus. 3-Valence electrons: Electrons in the outermost shell, which determines the chemical reactivity of the element. 4-The Atomic weight: Weight of Protons + Neutrons inside the nucleus  1-Inter-atomic Bonding (primary bonds): Atoms achieve a stable state by having eight electrons in their outer shell (as in inter 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. -In solids, atoms are held together by either primary or secondary bonds.  Types of bonds Primary Bonds Secondary Bonds (Strong, Chemical) (weak , physical) Ionic Bond Covalent Bond Metallic Bond A. Primary Bonds: Def.: A bond that is formed between atoms and involves exchange or sharing of electrons. -Primary bond are strong chemical bond due involvement of valence electrons. -There are three types of primary bonds: 1) Covalent 2) Ionic 3)Metallic 1) Covalent Bonds: Def: It is sharing of electrons. - The hydrogen molecule, H2, is an example of covalent bonding. - The two atoms approach one another and the orbitals of the electrons begin to overlap, - The two electrons are shared between the two nuclei. The shared electrons will spend most of their time in the region where the orbitals overlap. - Double and triple covalent bonds are possible, where two or three pairs of electrons are shared between atoms. - Examples of covalent bonds are fluorine, hydrogen, diamond ,polymers and ethylene. Characteristics of covalent bonds: Very strong. Directional Insulators. Resist inorganic solvents. 2) Ionic Bonds: result from the electrostatic attraction between ions of unlike charge. -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 then the attraction of positive and negative ions results in the stable compound Na+ Cl-. Characteristics of ionic solids: 1-Heat resistant and insulator as solid. 2-Insoluble in organic solvents. 3-Easily dissolved in ionic solutions such as water, acids and alkalis, and dissociate into their constituent ions in solutions which in their turn can conduct an electric current. 4-Non directional 5-Basic bond for glasses and ceramics 6-Low coefficient of thermal expansion e.g In dentistry, ionic bonding exists in some dental materials, such as gypsum and Zinc phosphate cements. 3)Metallic Bonds: Def. : It is the attraction between +ve cores and free electrons or electron cloud". -It occurs in metals. Characteristics of the metallic bonds: The free mobility of electrons contributes to the following properties of metals: 1-High thermal and electrical conductivity due to the presence of free electrons. 2-Opaque due to absorption of light by free electrons. 3-Lustrous due to reflection of light by free electrons 4-High strength and hardness. 5-It leads to crystalline arrangement in metals 6- Low coefficient of thermal expansion B. Secondary Bonds (Van der Waals forces) "intermolecular bonds" "Dipole bonds": Def.: A bond that involves attraction between molecules. -These forces are physical, weak and arise from the polarization of molecules i.e. formation of electrical dipoles followed by physical attraction between the opposite poles. -Hydrogen bridges between water molecules are the most important example. Characteristics of secondary bonds: A solid whose molecules are bonded together by Vander Wall forces has: 1. Low strength and hardness. 2. Low thermal resistance.  Distinction should be made between: Atomic solids, such as diamond, and molecular solids, such as polymers, where in molecular solids, the covalently bonded molecules are held together by Van der Waals forces which control the properties. Molecular Atomic solids solids Primary Bonds Secondary forces hold No Secondary forces molecules High strength and Low strength and hardness hardness High melting temperature Low melting temperature Low coefficient of High coefficient of thermal thermal expansion expansion Example: Metals, diamond Example: Polymers, waxes Ceramics  2-Inter-atomic distance (I.A.D.): The space between atoms is caused by: 1- Inter-atomic repulsive force which results from the electrostatic fields of each atom. 2- Inter-atomic attractive forces which results from different types of bonding. The equilibrium distance is that distance at which the repulsive and attractive forces are equal. N.B. Application of an external force to a solid can displace atoms from the equilibrium position, and change the I.A.D.  State of Matter Matter can be classified into gas, liquid, and solid. A. Gases B. Liquids C. Solids The molecules in gases can In liquids, In solids, the atoms move freely and their with the have 1-largest interatomic distance 1-there is less inter- 1-the least I.A.D. in between atomic distance 2-the energy is high. 2-Less energy 2-the least energy.  3-Mannar of arrangement of atoms. The structure of solids Properties of materials depend on the arrangement of their atoms. Types of Solids: a) Crystalline solids. b) Amorphous solids. a) Crystalline Solids: Def.: Solids in which their is a regularity and repetition in the arrangement of their atoms i.e (atoms are regularly arranged in a space lattice). A space lattice :the arrangement of atoms in three dimensions such that every atom has a position similar to every other atom. The atoms may be held together by ionic bonds as in sodium chloride, covalent bonds as in diamond, or metallic bonds as in metals.  Types of crystal lattice or crystal systems: -Atomic packing in a crystal may take many configurations; the simplest way to study it is to consider a unit cell. Unit cell :which is the smallest repeating unit in the space lattice. -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. -Unit cells are classified according to: a) The length of their axes (a ,b , c ). b) The interfacial angles( , ,  )  System Axes Axial angles Cubic a=b=c α=β=γ=90° Tetragonal a=b≠c α=β=γ=90° Orthorhombic a≠b≠c α=β=γ=90° Monoclinic a≠b≠c α=β= 90°≠γ Triclinic a≠b≠c α≠β≠γ≠90° Hexagonal a=b≠c α=β=90°;γ=120 Rhombohedral a=b=c α≠β≠γ≠90° Types of crystal lattice systems  1-Cubic system: The cubic space lattice is characterized by having axes that have equal lengths and they meet at right angle Cubic a=b=c α=β=γ=90° There are three types of the cubic system:  Hexagonal a=b≠c α=β=90°;γ=120 2-Hexagonal System: Simple hexagonal Close packed Hexagonal 19 b) Amorphous Solids: Amorphous means without specific form or shape. -Def.: Solids in which there is no regularity and no repetition in the arrangement of their atoms. -Gases and liquids are amorphous substances. -Some solids like glass and waxes are amorphous because of the random arrangement of their atoms, yet their atoms may form a short range order arrangement without repetition.  Comparison between crystalline and amorphous materials Crystalline solids Amorphous solids 1) Have definite melting temperature. 1) No definite melting temperature (gradually soften on heating and gradually harden on cooling).. The temperature at which they first form a rigid mass upon cooling or soften upon heating is called glass transition temperature Tg 2) No regular unit cell but may have a short range of regularity 2) Have regular unit cell with repetition. but no repetition. i.e. no repetition no regularity 3) Low internal energy. 3) Higher internal energy due to random arrangement of atoms.  4-Atomic Packing Factors: Definition: It is the fraction of the space of the structure unit occupied by the atoms and is calculated by: volume of atoms inside the unit cell Atomic packing factor= volume of unit cell A-For the simple cubic system = 0.54 which indicates that nearly 50% of the space is free. B-Body centered cubic (bcc) = 0.68 C-Face centered cubic (fcc) = 0.74 D-Closed packed hexagonal (cph) = 0.74 -Materials having higher atomic packing factor usually have higher density , stability and strength properties. -Most dental alloys crystallize in fcc or hcp space lattices. simple cubic Body centered cubic Face centered cubic Closed packed hexagonal  Imperfections in crystalline solids Theoretical strength Vs Actual strength Materials contain some defects or imperfections which decrease the actual strength  1- Point Defect: Vacancy: Interstitial Impurity: A missing atom due to imperfect Extra atom may be lodged packing during crystallization within the crystal Thermal vibrations when heated Strength Strength 2- Line Defect: e.g: Dislocation: displacement of a row of atoms from their normal positions in the lattice Strength 3- Plane Defect: e.g. Grain Boundaries in metals Strength  Polymorphism the existence of the material in different physical forms while having the same chemical structure For Organic materials → Isomerism e.g Gutta percha For Inorganic materials → Allotropy e.g Silica (SiO2) Polymorphism The existence of the same chemical compound in different crystalline (physical) forms. Polymorphism Allotropy Isomerism Occur in crystalline inorganic materials Occur in amorphous organic materials e.g. Silica in dental investment e.g. Natural rubber and gutta perca Silica (SiO2) -It is an important example for allotropy in dentistry. -It exists in nature in four different allotropic forms, which are; Quartz, Tridymite, Crystobalite and Fused quartz. -Each form has different physical properties due to their difference in crystalline structure but all are chemically SiO2. -On heating of the four forms ,two types of transformation take place:  * Polymorphism: e.g Silica 1-Reconstructive Transformation: 870ºC 1470ºC 1713ºC Fused Quartz Tridymite Crystobalite Quartz Hexagonal Rhombohedral Cubic Amorphous  2-Displacive transformation 870ºC 1470ºC 1713ºC β- β- Fused β- Quartz Quartz Tridymite Crystobalite Hexagonal Rhombohedral Cubic Amorphous 573ºC 160ºC 220ºC α-Quartz α-Tridymite α-crystobalite (SiO2) (SiO2) (SiO2)  Comparison between reconstructive and displacive transformation Reconstructive Transformation Displacive Transformation -Break down of atomic bonding -No break down of atomic bonding followed by reconstruction of only displacement of atoms giving new space lattice. the same space lattice but with larger volume. -Needs high thermal energy. -Needs less thermal energy. -Slow transformation. -Rapid transformation. --The manufacturer selects one -The selected type compensates of the types to be used in the for the solidification shrinkage of dental investment the metal during casting.  Correlation between atomic structure and materials properties: 1.Density is controlled by atomic weight, atomic radius, and the atomic packing factor. 2.Thermal expansions of materials with comparable atomic packing factors vary inversely with their melting temperature. 3.Electrical and thermal conductivity are very dependent on the nature of the atomic bonds [ionic/covalent/metallic]. 4.Melting and boiling temperatures can be correlated with the strength of the bond. Generally, materials with weaker bonds have low melting point 5.Strength can be correlated with the type of the bond. 6-The arrangement of atoms classify the materials into crystalline or amorphous structure. 7-Atomic structures are generally stronger than molecular structures because primary bonds control the properties.  Self assessment questions I. Choose the correct answer(s): 1-Fraction of space occupied by the atoms in a unit cell of a space lattice is: a. Interatomic distance. b. coordination number. c. Atomic packing factor. d. Atomic weight. 2-Amorphous solids are characterized by having: a. High internal energy. b. Random and short order arrangement c. Definite melting temperature. d. Specific shape and form in their structure. 3-Crystalline solids are characterized by having: a. Low internal energy. b. Random and short order arrangement c. Glass transition temperature. d. Specific shape and form in their structure 4-For secondary bond to occur, there must be: a. Electrons cloud. b. Electrons share. c. +ve core. d. Dipole. 5-Metals are electrical conductor due to presence of: a. Cloud of electrons. b. +ve & -ve ions. c. Shared electrons. d. All of the above. II. Give reasons for: (1) Theoretical strength of the materials is higher than the actual strength. (2) High electric conductivity of metals. (3) Molecular solids are weaker than atomic solids (4) Ceramics are heat resistant. (5) Ceramics have high melting point.