structure of matter mahi .pptx

Structure of matter By: Dr Mahi Mansour Science of dental materials The science of dental materials is the study of the properties of the materials used for the fabrication of dental appliances and tooth restorations.  Structure of matter It is necessary to under...

Structure of matter By: Dr Mahi Mansour Science of dental materials The science of dental materials is the study of the properties of the materials used for the fabrication of dental appliances and tooth restorations.  Structure of matter It is necessary to understand that any material begins with an “ Atom” An atom is a particle of matter that uniquely defines a chemical element. An atom consists of a central nucleus that is surrounded by one or more negatively charged electrons. The nucleus is positively charged and contains one or more relatively heavy particles known as protons and neutrons.  Atomic structure 1.Electrical state of atom is Neutral as the positive charges = the negative charges. 2. Atomic number = Number of protons. 3. Valence electrons: Electrons in the outermost shell, which determines the chemical reactivity of the element. 4. The Atomic weight = Protons + Neutrons inside the nucleus.  Types of bonds Secondary Bonds (Van der Primary bonds Waals forces) ‘interatomic bonds’ "intermolecular bonds" "Dipole bonds": Ionic bond Covalent bond Metallic bond The atoms try to achieve the highly stable structure of having 8 electrons in outer most shell. Atoms may either receive or release electrons or share electrons by making a primary bond.  Ionic bond Ionic bonds are formed as a result of electrostatic attraction between positive and negative ions.  Characteristics of Ionic bonds Heat resistant bond (a high amount of energy is needed to break the bond between the atoms) Strong Insoluble in organic solvents Soluble in ionizing solvents such as water, acids and alkalies Insulator as solid structure, but in solution and in molten condition, they can conduct an electric current easily Basic bond for glasses and ceramics  Covalent bond These bonds are formed by the sharing of valence electrons between the two bonded atoms Characteristics of covalent bonds Heat resistant bond Strong Directional: Since the electrons will spend most of their time in the region, where the orbits overlap. Insoluble in water, but soluble in organic solvents Insulators Basic bond for diamonds  Metallic bond A solid metal bond consists of positive ions, held together by a "cloud of free electrons". Attraction between positive cores and negative free electrons Characteristics of metallic bonds Heat resistant bond. Strong Due to the presence of free electrons 1.High electrical and thermal conductivity. 2.Opacity: absorption to all incidence light. 3.Lustrous: reemission of the absorbed light  Secondary Bonds (Van der Waals forces) “Intermolecular bonds" "Dipole bonds": These physical forces are much weaker and cannot support high temperature. Van der Waals forces result from polarization of molecules to form dipole (+ve pole and –ve pole). Physical attraction between the opposite poles will occur. States of matter Due to the previous discussed types of bond, all matters exist in one of three forms: Solids , liquids and gases They differ from each other in: 1.Interatomic distance. 2.The kinetic energy of the molecule.  Interatomic distance Two factors influence the distance between atoms in a solid: a. Forces of attraction due to inter- atomic bonding. b. Force of repulsion occurs between the electrons of neighboring atoms. 1. These forces are equal and opposite in direction at the equilibrium position. 2. The potential energy of the system is a minimum. N.B: Change from gas to liquid to solid, is accompanied by contraction and loss of energy. i.e. gases have the greatest interatomic distance and the highest energy, whereas the solids have the minimum interatomic distance and the least energy. Classification of solids  Classification of solids Types of bond Arrangemen between the t of atoms atoms 1. Atomic solid 1. Crystalline solid 2. Molecular 2. Amorphous solid solid Atomic and molecular solids  Atomic Solids: Primary bond exist between atoms and molecules. The intermolecular forces are covalent bonds as well. Characterized as being very hard with very high melting points and being poor conductors. Primary bonds control the properties Atomic solid Molecular Solids: Primary bond exists in the individual molecule between atoms [ inter- atomic], whereas secondary bond exists between the molecules [inter- Molecular] (example: Polymer). Secondary bonds dominate and control the properties in the molecular solids Molecular solids are weaker than the atomic solids  Comparison between atomic and molecular solids Properties Atomic solids Molecular solids Primary bond exists between Primary bond exists between - the atoms and the molecules of the atoms, while secondary the solid bond exists between the molecules of the solid Mechanical properties High strength and hardness Low strength and hardness Thermal properties High melting temperature Low melting temperature Low coefficient of thermal High coefficient of thermal expansion and contraction expansion and contraction ARRANGEMENT OF ATOMS  COMPARISON BETWEEN AMORPHOUS AND CRYSTALLINE SOLIDS Properties Crystalline solids Amorphous solids Arrangement of Atoms are regularly arranged There is no long-range repeating with repetition in 3 dimension structure but a random atoms in space [Space or Crystal arrangement of atoms. lattice] There is tendency for short order (i.e. repetition + regularity) arrangement. i.e. no repetition no regularity Minimum Internal energy Internal energy More stable structure Higher Internal energy Less stable structure Melting Definite melting temperature No definite melting temperature but gradual transition. characteristics The temperature above which the material is soft and below which the material is hard, is termed: the glass-transition temperature (Tg).  Regarding the crystalline solids Space lattice or crystal lattice is defined as: The three-dimensional network of imaginary lines connecting atoms is called the space lattice. A crystal is an arrangement in three dimensions of atoms or molecules in repetitive patterns. A unit cell is defined as the smallest repeating unit that is contained in a crystal. CUBIC SYSTEMS Simple cubic system The simple cubic (SC) unit cell can be imagined as a cube with an atom on each corner. Now we need to count how many atoms are in each unit cell. It may look like there are 8 atoms because there are 8 corners ?? Simple cubic system How many atoms are in each unit cell ?????????  But actually, the cell only intersects 1/8th of each atom. 8 x 1/8 = 1 So there is only one atom per unit cell. In fact, an equally valid way of drawing a simple cubic unit cell is like this: Body-centered cubic system Now we need to count how many atoms are in each unit cell Body-centered cubic system The Body-Centered Cubic (BCC) unit cell can be imagined as a cube with an atom on each corner, and an atom in the cube’s center. It is one of the most common structures for metals. BCC has 2 atoms per unit cell face-centered cubic system Now we need to count how many atoms are in each unit cell ???? face-centered cubic system The Face-Centered Cubic (FCC) unit cell can be imagined as a cube with an atom on each corner, and an atom on each face. It is one of the most common structures for metals. FCC has 4 atoms per unit cell. HEXAGONAL SYSTEMS  Atomic packing factor It tells how much space is It is the fraction of the space of the occupied by the atoms and how much space is filled structure unit occupied by the atoms with voids. Lower the voids, higher the packing fraction. and is calculated by: APF = Volume of atoms inside the unit cell Volume of unit cell  Crystal imperfections Real crystals are never perfect: there are always defects. Controlling the defects is one of the main goal of materials science and engineering. Imperfecti ons Planar Point defects defects Grain boundaries Vacancy atoms Impurities: Line Substitutional defects and Interstitial atoms Dislocatio ns  Point defects 1. Vacancies: vacant atomic sites in a structure. 2. Interstitial : They are “extra" atoms positioned between atomic sites A small void space that under ordinary circumstances is not occupied.  LINE DEFECTS They are linear defects around which the atoms of the crystal lattice are misaligned  PLANE DEFECTS A Grain Boundary is a general planar defect that separates regions of different crystalline orientation (i.e. grains) within a polycrystalline solid. These crystalline imperfections play an important role in the mechanical behavior of the crystalline material. Polymorphism Polymorphism Polymorphic materials have more than one crystal structure when changing the surrounding physical condition. The polymorphic forms have the same chemical composition but different physical properties. Silica It exists in nature in four different allotropic forms, which are; Quartz, Tridymite, Crystobalite and Fused quartz. Each form has different physical properties, but all are chemically SiO2.  On heating of the four forms two types of transformation take place: I. Reconstructive transformation: Quartz 870ºC Tridymite 1470ºC Cristobatite 1710ºC Fused silica (Hexagonal) (Rhombohedral) (Cubic) (Amorphous) ii. Displacive transformation: It is the transformation from α form of silica to  form; this is associated with an expansion. 575ºC a. Low quartz High quartz (α-quartz) (-quartz) 160ºC b. Low tridymite High tridymite (α-tridymite) (-tridymite) 220ºC c. Low cristobalite High cristobalite (α-cristobalite) (-cristobalite)  Reconstructive Displacive transformation transformation Break down of atomic bonding followed No break down of atomic bonding only by reconstruction of new space lattice. displacement of atoms giving the same space lattice but with larger volume. -Slow transformation. -Rapid transformation. -Needs high thermal energy. Needs less thermal energy. -The manufacturer selects one of the The selected type compensates for the types to be used in the dental investment solidification shrinkage of the metal during casting.