Structure of Matter PDF

Structure of Matter Edited by Dr. Hadiah ElBakry Structure of Matter Generally, the physical, mechanical and chemical properties of any material are dependent mainly on:  Types of bonds between the atoms and molecules. ...

Structure of Matter Edited by Dr. Hadiah ElBakry Structure of Matter Generally, the physical, mechanical and chemical properties of any material are dependent mainly on:  Types of bonds between the atoms and molecules.  Inter-atomic distance.  Manner of arrangement of atoms.  Atomic packing. 1 The structure of atoms and atomic bonding 1.1 Structure of atoms The atom is the basic unit of the structure of any material. It consists of: o Central positive nucleus which is composed of positively charged protons and uncharged neutrons. o Negatively charged particles (electrons) revolving around the nucleus in definite orbits (state of energy levels or shells). Valence electrons: Electrons in the outermost shell, which determines most of the physical and chemical reactivity of the element. The number of protons in any atom should equal that of the electrons. This number represents the atomic number of the element. The atomic weight of an atom is nearly proportional to the weight of protons and neutrons in its nucleus. It affects the density and specific heat, while it has very little influence on the mechanical properties. 1.2 Types of bonds Inter-atomic bonds (Primary bonds). Inter-molecular bonds (Secondary bonds). 1.2.1 Inter-atomic bonds (Primary bonds): In any element, except the inert gases, the atoms try to achieve the most stable configuration of having eight electrons in the outer shell. This could be obtained through one of the following: 1|Page Structure of Matter Edited by Dr. Hadiah ElBakry 1. Receiving extra electrons to complete the outer shell and becoming a negatively charged ion. 2. Releasing electrons so that the outer shell has eight electrons and becoming a positively charged ion. 3. Sharing electrons so that the outer shell of two or more atoms is completed. The electronic configuration which result from the previous three methods will give rise to strong atomic attraction or bonding called primary bonds. Within the molecule, the atoms are held together by strong intra-molecular bonds “inter-atomic bonds”. Primary bonds strong chemical bonds due to the involvement of the valence electrons. Types of Primary bonds: 1.2.1.1 Covalent bond: It arises when two atoms share their electrons so that each outermost shell achieves a highly stable configuration. The hydrogen molecule is an example of covalent bonding, Figure (1). The two atoms approach one another and the orbitals of the electrons begin to overlap, a molecular orbital is formed where 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. Examples of covalent bonds are fluorine, hydrogen, and ethylene. Figure 1. Two hydrogen atoms combine through covalent bonding to form Hydrogen gas  Characteristic of covalent bonds are: Very strong. Insulators. Basic bonds for polymers. Resist inorganic solvents. 2|Page Structure of Matter Edited by Dr. Hadiah ElBakry 1.2.1.2 Ionic bonds: An ionic bond results from the electrostatic attraction between ions of unlike charges. The classic example is sodium chloride NaCl. Figure (2). Na (2,8,1) - e- Na+(2,8) atomic no. 11 ----------- cation. Cl (2,8,7) + e- Cl- (2,8,8) atomic no. 17 ----------- anion. Figure 2. Formation of an ionic bond between sodium and chlorine  Characteristic of ionic bonds are: Heat resistant and insulators as solids. In ionic solution, they dissociate into their constituent ions and conduct electricity. Insoluble in organic solvents. Basic bond for glasses and ceramics. 1.2.1.3 Metallic bond: Metal atoms have valance electrons that are rather loosely held and these electrons are free to move around all the atoms. The metal consists of positive ion cores held together by a cloud of free electrons. Therefore, the metallic bond can be considered an attraction between the positive ion cores and the negative unattached electrons. In this situation, the electrons diffuse around the +ve cores with a great degree of freedom. This diffuse nature is responsible for the characteristics of metals, Figure (3) Figure 3. Formation of a metallic bond, showing a cloud of electrons surrounding the nuclei 3|Page Structure of Matter Edited by Dr. Hadiah ElBakry  Characteristics of metallic bonds: High electrical conductivity. High thermal conductivity. Opaque because the free electrons may absorb light. Reflective or lustrous (surface luster) because the electrons reemit light. It leads to crystalline arrangement in metals. 1.2.2 Secondary bonds /Van der Waal forces (Dipole bonds): In many liquids or solids, it may find that the primary strong bonds are not controlling totally the properties and that the molecules are attracted by other type of physical or weaker bonds called dipole bonds. Electric dipoles are electric imbalance in the molecule resulting from sharing electrons (covalent bonding). The molecule may acquire a slight negative charge during rotation of the electrons around the nucleus and the other end also acquired a slight positive charge leading to the formation of dipoles. These dipoles allow molecules to interact with one another and to form weak bonds. Hydrogen Bridge in water molecules, Figure (4): Figure 4. Hydrogen Bridge in water molecules i. The oxygen atom and two hydrogen atoms have shared electrons. ii. Van der Waals forces result since the “exposed” hydrogen nucleus is attracted to the negative oxygen atom whose nucleus is “protected” by the unshared electrons. 1.3 Inter-atomic distance (I.A.D.): The space between atoms is caused by inter-atomic repulsive force which results from the electrostatic fields of each atom, in addition to the 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. i.e. The sum of the two forces provides the basis for bonding energy and equilibrium inter-atomic distance. 4|Page Structure of Matter Edited by Dr. Hadiah ElBakry 2 State of Matter: Matter can be classified into gas, liquid, and solid. The molecules in gases can move freely and their energy is higher with the largest interatomic distance in between. In liquids, there is less inter-atomic distance and less energy as the atoms or molecules are short ordered arranged. In solids, the atoms have the least I.A.D. with the least energy. 3 The structure of solids: The properties of materials depend on the arrangement of their atoms. Such arrangements may be classified as: Crystalline solids Amorphous solids Also, solids can be classified according to type of bond between atoms as: Atomic solid Molecular soids 3.1 Crystalline solids In many solids, the constituent atoms are regularly arranged with repetition in three dimensions in what is called a crystal lattice or space lattice. A space lattice can be defined as 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. 3.1.1 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. A unit cell is the smallest repeated unit in a crystal lattice. Atomic arrangement may take one of seven main crystal patterns which may have other seven subdivisions to make fourteen (14) possible arrangements, Figure (5) and Table (1). Only few are of dental interest. The type of space lattice is defined by the length of the three axes of the unit cell and the angles between them. 5|Page Structure of Matter Edited by Dr. Hadiah ElBakry Figure 5. Types of crystal lattice systems Table 1. Types of Crystal lattice systems 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° 3.1.1.1 Cubic system: The cubic space lattice is characterized by having axes that have equal lengths and they meet at right angle, Figure (6). There are three types of the cubic system: 6|Page Structure of Matter Edited by Dr. Hadiah ElBakry a. Simple cubic system (sc): This structure is theoretical for pure metals but provides us with good starting point for understanding. In the simple cubic unit cell, each atom at each of the eight corners of the cube is associated with eight surrounding unit cells. Therefore, each atom is participating in eight unit cells. In such a way, each atom has 1/8 of its volume in each of these eight cells. The simple cubic structure in this way contains one metal atom per unit cell 8 atoms x 1/8 at each corner = one atom. Figure 6. Simple cubic system. b. Body centered cubic (bcc): In the bcc, the unit cell has an atom at each corner of the cube and another atom at the center of the unit cell, Figure (7). There are two atoms per unit cell in bcc structure where: 8 atoms at each corner x 1/8 + one atom in the center = 2 atoms. Figure 7. Body centered cubic. 7|Page Structure of Matter Edited by Dr. Hadiah ElBakry c. Face centered cubic (fcc): In such arrangement, there is an atom at each corner of the unit cell, and one at the center of each face of the cube. At the center of each face, the atom is shared between two unit cells and thus its value will be half atom, Figure )8). Therefore, the number of atoms in fcc system unit cell are four: (8 x 1/8) + (6 x 1/2) = 4 atoms The face centered cubic system is more common among metals, e.g. Gold (Au), Silver (Ag), Copper (Cu), Platinum (Pt). Figure 8. Face centered cubic system 3.1.1.2 Hexagonal crystals: a. Simple hexagonal system: In which a = b ≠ C with γ = 120° and α = β = 90°. The atoms can be imagined at the corners with one atom at the upper face and another at the lower face, Figure (9). At each corner of the hexagonal system the atom value can be considered 1/6 and at the face as 1/2 atom so the value of simple hexagonal unit cell is three atoms. Yet, metals do not crystallize in the simple hexagonal pattern because the packing factor is too low which represents less stable condition. Instead a more closed packed structure is formed. Figure 9. Simple Hexagonal Systems 8|Page Structure of Matter Edited by Dr. Hadiah ElBakry b. Closed packed hexagonal (hcp): It can be considered as simple hexagonal system with three unshared atoms at the same plane in the center of the hexagonal system. Thus, the atoms are more packed, Figure (10). E.g. Zinc (Zn). 2 (1/6 x 6) + (1/2 x 2) + 3 atoms = 6 atoms. Figure 10. Closed packed hexagonal system 3.1.2 Atomic packing factor: Some of the space of the structural unit is not occupied by the atoms. The fraction of space occupied by the atoms is called the atomic packing factor and is calculated by: Atomic Packing factor = Volume of atoms inside the unit cell Volume of unit cell For the simple cubic system sc, it equals 0.54 which indicates that nearly 50% of the space is free. Other arrangements (commonly occur in metals) such as body centered cubic bcc, Face-centered cubic fcc have 0.68 and 0.74 atomic packing factor, respectively. Materials having higher atomic packing factor usually have higher densities, stability and strength properties. Most of the dental alloys crystallization into fcc and hcp space lattice. 3.1.3 Imperfections in crystalline solids: The previous information about the crystalline structures allows computing their theoretical strength. However, this theoretical strength is always much higher than the actual strength found from experimental work. This is because nature is not perfect, materials are bound to contain some defects or imperfections that decrease the strength. 9|Page Structure of Matter Edited by Dr. Hadiah ElBakry Types of crystalline imperfections: 3.1.3.1 Point defects: The simplest point defects are: (i) Vacancy, Figure (11 a) in which an atom is missing within a crystal. Such defects can be a result of imperfect packing during the original crystallization or they may arise from thermal vibrations of the atoms at elevated temperatures. Vacancies may be single, two or more. (ii) Interstitial impurities, Figure (11 b) in which an extra atom may be lodged within a crystal structure. Such imperfections produce atomic distortion within the crystal lattice. a b Figure 11. Point defects (a) Vacancy and (b) Interstitial impurities 3.1.3.2 Line defects: The most common type of line defect within a crystal is a dislocation. Dislocation is defined as the displacement of a raw of atoms from their normal positions in the lattice, Figure (12). Figure: 12 Line defects 10 | P a g e Structure of Matter Edited by Dr. Hadiah ElBakry 3.1.3.3 Plane defects: Such as grain boundaries in metals, Figure (13). Figure 13. Plane defects 3.1.4 Polymorphism: It is the existence of the material in different physical forms while having the same chemical structure. These different polymorphic forms may be called allotropic forms. Most commonly the term allotropy is used to refer to the phenomena of polymorphism in inorganic material, while isomerism to the phenomena in organic materials. Silica (SiO2) is an important example for allotropy in dentistry. It exists in nature in four different allotropic forms, which are: a- Quartz (Hexagonal), b- Tridymite (Rhombohedral), c- Crystalobailite (Cubic), d- Fused quarts (Amorphous). Each form has different physical properties as density, but all are chemically SiO2. On heating the four forms, 2 types of transformation take place: 3.1.4.1. Reconstructive transformation: 870˚C 1470˚C 1713˚C Quartz Tridymite Crystobalite Fused quartz (Rhombohedral) (Hexagonal) (Cubic) (Amorphous)  Reconstructive transformation is characterized by: a) It involves bond breakage. b) It needs more time. c) It occurs more slowly. 11 | P a g e Structure of Matter Edited by Dr. Hadiah ElBakry 3.1.4.2. Displacive transformation: In which the atoms become displaced with change in temperature and no breakage of bonds takes place. 573˚C a) Low quartz High quartz (α - quartz) (β - quartz) 160˚C b) Low tridymite High tridymite (α - tridymite) (β - tridymite) 220˚C c) Low cristobalite High cristobalite (α -cristobalite) (β -cristobalite)  Displacive transformation is characterized by: a) No breakage of bonds. b) It occurs at specific temperature. c) The transformation is rapid. d) Accompanied by expansion. 3.2 Amorphous solids There are many solids in which the molecules are distributed at random without regularity or repetition in their arrangement so there is no specific form or shape in their structure. These solids are termed Amorphous, which means without shape, Figure (14) and Table (2). Even in this case there is tendency for short-range order arrangement, e.g. Glasses. The structural arrangements of the non-crystalline solids do not represent as low internal energies as do crystalline arrangements. They do not have a definite melting temperature, but rather they gradually soften as the temperature is raised and gradually harden as they cool. The temperature at which they first form a rigid mass upon cooling or soften upon heating is called glass transition temperature Tg. Figure 14. Crystalline vs. Amorphous solids 12 | P a g e Structure of Matter Edited by Dr. Hadiah ElBakry Table 2. Comparison between crystalline and amorphous structure Crystalline solid Amorphous solid Arrangement of -Atoms are regularly - There is no long range repeating atoms arranged with repetition in 3 structure but a random arrangement dimension in space i.e. of atoms. There is tendency for repetition + regularity short order arrangement [super cooled liquid]. i.e. no repetition no regularity Internal Energy - Minimum Internal energy - Higher internal energy - More stable structure - Less stable structure Melting -Definite melting temperature - No definite melting temperature Characteristics  Atomic solids and Molecular solids: 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, Table (3). Table 3. Comparison between atomic and molecular solids Atomic solid Molecular solid Types of bonds -Primary bond exists between the -Primary bond exists atoms as well as between the between the atoms, while molecules of the solids secondary bond exists between the molecules of the solids Mechanical -High strength -Low strength Properties -High hardness -Low hardness Thermal -High melting temperature -Low melting temperature Properties -Low coefficient of thermal -High coefficient of thermal expansion and contraction expansion and contraction Examples -Metals and ceramics -Polymers 13 | P a g e Structure of Matter Edited by Dr. Hadiah ElBakry 4 Correlation between atomic structure and properties: The properties of materials depend basically on the type of bonds which dominate the structure, and in turn the interatomic distances, 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. 3. Thermal expansion of materials with comparable atomic packing factors vary inversely with their melting temperature. The higher the melting temperature, the less the coefficient of thermal expansion. 4. Electrical and thermal conductivity are very dependent on the nature of the atomic bonds. Thermal conductivity is higher in materials with metallic bonds. 5. Strength can be primarily governed by the type of bond, although the arrangement of atoms controls the deformation and resistance to stresses. 6. Crystalline structures have lower energy level while amorphous structures have higher energy due to irregular arrangement. Therefore, the amorphous structures do not have definite melting temperature but rather glass transition temperature (softening temperature), i.e. they soften before melting. 7. Atomic solids are generally stronger than molecular solids because primary bonds control the properties. 14 | P a g e