Interatomic Bonding Lecture 5 PDF

Interatomic Bonding Cont’d Lecture 5 1  Recap – Atomic Bonding in Solids Interatomic forces (between the atoms) bind the atoms together If the distance between the atoms is large, the interaction is negligible As the atoms come closer, e...

Interatomic Bonding Cont’d Lecture 5 1  Recap – Atomic Bonding in Solids Interatomic forces (between the atoms) bind the atoms together If the distance between the atoms is large, the interaction is negligible As the atoms come closer, each atom exerts forces on the other atoms – These are attractive and repulsive forces that are developed between them – The magnitude of each force is a function of interatomic distance 2  Recap – Atomic Bonding in Solids 𝐹𝐴 depends on the bonding between the two atoms. As r becomes smaller, 𝐹𝑅 comes into play as the outer shells overlap. At 𝑟 = 𝑟0 and 𝐹𝑁 = 0, a state of equilibrium is achieved. 3  Covalent Bonds Unlike ionic bonding This type of bonding is directionality sensitive Atoms must come at a specific angle! WHY? 4  Bond Hybridization Carbon can form sp3 hybrid orbitals from overlapping s and p orbitals Fig. 2.14, Callister & Rethwisch 9e. (Adapted from J.E. Brady and F. Senese, Chemistry: Matter and Its Changes, 4th edition. Reprinted with permission of John Wiley and Sons, Inc.) Fig. 2.13, Callister & Rethwisch 9e. 5  Covalent Bonding: Carbon sp3 Example: CH4 C: has 4 valence e-, needs 4 more H: has 1 valence e-, needs 1 more Electronegativities of C and H are comparable so electrons Fig. 2.15, Callister & Rethwisch 9e. are shared in covalent bonds. (Adapted from J.E. Brady and F. Senese, Chemistry: Matter and Its Changes, 4th edition. Reprinted with permission of John Wiley and Sons, Inc.) 6  Examples of Covalently Bonded Molecules Molecules containing dissimilar atoms, such as CH4, H2O, HNO3 and HF, are covalently bonded. This type of bonding is found in elemental solids such as: – Diamond(carbon), silicon, and germanium and other solid compounds composed of elements that are located on the right-hand side of the periodic table, such as gallium arsenide (GaAs), indium antimonide (InSb), and silicon carbide (SiC). 7  Covalent Bonding The number of covalent bonds that is possible for a particular atom is determined by the number of valence electrons For N’ valence electrons, an atom can covalently bond with at most 8 - N’ other atoms. – For example, N’ = 7 for chlorine, thus 8 - N’ = 1 – one Cl atom can bond to only one other atom, as in Cl2. – Similarly, for carbon N’ = 4, and each carbon atom has 8 - 4 = 4 electrons to share. Diamond is simply the three-dimensional interconnecting structure wherein each carbon atom covalently bonds with four other carbon atoms 8 9  Covalent Bonding Covalent bonds can be very strong (ex. diamond)… very hard and melting temps > 3500 degrees Celsius. Or weak, ex. bismuth melts ~270 degrees Celsius. Polymeric materials typify this bond, the basic molecular structure being a long chain of carbon atoms that are covalently bonded together with two of their available 10  Ionic-Covalent Mixed Bonding It is possible to have interatomic bonds that are partially ionic and partially covalent – very few compounds exhibit pure ionic or covalent bonding The degree of either bond type depends on the relative positions in the periodic table (difference in their electro- negativities) – The wider the separation from the lower left to the upper-right-hand corner the more ionic the bond – Conversely, the closer the atoms are together the greater the degree of covalency 11  Ionic-Covalent Mixed Bonding Ionic-Covalent Mixed Bonding % ionic character = x (100%) where XA & XB are Pauling electronegativities Ex: MgO XMg = 1.3 XO = 3.5 12  Metallic Bonding Found in metals and their alloys These valence electrons are not bound to any 1 atom in the solid and are free to drift throughout the entire metal. They may be thought of as belonging to the whole structure or forming a “sea of electrons” or an “electron cloud.” 13  Metallic Bonding Metallic bond – electrostatic attraction between +ve ion cores and delocalized electrons. The remaining non-valence electrons and atomic nuclei form what are called ion cores. which possess a net positive charge equal in magnitude to the total valence electron charge per atom. these free electrons act as a “glue” to hold the ion cores together. 14  Metallic Bonding Bonding may be weak or strong; energies range from 68 kJ/mol (0.7 eV/atom) for mercury to 850 kJ/mol (8.8 eV/atom) for tungsten. Their respective melting temperatures are: Some general behaviors of the various material types (i.e., metals, ceramics, polymers) may be explained by bonding type 15  Secondary Bonding (weak bonds) Due interaction between dipoles (due to asymmetry in electron cloud distribution) Fluctuating dipoles asymmetric electron ex: liquid H 2 clouds H2 H2 + - + - H H H H secondary secondary bonding Adapted from Fig. 2.20, Callister & Rethwisch 9e. bonding (Electrostatic attraction) Van der waals Permanent dipoles-molecule induced Hydrogen bonds secondary -general case: + - bonding + - Adapted from Fig. 2.22, Callister & Rethwisch 9e. secondary -ex: liquid HCl H Cl bonding H Cl -ex: polymer secondary bonding Although C-C bond is strong, melting temps of polymers tend to be low, WHY? 16  Secondary Bonding: Van der Waals Van der Waals Bonding – physical bonds are weak in comparison to the primary or chemical ones – bonding energies are typically on the order of only 10 kJ/mol (0.1 eV/atom) – bonding exists between virtually all atoms or molecules but its presence may be obscured if any of the three primary bonding types is present – forces arise from atomic or molecular dipoles – bonding results from the coulombic attraction between the positive end of one dipole and the negative region of an adjacent one 17  Secondary Bonding: Hydrogen Bonds Dipole interactions occur between induced dipoles, between induced dipoles and polar molecules and between polar molecules Hydrogen bonding, a special type of secondary bonding, is found to exist between some molecules that have hydrogen as one of the constituents. 18  Running Back: Covalent Bonding Covalent bonds can be very strong (ex. diamond)… very hard and melting temps > 3500 degrees Celsius. Or weak, ex. bismuth melts ~270 degrees Celsius. Polymeric materials typify this bond, the basic molecular structure being a long chain of carbon atoms that are covalently bonded together with two of their available 19  Properties From Bonding: Tm Bond length, r Melting Temperature, Tm Energy r Bond energy, Eo ro r Energy smaller Tm unstretched length ro larger Tm r Eo = Tm is larger if Eo is larger. “bond energy” 20  Properties From Bonding: α Coefficient of thermal expansion, α length, L o coeff. thermal expansion unheated, T1 ΔL ΔL = α (T2 -T1) heated, T 2 Lo α ~ symmetric at ro Energy α is larger if Eo is smaller. unstretched length ro r E larger α o E smaller α o 21  Summary: Bonding Type Bond Energy Comments Ionic Large! Nondirectional (ceramics) Covalent Variable Directional large-Diamond (semiconductors, ceramics small-Bismuth polymer chains) Metallic Variable large-Tungsten Nondirectional (metals) small-Mercury Secondary smallest Directional inter-chain (polymer) inter-molecular 22  Summary: Primary Bonds Ceramics Large bond energy (Ionic & covalent bonding): large Tm large E small α Metals Variable bond energy (Metallic bonding): moderate Tm moderate E moderate α Polymers Directional Properties (Covalent & Secondary): Secondary bonding dominates small Tm small E large α 23