Unit IV Chemical Bonding: PDF

Unit IV: Chemical Bonding – The Formation of Materials AMOL M. KAPSE  Part A The Formation of Materials This section covers chemical bonding and its effect on the chemical properties of the elements. Ionic bonding & covalent bonding are compared in terms o...

Unit IV: Chemical Bonding – The Formation of Materials AMOL M. KAPSE  Part A The Formation of Materials This section covers chemical bonding and its effect on the chemical properties of the elements. Ionic bonding & covalent bonding are compared in terms of the octet rule and valence bond theory. Polar and non-polar covalent bonds. Molecular orbital theory is introduced to explain magnetism, bond order and hybridization helpful in Carbon chemistry. Intermolecular forces, including hydrogen bonding, are discussed with a special Case Study focusing on the special properties of water.  Part A The Formation of Materials ATOMS AND IONS: Cation : positively charged ion Anion : negatively charged ion Periodic property: Gain & loss of electron Atoms will gain or lose electrons to form ions that have electronic configurations which are more stable than the electronic configurations of the parent atoms. This is to achieve closed valence shell.  Elements with Multiple Ionic Forms Element Group Ionic Forms Titanium 4 4 +, 3 +, 2 + Vanadium 5 5 +, 4 +, 3 +, 2 + Niobium 5 5 +, 3 + Chromium 6 6,+ 3 +, 2 + Manganese 7 7 +, 4 +, 3 +, 2 + Technetium 7 7 +, 6 +, 4 + Rhenium 7 7 +, 6 +, 4 + Iron 8 3 +, 2 + Osmium 8 3 +, 4 + Cobalt 9 3 +, 2 + Iridium 9 4 +, 3 + Nickel 10 2 +, 3 + Palladium 10 2 +, 3 +  Why is this variability? Because the removal of all the valence electrons to get to the noble gas configuration would require a great deal of energy for transition metals with many valence electrons. The additional ionic forms are achieved by losing a smaller number of electrons, which is more energy- efficient. the resulting electronic configuration is still more stable than that of the parent atom  DETERMINING THE ELECTRONIC CONFIGURATION OF AN ION Q) What is the electronic configuration of O2—? A) [He]2s22p6 Also find the same for Ni(II) & Fe(II)  Naming Anions: suffix “ide” to the root of the name of the parent element. Cl— is the chloride ion, N3— is the nitride ion, Cations : with one ionic form -adding the word “ion” after the element name. Na+:sodium ion Zn2+ :zinc ion. Cations : with multiple ionic forms -name of the element followed immediately by a Roman numeral in parentheses, called the Stock number, to indicate the charge of the ion. For example, Fe2+ is iron(II) and Fe3+ is iron(III).  Ionic Radius Ionic radius is also a periodic property Atomic radius (blue) and ionic radius (green) in picometers as a function of atomic number for elements 1 through 95. Anions: larger ionic radius than the parent atom Cations: smaller ionic radius than the parent atom  What’s the Ionic radius trend in Transition elements ???? The ionic radius of the transition metals is more variable than the atomic radius of the neutral atoms. This is because ionic charge also affects the ionic radius. The higher the ionic charge, the stronger the nuclear attraction and the smaller the ionic radius.  Summary Cations are smaller than their parent atoms. Anions are larger than their parent atoms. Anions are generally larger than cations. Increasing charge leads to decreasing ionic radius. Ionic radius increases going down a group. Ionic radius generally decreases going across a period.  Exercise List the following ions in order of increasing size: Cs+, K+, F—, & Cl—. Determine the position of the ions in the periodic table. Cs+ and K+ are both in group 1; K+ is in period 4 and Cs+ is in period 6. F— and Cl— are both in group 17; F— is in period 1 and Cl— is in period 2. Determine how the position of the ions is related to their size. Since ionic radius increases going down a group: Cs+> K+ and Cl— > F—. Cs+ and K+ in periods 4 and 6 have more electronic shells than F— and Cl— in period 1 and 2. Determine how the ionic charge is related to their size. Anions are smaller than cations: F—< Cl— < K+< Cs+.  IONIC BONDING Completely transfer one or more electrons from one atom to another. creates two oppositely charged ions electrostatic attractions - create an ionic bond force (F) acting between two charged particles with charges q1 and q2 is described by Coulomb’s law as; “d” - distance between the two charged particles “k” - constant with the value of 9 × 109N m2 C—2. Modified version of this law to describe the potential energy (E) between two ions, with charges q1 and q2. Negative potential energy (E= -): ions will attract, Positive potential energy (E= +): ions will repel Potential energy (E) as a function of the distance (d) between two oppositely charged ions. The values of the ionic bond length and the ionic bond strength are shown by dotted lines.  Octet rule. The tendency for atoms in the s- and p-blocks to combine in such a way that each atom acquires eight electrons in its valence shell is known as the octet rule. Eg: NaCl  DETERMINING THE CHEMICAL FORMULA OF AN IONIC COMPOUND USING ELECTRON DOT REPRESENTATIONS What is the chemical formula of the ionic compound formed from the reaction between sodium and sulfur? 1. Determine the electron configurations of the neutral atoms. sodium¼[Ne]3 s1. sulfur¼[Ne]3s23p4. 2. Determine the electron dot representations for the neutral atoms. S and Na 3. Determine the combination of atoms that will result in a closed valence shell. Sulfur needs two electrons to complete the valence octet. This requires two sodium atoms. 4. Write the chemical formula. Na2S  COVALENT BONDING Covalent bonding: elements with similar electro- negativities Closed valence shell is achieved a by sharing of one or more electrons. Eg: H2, Cl2 etc. Covalent bond that results when two atoms share one electron pair =single bond. two electron pairs = double bond three electron pairs = triple bond Atoms with same electro-negativities: Share the electrons equally: Forms non- polar covalent bonds Atoms with different electro-negativities: Share the electrons unequally: Forms polar covalent bonds A bond (or molecule) with partial positive charge on one end and a partial negative charge on the other end is called a dipole. The measure of the bond polarity is the dipole moment of the bond. The bond dipole: modeled as two partial opposite charges δ+ and δ- that are equal in magnitude and separated by distance d as shown below. The dipole moment (μ) of the bond : Its is the product of the magnitude of the charge on either atom (δ) and the bond length (d) as; μ= δ x d SI unit for a dipole moment: coulomb-meter (C m). Convenient unit for covalent bond polarities: debye (D), 1D= 3.34 x10-30C m. Pauling Sacle “Thumb Rule” for bond behavior in chemical reactions Electro-negativity difference between bonded atoms: is >0.4, the bond will behave as polar. Electro negativity difference is 0.4 or less, the bond will behave as non-polar. Electro-negativity difference is >2, the bond will behave as ionic.  MIXED COVALENT/IONIC BONDING Polyatomic ions, also called molecular ions, are ions that are made up of two or more different atoms covalently bonded tightly together so that they can behave as a single unit. Polyatomic ions have a charge because the group of atoms making up the molecule has either gained or lost electrons. So, the polyatomic ion is a covalently bonded molecule that behaves like an ion in forming ionic compounds since it has a charge.  MOLECULAR ORBITALS Atomic orbitals overlap to give Molecular orbitals Atoms share electrons in covalent bonds so the electrons no longer reside in the atomic orbitals instead occupy Molecular orbitals. Total number of molecular orbitals =total number of atomic orbitals involved in bonding Molecular orbitals if lower in energy than atomic orbitals (involved): called as bonding molecular orbital Molecular orbitals if higher in energy than atomic orbitals (involved): called as anti-bonding molecular orbital Electrons in bonding orbitals: stabilize the molecule Electrons in the antibonding orbitals destabilize the molecule Formation of Anti-bonding molecular orbitals: Out of phase combination of atomic orbitals. Filled anti-bonding orbitals : decrease in the electron density between the atoms. Electron filling in Molecular orbital is as per Aufbau principle, the Pauli Exclusion Principle, and Hund’s Rule.  MO for Helium (molecule ???) A molecular orbital diagram for the H2 molecule.  MO Shapes The shapes of bonding and antibonding molecular orbitals  Bond Order Single covalent bonds = 1 sigma MO. Double bonds= 1 sigma MO and 1 pi MO. Triple bonds = 1 sigma MO and 2 pi MO. Bond Order: The number of bonds between 2 atoms. It can be calculated from MO diagram as; Bond order= ??? MO for Helium MO for Oxygen molecule MO for Nitrogen Molecule Electronic configurations  Part B Engineering Materials Nano material When at least one of the dimensions of the particles of a material , is in the range 1 t0 100nm, then the material is known nano material. The mechanical., thermal, optical, electrical, magnetic etc..properties changes, when the size of the material particles comes in the Nano range  Allotropes of Carbon Allotropes of Carbon Graphite Diamond Fullerene Carbon nano tubes  STRUCTURE OF GRAPHENE Graphene is a crystalline allotrope of carbon with 2- dimensional properties. All C are sp3 hybridised. Each atom has four bonds: one σ bond with each of its three neighbors and one π-bond that is oriented out of plane. C-C bond length is 1.42Å. Graphene's stability is due to its tightly packed carbon atoms and a sp2 orbital hybridization – a combination of orbitals s, px and py that constitute the σ-bond.  PROPERTIES OF GRAPHENE Graphene is chemically the most reactive form of carbon. It burns at a very low temperature. It has high electron mobility due to resonance. It is a good conductor of electricity. Electrons are able to flow through graphene more easily than copper. It is a perfect thermal conductor. Strongest material ever discovered per unit weight. Very light at 0.77 milligrams per square metre, paper is 1000 times heavier. Also, graphene is very flexible, yet brittle. It is also completely impermeable to gas. It has the ability to transmit up to 98% of light.  APPLICATIONS OF GRAPHENE Bio medical- reading the sequence of chemical bases in a DNA strand Integrated circuits- High Performance Processor,Terahertz-speed transistor Optical Electronics-touchscreens, liquid crystal displays, organic photovoltaic cells, and OLED Filters-Desalination,Ethanol distillation Energy Storage Devices: Supercapacitors Graphene nanoribbons IR detectors & Chemical sensors Single-molecule gas detection Piezoelectric materials Energy Harvesting Composite Materials Liquid Cells for Electron Microscopy Thermal management materials Optical Modulators  Types of CNT’s Types of CNTs Wrapping of Graphene sheet to form CNTs Mr.Amol Kapse 38  Properties of Carbon Nanotubes  CNTS have high thermal conductivity  CNTS have high electrical conductivity  CNTS aspect ratio  CNTS are very elastic ~18% elongation to failure  CNTS have very high tensile strength  CNTS are highly flexible — can be bent considerably without damage  CNTS have a low thermal expansion coefficient  CNTS are good electron field emitters  Synthesis of CNTs Electric arc discharge process: An electric arc is struck between 2 graphite electrodes under the following conditions Conditions Electrodes: Graphite (diameter 5-25 mm) Gap between electrodes: ~1 mm Current applied: 50- 100 amp Voltage: 15-25 volts Environment: He gas (100 -500 torr pressure for CNTs & below 100 torr. for fullerenes)  Synthesis of CNTs Chemical Vapour Deposition: Hydrocarbon gas cracked to produce carbon black containing hexagonal rings which favours formation of CNTs Conditions Gas used: Benzene, Hexane or methane Pressure: 0.1 to 1 torr. Catalyst: Fe/Co/ Ni/ Pt Furnace temp: ~ 1000oC Note: there is no formation of amorphous nanoparticles  APPLICATIONS OF CARBON NANOTUBES 1. CNTS field emission 2. CNTS thermal conductivity 3. CNTS energy storage 4. CNTS conductive properties 5. CNTS conductive adhesive 6. CNTS thermal materials 7. Molecular electronics based on CNTS 8. CNTS structural applications 9. CNTS fibers and fabrics 10. CNTS biomedical applications 11. CNTS air & water filtration  Fullerene Structure 1.Fullerenes are similar in structure to graphite, which is composed of stacked graphene sheets of linked hexagonal rings. It has football like shape so it is also known as Bucky ball 2.It shows sp2 hybridization and has trigonal planar shape. Fullerenes consist of 20 hexagonal and 12 pentagonal rings 3.Each carbon atom is bonded to three others and is sp2 hybridized. 4.The C60molecule has two bond lengths - the 6:6 ring bonds can be considered "double bonds" and are shorter than the 6:5 bonds. 5. There are 30 double bonds  Applications of C60  Antioxidants: Fullerenes can make excellent antioxidants, a single C60 molecule can interact with up to 34 methyl radicals before being used up.  Antiviral Agents: Fullerenes have grabbed quite a bit of attention due to their potential as antiviral agents.  Photosensitizers Therapy : Photodynamic Therapy (PDT) is a form of therapy of using non-toxic light sensitive compound. The highly water-soluble C60-N vinylpyrrolidone copolymer is used as an agent.  Solar Cells : Fullerenes, due to their high electron affinity and ability to transfer these electrons, make excellent acceptors.  In Protective Eye wear: Fullerenes have optical limiting properties. This refers to its ability to decrease the transmittance of light incident to it.  Hydrogen Gas Storage: molecular structure of fullerenes enables them to hydrogenate and dehydrogenate quite easily. The carbon rings in fullerene are conjugated with C=C double bonds  Polymers Polymer :-poly –”many” + -mer-”parts or units” A polymer is a large molecule, or macromolecule, composed of many repeated subunits. Polymers range from familiar synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins that are fundamental to biological structure and function.  Monomers The word monomer comes from mono-(one) and-mer (part). Monomers are small molecules which may be joined together in a repeating fashion to form more complex molecules called polymers. Polymer nomenclature is generally based upon the type of monomer residues comprising the polymer.  Polymerization Polymerization is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three- dimensional networks.  Polymerization reactions Addition polymerization-where monomers add on to each other with the addition of a catalyst, these are usually alkenes such as ethene and propene. Alkenes can act as monomers because they have a double bond. Condensation Polymerization -This is when monomers join or polymerise with a by-product such as water, carbon dioxide or ammonia. This usually requires two different types of monomers that join alternately.  Degree of polymerization & Functionality of Polymers The degree of polymerization, or DP, is the number of monomeric units in a macro molecule or polymer. According to IUPAC, the functionality of a monomer is defined as the number of bonds that a monomer's repeating unit forms in a polymer with other monomers. The functionality of a monomer means its number of polymerizable groups, and affects the formation and the degree of cross-linking of polymers. A mono-functional molecule possesses one function, a di-functional two, a trifunctional three, etc.  CONDUCTING POLYMERS Definition: Conductive polymers or, more precisely are organic polymers that conduct electricity There are two types of conduction polymers.  Extrinsic conducting polymers  Intrinsic conducting polymers Some polymers can be made conductive by doping with an electron donor or acceptor. e.g. Examples  Doping Doping is used to make polymers conductive or increase their conductivity. Doping is the addition or removal of electrons to a material. Types of doping techniques:  oxidative doping (p-type).  reductive doping(n-type).  protonic doping  Electroluminescent polymer The property in which a material produces bright light of different colours when stimulated electronically is known as electroluminescence. The material which shows electroluminescence is called as electroluminescent polymer. Construction & Working  Biomaterials A material intented to interface with biological systems to evaluate, treat, augment or replace any tissue, organ or function of the body.  Bioceramics  Metallic biomaterials  Biocomposite  Biologically based (derived) biomaterials  Polymeric biomaterials  Biocompatibility Biocompatibility: The ability of a material to perform with an appropriate host response in a specific application. Host response: the reaction of a living system to the presence of a material  Polymeric Biomaterials: Adv & Disadv  Easy to make complicated items  Tailorable physical & mechanical properties  Surface modification  Immobilize cell etc.  Biodegradable Absorb water & proteins etc. Surface contamination Wear & breakdown Biodegradation Difficult to sterilize  Examples of Polymeric Biomaterials PMMA PVC PLA/PGA PE PP PA PTFE PET PUR Silicones  Bioceramic: Advantages and disadvantage  High compression strength  Wear & corrosion resistance  Can be highly polished  Bioactive/inert High modulus (mismatched with bone) Low strength in tension Low fracture toughness Difficult to fabricate  Examples of Bioceramic Alumina Zirconia (partially stabilized) Silicate glass Calcium phosphate (apatite) Calcium carbonate  Metallic Biomaterials: Advantages & Disadvantages  High strength  Fatigue resistance  Wear resistance  Easy fabrication  Easy to sterilize  Shape memory High moduls Corrosion Metal ion sensitivity and toxicity Metallic looking  Examples of Metallic Biomaterials Stainless steel (316L) Co-Cr alloys Ti6Al4V Au-Ag-Cu-Pd alloys Amalgam (AgSnCuZnHg) Ni-Ti Titanium  General Criteria for materials selection Mechanical and chemicals properties No undersirable biological effects Carcinogenic, toxic, allergenic or immunogenic Possible to process, fabricate and sterilize with a good reproducibility Acceptable cost/benefit ratio  Biomaterials applications Dental implant Tooth fillings Vascular implants Drug delivery, bone fixing pine, suture Bone defect fillings Hip joint prosthesis bone plate Scaffolds for tissue engineering Contact lens