Recap Last Lecture. Chemical Bonding. Ionic Bonding. Ionic Bonding

Transcription

1 Recap Last Lecture Chemical Bonding Periodic Trends in ionisation energy, atomic radius and electronegativity result from the influence of effective nuclear charge. If a system has a lower energy when the atoms are close together than when apart, then bonds exist between those atoms. A bond is an electrostatic force that holds the atoms of elements together in a compound. M.Silberberg: Chemistry The Molecular nature of Matter and Change 4th Ed Effective Nuclear Charge 1 Ionic Bonding 2 Ionic Bonding Electrons are transferred to form cations and anions. An ionic bond is the electrostatic attraction between oppositely charged ions. Electronegativity differences >2 generally result in ionic bonds. 3 The interactions are isotropic - i.e. non-directional. Structure of crystal lattice depends on ionic radii and ratio of ions. The magnitude of the attraction depends on the size and charge of the ions (charge density). 4 1

2 Ionic Solids Solids are hard, crystalline, brittle and have a high melting point. The solid does not conduct electricity. Metallic Bonding If elements have relatively low ionization energies then the valence electrons become mobile giving rise to a sea of electrons or metallic bonding, An ionic compound does conduct electricity as a molten liquid, or in solution for soluble compounds, when ions are released to carry the current. 5 6 Metallic Bonding Good electrical conductivity in solid and liquid. Metals are malleable and ductile. Melting point variable but often low as attraction between nuclei and mobile electrons not really broken when melted. Boiling point typically high due to need to overcome attraction between nuclei and In a covalent bond electrons are shared between two atoms. The distance at which the energy is minimised is the bond length. 7 8 mobile electrons. 2

3 Electronegativity differences <2 generally result in covalent bonds. So, compounds between non-metals are predicted to be covalent. A single covalent bond: one pair of electrons; A double bond: two pairs of electrons; A triple bond: three pairs of electrons. C O C O C N 9 There is a broad inverse correlation between the strength of a bond and its length. Bond Energy (kj mol -1 ) Bond Length (pm) C Cl-Cl Br-Br C-C C=C C C Two Types of Covalent Solid If a bond is formed between two different atoms, the electron pair of the bond will be attracted towards the atom with the higher electronegativity. This results in a polar bond. 11 Network covalent solids, e.g. diamond very hard, high mp, (infinite network of covalent bonds, much energy required to break bonds), nonconducting (electrons not mobile, no ions in melt). Molecular covalent, e.g. P 4, I 2, CO 2 soft, low mp (small molecules with only weak forces between them), nonconducting (electrons not mobile, no ions in melt). 12 3

4 The Bonding Spectrum Types of Formula Ionic Bonds Polar Covalent Bonds Covalent Bonds Empirical formula: Lowest ratio of atoms of different types present. Ionic Compounds, eg NaCl, K 2 O Elements, always used except for diatomic elements ( 2, N 2, O 2, F 2, Cl 2, Br 2, I 2 ) Electrons transferred Electrostatic attraction between cations & anions Non-directional Electrons shared Attraction between electrons and nuclei Defined bond axis Molecular formula: Actual number of atoms of each type in molecule. Implies covalent bonding Always a simple multiple of empirical formula eg C 6 12 O Applications Learning Outcomes: Nitinol is an alloy of Ni and Ti that has shape memory. The alloy is programmed into a shape at an elevated temperature in which the crystal structure is face-centered cubic (fcc). On cooling the alloy transforms to a body-centred tetragonal structure. It can be reshaped. Now warming above a transformation temperature reforms the fcc structure and the original shape. An arterial stent can be programmed, cooled and crumpled into a small volume for surgery. Warming to body temperature it regains the programmed shape opening the 15 artery. By the end of this lecture, you should: be able to predict the type of bonding between two elements. explain the characteristics of ionic, metallic and covalent bonding. be able to distinguish between network and molecular covalent compounds. be able to complete the worksheet (if you haven t already done so ) 16 4

5 Questions to complete for next lecture: 1. When a material is heated the energy provided results in increased motion of the particles present. Why are metals good conductors of heat? 2. Silicon has a melting point of 1410 C and phosphorus has a melting point of 44 C. What does this suggest about the nature of bonding in the two solids? 3. Silicon carbide (SiC) is a solid at temperatures up to about 3000 C. What does this suggest about the type of structure of this material? 4. What condition is necessary for a bond to be polar? 5. Classify the bonds between the following pairs of molecules as covalent, polar covalent or ionic : and Br; O and O; Mg and Cl; P and O; I and I 17 5