Recap – Last Lecture Periodic Trends in ionisation energy, atomic radius and electronegativity result from the influence of effective nuclear charge. 1 Effective Nuclear Charge

2 Chemical Bonding 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 4 th Ed

3 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.

4 Ionic Bonding 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).

5 Ionic Solids Solids are hard, crystalline, brittle and have a high melting point. The solid does not conduct electricity. An ionic compound does conduct electricity as a molten liquid, or in solution for soluble compounds, when ions are released to carry the current.

6 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,

7 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 mobile electrons.

8 Covalent Bonding In a covalent bond electrons are shared between two atoms. The distance at which the energy is minimised is the bond length.

9 Covalent Bonding 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.

10 Covalent Bonding There is a broad inverse correlation between the strength of a bond and its length. BondEnergy (kJ mol -1 )Bond Length (pm) H-H43274 C-H Cl-Cl Br-Br C-C C=C C≣CC≣C

11 Covalent Bonding 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.

12 Two Types of Covalent Solid Network covalent solids, e.g. diamond – very hard, high mp, (infinite network of covalent bonds, much energy required to break bonds), non- conducting (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), non- conducting (electrons not mobile, no ions in melt).

13 The Bonding Spectrum Polar Covalent Bonds Ionic Bonds Covalent Bonds Electrons transferred Electrostatic attraction between cations & anions Non-directional Electrons shared Attraction between electrons and nuclei Defined bond axis

Types of Formula 14 Empirical formula: Lowest ratio of atoms of different types present. Ionic Compounds, eg NaCl, K 2 O Elements, always used except for diatomic elements (H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2 ) Molecular formula: Actual number of atoms of each type in molecule. Implies covalent bonding Always a simple multiple of empirical formula eg C 6 H 12 O 6

15 Applications 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 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 Learning Outcomes:

17 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’: H and Br; O and O; Mg and Cl; P and O; I and I