CH 06 - Lecture 10 PDF

CHEM 231 Inorganic Chemistry I, 3.0 Credits Chapter 6, Lecture 10 Dr. Mohamed Ahmed Ass. Prof. @ Chemistry Department, UAEU  Structures and energetics of metallic and ionic solids 6.1 Introduction 6.2 Packing of spheres 6.1 Introduction...

CHEM 231 Inorganic Chemistry I, 3.0 Credits Chapter 6, Lecture 10 Dr. Mohamed Ahmed Ass. Prof. @ Chemistry Department, UAEU  Structures and energetics of metallic and ionic solids 6.1 Introduction 6.2 Packing of spheres 6.1 Introduction In the solid state, both metallic and ionic compounds possess ordered arrays of atoms or ions and form crystalline materials with lattice structures. Differences in bonding between their building blocks result in quite distinct properties for metallic and ionic solids. In metals, the bonding is essentially covalent. The bonding electrons are delocalized over the whole crystal, giving rise to the high electrical conductivity. Ionic bonding in the solid state arises from electrostatic interactions between charged species (ions), e.g. Na+ and Cl- in rock salt. Ionic solids are insulators. Mix character bonding: ionic contributions to ‘covalent’ bonding. And covalent character in a predominantly ionic compounds. 6.2 Packing of spheres “spherical atoms” Cubic and hexagonal close-packing Close-packed, the most efficient way in which to cover the floor of the box (the most efficient use of the available space where 74% of the space is occupied). Spheres that are not on the edges of the assembly are in contact with six other spheres within the same layer. Hexagon 6.2 Packing of spheres “spherical atoms” Cubic and hexagonal close-packing Close-packed of the 2nd layer, we will occupy only other hollow. There are two types of hollows after adding layer B, one lies over red sphere in layer A and three lie over hollows in layer A. Layer A Layer AB Layer ABC Close-packed of the 3ed layer, we will construct ABC ABC ABC or ABA ABA ABA repeating Layer AB Layer ABA configurations.  6.2 Packing of spheres “spherical atoms” Hexagonal and cubic close-packing Hexagonal close-packing Cubic close-packing (hcp) (ccp) o The ABABAB... and ABCABC... packing arrangements are called hexagonal close-packing (hcp) and cubic close-packing (ccp), respectively. o In each, any given sphere is surrounded by (touches) 12 other spheres and is said to have 12 nearest neighbors, to have a coordination number of 12, or to be 12-coordinate.  6.2 Packing of spheres “spherical atoms” The unit cell: hexagonal and cubic close-packing ❑ A unit cell is a fundamental concept in solid state chemistry. It is the smallest repeating unit of the structure which carries all the information necessary to construct unambiguously an infinite lattice. ccp or fcc ❑ Cubic close-packing is also called face-centred cubic (fcc) packing. ❑ The relationship between the ABABAB sequence and the hcp unit cell is easily recognized; hcp consists of parts of three ABA layers. ❑ However, it is harder to see the ABCABC sequence within the ccp unit cell since the close-packed layers are not parallel to the base but lie along the body-diagonal of the cube. hcp  6.2 Packing of spheres “spherical atoms” The unit cell: hexagonal and cubic close-packing ❑ Close-packed structures contain octahedral and tetrahedral holes (sites). ❑ There is one octahedral hole per sphere, and there are twice as many tetrahedral as octahedral holes in a close-packed array. ❑ The octahedral holes are larger than the tetrahedral holes.  6.2 Packing of spheres “spherical atoms” Non-close-packing: simple cubic vs. body-centred cubic arrays ✓ 74% found as efficient used space for a close-packed arrangement. On the other hand, spheres are not always Simple cube packed as efficiently as in close-packed arrangements. ✓ In simple cube: each sphere has a coordination number of 6. ❑ If the eight spheres in the cubic cell are pulled apart slightly, another sphere is able to fit inside the hole. The result is the body-centred cubic (bcc) arrangement. ❑ The coordination number of each sphere in a bcc lattice is 8. Body-centred Cube  Worked example: Packing efficiency Show that in a simple cubic lattice, (a) there is one sphere per unit cell, and (b) approximately 52% of the volume of the unit cell is occupied.