Lecture 14: Structure and Bonding in Metals

Before coming to class:
  1. Carefully read the information on this page and in sections 5.2 and 5.3 from your textbook.

  2. Watch the 15 minute video lecture Structure and Bonding in Metals on the MediaSite page.

Introduction

The reaction of some atmospheric components with solid element oxides produces the carbonate minerals on the Earth's surface.

CO2 + H2O + CaO CaCO3


Other minerals in the crust are silicates.

This week we will cover the structure and properties of some of the inorganic compounds in the Earth.

Regions of the Earth

First, let's review the structure of the Earth. The planet is made up of three main shells: the very thin, brittle crust, the mantle and the core. The core forms only 15 percent of the Earth's volume, whereas the mantle occupies 84 percent. The crust makes up the remaining 1 percent. The composition of the crust is quite different from that of the Earth as a whole. Heavy elements are segregated towards the center and lighter elements towards the surface.

Most Abundant Elements
in the Earth		Most Abundant Elements
in the Earth's Crust
  1. Fe - 35%
  2. O - 30%
  3. Si - 15%
  4. Mg - 13%
  5. Ni - 2.4%
  6. S - 1.9%
  7. Ca - 1.1%
  8. Al - 1.1%
  1. O - 46%
  2. Si - 28%
  3. Al - 8%
  4. Fe - 6%
  5. Mg - 4%
  6. Ca - 2.4%
  7. K - 2.3%
  8. Na - 2.1%

Crust

The rocks that make up the crust can be classified into three types.
  • Sedimentary Rocks - rocks that are formed from the lithification of sediments, chemical precipitation or by direct biogenic deposition. Some common types are sandstone, shale, coal, limestone and coral.
  • Igneous Rocks - rocks that cool from a magma. The two most common types are granit and basalt.
  • Metamorphic Rocks - rocks that have been altered by high pressures, temperatures and/or chemical reaction while still in the solid state. Two common types are marble, which comes from limestone, and slate which comes from shale.


Mantle

The mantle is composed mainly of iron and magnesium silicates. The temperature increases with depth from 870 deg to 2200 deg C.

Core

The core is mainly composed of hot (greater than 5000 deg C!) metallic nickel and iron. The outer core is liquid but the inner core is solid due to the higher pressure.

Properties of Metals

Properties of metals include:
  • They are good conductors of heat and electricity
  • They are usually opaque to light
  • They have metallic luster


Close-packed Structures

Many metals have a close-packed structure. That is, the metal atoms pack as closely as possible to mimimize empty space. Close packed structures begin with a hexagonally packed layer. Imagine that each metal atom has 6 metal atoms packed around it.



In any close packed array of atoms or ions, there are both octahedral and tetrahedral holes where smaller atoms or ions could reside.

In this figure, there are three layers of atoms or ions. Look at the first layer. There are holes surrounded by 3 of these anions.

We add a second layer (red) so that each atom or ion fits into a depression in the layer below it. Some of the holes in the first layer are capped by another atom or ion in the second layer. These are tetrahedral holes. Other holes are not capped in this way. A bigger atom or ion could fit into these octahedral holes which are surrounded by 3 anions from one layer and 3 from another layer.

A third layer covers these octahedral holes in the ABC layering of the cubic close packed structure (ccp). If the third layer were a position identical to the first layer, the structure would have hexagonal close packing (hcp).


    The smallest unit that when repeated gives the structure of the material is the unit cell. Three unit cells of hexagonal close packed (hcc) structure are below.The unit cell is indicated by the rectangle. You can see that the bottom and the top layers are the same for this ABA... packing.


    The layers in the unit cell of a cubic close packed structure is more difficult to see because they are on the diagonal of the cell. At one vertex there is one atom from layer A and at the opposite vertex is another atom of the next layer A so the unit cell contains parts of ABCA layers.



Other Structures

Some metals pack in structures that are less compact than the two above. Cubic structures have one layer directly on top of another. Body-centered cubic packing is like cubic packing with an additional atom in each cubic hole.The smallest unit that when repeated gives the structure of the material is the unit cell. The unit cell for cubic, body-centered cubic, and face-centered cubic are shown below. Face-centered cubic is another name for cubic close packed.



The unit cells can be classified according to their dimensions (height, width, length; a, b, c) and angles.


Physical Methods

X-ray diffraction is one physical method of characterizing crystalline solids. Single wavelength radiation in the X-ray region of the spectrum interacts with the electron density around atoms to diffract and reinforce it or to cancel it . Different atoms diffract X-rays differently. By analyzing the pattern of spots of reinforced X-rays that occur when a crystal is rotated in an X-ray beam, we can calculate the three dimensional structure of the unit cell.

A powdered crystalline sample diffracts X-rays as well. Because the crystals will be randomly oriented in the sample, the X-ray powder pattern is a series of concentric rings. Analysis of the powder pattern provides the spacing of planes of atoms in the crystalline lattice but does not give detailed information on the 3 dimensional structure of the unit cell. However, individual crystalline solids have characteristic powder patterns and this can be used to identify a solid sample.

Bonding in Metals

The bonding in metals is not directional. Each metal atom uses its valence orbitals to combine with the atomic orbitals of all of its neighbors and these neighboring atoms, in turn, combine with all of its neighboring atoms. The molecular orbitals that are formed extend over the entire material.

When 2 atomic orbitals combine, they form 2 molecular orbital. Three atomic orbitals make 3 molecular orbitals. When there are n atomic orbitals (and n is a very large number) there are n very closely spaced molecular orbitals. Electrons are free to travel throughout the whole material because the orbitals that contain them also extend throughout.



In metals, there is no gap between the bonding and antibonding orbitals. At any temperature above absolute zero, some electrons from the conduction band are promoted to the valence band. Metals are conductors for this reason.



The bonding in metals doesn't require specific bond angles. Because of this, metals retain their intermolecular interaction when the solid is deformed. Metals bend and can be drawn out into wires.