Chapter 11 Structure of solids continued

Structure and Bonding in Metals Metals have: –High thermal and electrical conductivity –Are malleable –Are Ductile The reason for this is they are like small spheres packed together and bonded equally with other metal atoms in all directions.

Body-centered & Face-centered Crystal Lattice

Closest Packing The structural model has uniform spheres as atoms packed in a manner that most efficiently uses the available space. The top layer does not lie directly on the spheres below but in the spaces available.

Hexagonal close packing When the atoms in the third layer lay over the atoms in the first layer. The unit cell here is body centered.

Other examples in nature of Hexagonal Close Packing

Cubic Close packing When the first and the fourth layer line up with one another. The unit cell shown is face centered cubic.

Bonding Model for Metals Metals qualities are best explained by the electron sea model. This envisions a regular array of organized cations surrounded by delocalized sea of electrons. This allows the movement of electrical current, and the metal ions can be easily moved around as a metal is hammered into a shape.

Metal Strength Sodium, potassium and lithium are soft metals that may be cut with a spoon! They have only one valence electron each. Chromium and iron are much harder metals each with 6 and 8 valence electrons respectively. What about mercury?

Discussion Mercury hangs on to its valence 6s electrons very tightly. Mercury-mercury bonding is very weak because its valence electrons are not shared readily. (In fact mercury is the only metal that doesn't form diatomic molecules in the gas phase). Hg [Kr] 4d 10 4f 14 5s 2 5p 6 5d 10 6s 2

Other notes: Metal alloys are a substance that contains a mixture of elements and has metallic properties. There are two types of alloys: –Substitutional alloy –Interstitial alloy

Substitutional Interstitial

Bonding in Molecular Solids Molecular solids are held together by intermolecular forces. London forces, Dipole-dipole and hydrogen bonding. The properties of the molecular solids depends not only on the strength of these forces but also on the ability of the molecules to closely pack. Examples: Ar, CO 2, and H 2 O

Network Solids Many atomic solids form strong directional covalent bonds. This allows the formation of “giant” molecules. Silicon and Carbon form some of the most important network solids. Diamond and graphite are both made of carbon. Yet diamond is a poor conductor and graphite can conduct electricity.

Why? Diamond is carbon bound in a tetrahedral shape to other carbons (sp3). This localizes the electrons and prevents conduction. Graphite is layers of 6 carbon rings with some delocalized electrons between the sheets of rings. Aka. sp2 hybridization with pi-bonds.

This is why!

Silica Silica (SiO 2 ) crystal when heated to 1600 °C and cooled rapidly an amorphous solid called glass is formed.

Ionic Solids These are stable high melting substances held together by strong static forces between oppositely charged ions. Most are binary solids and can be modeled by closest packing spheres. The smaller cations fit in the holes created by closely packing the anions. The packing is done to maximize the oppositely charged particles and minimize the repulsions by ions with the same charge.

Shapes There are three types of holes in closest packed structures. Trigonal holes formed by three sphere in the same layer Tetrahedral holes formed when a sphere sits in the dimple of three spheres in an adjacent layer. Octahedral holes are formed by two sets of three spheres of the closest packed structure. The relative size of the wholes is : Trigonal<tetrahedral<octahedral