Metals & Metallic Bonding

Properties of Metals:

  • Conduct heat and electricity
  • Malleable - easily shaped
  • Ductile - can be drawn into a wire
  • Relatively high melting and boiling point
  • High density
  • Lustrous - reflective/shiny
  • Hard with high tensile strength - resists tearing
  • Low electro-negativities - lose electrons easily, once a metallic atom has lost an electron, it becomes a positively charged cation (think puss-ative, get it??) 
  • These properties can be linked to the way metals exist, in the metallic bonding model. 
Exceptions: 
Mercury: Hg: low melting point - liquid at room temperature.
Chromium: (Cr): Brittle
Group 1 metals: soft, lower density

Metallic Bonding Model:
Metals are composed of a regular 3D lattice of cations that are surrounded by a sea of delocalised electrons. The lattice is held together by electrostatic attractions between cations and delocalised electrons.

Cations surrounded by delocalised anions
The amount of electrons depends on the group the metal belongs to and how many it must lose to reach the valence shell equivalent of the nearest Nobel gas.
Limitations: The MBM does not explain the range of melting points, the different densities, the different electrical conductivities between different metals as well as the magnetic nature of Co, Fe and Ni

 d-Block/Transition Metals: 

  • Harder than metals of group 1 and 2
  • High tensile strength
  • Relatively unreactive
  • High melt point due to smaller atomic size and greater nuclear charge,
  • d subshell is being filled
  • Display similar chemical and physical properties because the valence shell of most transition metals have the configuration s2
  • Form coloured compounds
    • Iron (brown), cobalt (pink), copper (blue, red), nickel (green)
  • Form ions of various charges because the can lose from d and s subshell
    • Cu2+ and Cu3+  


 
Crystals: 
Atoms inside metals are arranged into regular crystal structures. The size and malleability of metals is dependent on the size of the crystals. Metals with smaller crystals are harder - less movement of layers of ions, but more brittle - more breaks between crystals.

Metal Modification:
Some metals can be used in their pure forms
Example: Aluminium: conducts heat and so useful in the production of cookware
Copper: conducts electricity and so is useful in electrical wiring. 
However, they are often modified to make their properties more useful/desirable.
Example: pure gold is soft and easily deformed, not good properties for jewellery.

Metals can be modified by
  • Alloying
  • Work Hardening
  • Heat treatment

Alloying:
This website has some interesting info on alloys
Involves melting and mixing metals together than allowing them to cool into a solid. This produces an alloy that has different properties to the original products.
In general they are: harder, less malleable and poorer electrical conductors
Substitutional alloys are made from elements that have similar chemical properties and size. Example: Australian 'silver' coins - Cu and Ni
Interstitial alloys are made of different sized elements. The smaller ones 'fit between' the larger ones. 

Work Hardening:
Ever bent a paper clip over and over? It starts by bending easily, but becomes tougher, and eventually snaps, this is due to 'work hardening'
Bending or hammering cold metals causes their crystal grains (little chunks or metal that stick together to form a bit bit of metal) to become smaller. Since bending is now more difficult, the metal is toughened or 'work hardened'
Work hardened metals are usually more brittle owning to the smaller crystals and increased number of dislocations (cracks) in the structure.

Heat Treatment:
Anneal:
Metals are heated then cooled slowly, larger metal crystals form resulting in softer metal and can restore ductility lost during work work hardening.
Quench:
Metals are heated till they are red hot, then cooled quickly in cold water. Small crystals form, resulting an a harder, but more brittle metal. 
Tempering:
Quenched then reheated to a low temperature and cooled slowly. This reduces brittleness but retains hardness.

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