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Metallic Bonding

Key Information & Summary

  • Metallic bonding joins a bulk of metal atoms.
  • Valence electrons are responsible for the metallic bond.
  • Valence electrons can move from one atom to another creating a cloud of delocalized electrons.
  • The positive metal ions are immersed in this cloud.
  • Electrostatic interactions are responsible for the metallic bond.
  • Metal bond determines the properties of metals.
  • Metals are strong solid with the exception of mercury, Hg, that is actually a liquid.
  • Metals can be drawn into thin wires, so we say that they are ductile.
  • Metals can be shaped in thin sheets quite easy, so we say metals are malleable.
  • Metals are shiny, good conductors of electricity and heat.

Metallic bond occurs among metal atoms and is typical of metallic materials.

Metal are the majority in the periodic table and all of them are characterized by several specific properties. One of the most important consists in having loosely electrons in the outer shell. These electrons are called valence electrons and they can be transferred or shared with other atoms easier than other electrons.

Valence electrons are responsible of the existence of the metallic bond. When metal atoms bond, they arrange in a lattice structure. The valence electron in this situation are electrostatically attracted not only by their nucleo but also from the other atoms. As a consequence they move from one atom to another creating a cloud of delocalized electrons. The positive metal ions are immersed in this cloud and they are electrostatically attracted by the negative electrons.  The mutual interaction of electrostatic forces between the cloud of electrons and metal ions is what keep the metals together and it describes in the metal bonding.

Usually each atom has 8 or 12 close atoms. The valence electrons in the metal atoms are usually less than four. It is not possible for the metal atoms to form 8 or 12 covalent bonds  because they do not have such large number of valence electrons. Thus, metal atoms cannot form a covalent bond with the neighboring atoms.

Properties

Thanks to the characteristic of the metal bonding, metals have peculiar properties that make these material very distinguishable. They are usually strong solid with the exception of mercury, Hg, that is actually a liquid. They can be drawn into thin wires, so we say that they are ductile. They can be shaped in thin sheets quite easy, so we say metals are malleable. They are shiny, good conductors of electricity and heat.

Strength

Metallic bonds are strong and require a great deal of energy to break. When a metal is molten, the metallic bond is still present but the structure is deformed. When the metal is heated up to the boiling point, the metal bond is broken. That means that boiling point is actually a way how we can estimate the strength of the metallic bond. Better than melting point.

The strength of a metallic bond depends on the number of delocalized electrons, on the intensity of the charge of the cation (metal) and on the size of the cation.

A strong metallic bond will be characterized by more delocalized electrons, which causes the effective nuclear charge on electrons on the cation to increase, in effect making the size of the cation smaller. The strength of metallic bonds for different elemental metals reaches a maximum around the center of the transition metal series, as these elements have large numbers of delocalized electrons.

Malleability and Ductility

Metal are able to deform under stress without cleave.

The cloud of electrons surrounding the ions acts like a cushion, and so when the metal is hammered on, for instance, the overall composition of the structure of the metal is not harmed or changed. The ions may rearrange but the cloud of electrons will adjust to the new formation and the metal will remain intact. The structure is able to deform instead of breaking because the layers of atoms can slide on each other. this effect is possible because the cloud of electron, which maintains the electrostatic interaction even if the ions are moving.

Opacity and luster

Metal are shiny and opaque because the free electrons can absorb photons in the cloud. Electrons on the surface bounce back light at the same frequency that the light hits the surface, therefore the metal appears to be shiny.

When a photon arrives at the metal’s surface it encounters the cloud. Most photons can be absorbed promoting some electrons to a higher energy level. As the electrons pass to a lower energy level, the photons are re-emitted, resulting in the characteristic metallic luster.

Electrical Conductivity

Conductivity is a fundamental property of a material that quantifies the ability of a material to let electric current flow. Since the electrons are free to move in cloud of the metallic bond, metal are characterized by high conductivity. If electrons from an outside source are pushed into a metal wire, the electrons move through the wire and come out at the other end.

Thermal conductivity

Thermal conductivity is the property of a material to conduct heat. This property varies across different types of metal. The thermal conductivity of a material is highly dependent on composition and structure. Metals are materials that allow energy in the form of heat, to be transferred in the material.

Different materials transfer heat at different rates and metal are quite fast, resulting often colder to the touch than fabric or wood (at the same temperature).

 

References and further readings:

https://www.slideserve.com/

https://www.youtube.com/watch?v=S08qdOTd0w0

http://www.chemistryrocks.net

https://en.wikipedia.org/wiki/Metallic_bonding

https://www.physics-and-radio-electronics.com/electronic-devices-and-circuits/introduction/metallicbond.html

https://chem.libretexts.org/Textbook_Maps/General_Chemistry/Book%3A_CLUE_(Cooper_and_Klymkowsky)/3%3A_Elements%2C_Bonding%2C_and_Physical_Properties/3.4%3A_Metals/Why_Are_Metals_Shiny%3F