4.5 METALLIC BONDING

ESSENTIAL IDEA Metallic bonds involve a lattice of cations with delocalized electrons. NATURE OF SCIENCE (2.2) Use theories to explain natural phenomena – the properties of metals are different from covalent and ionic substances and this is due to the formation of non-directional bonds with a “sea” of delocalized electrons.

UNDERSTANDING/KEY IDEA 4.5.A A metallic bond is the electrostatic attraction between a lattice of positive ions and delocalized electrons.

METALLIC BONDING Metallic bonding occurs when the delocalized electrons in metal atoms are attracted to the lattice of positive metal ions. “Delocalized” means that the electrons do not belong to any one metal nucleus but can spread themselves throughout the metal structure. Because the outer shell electrons in metals can easily be lost, positive ions are easily formed. When the metal atoms are together, the positive ions form a lattice where the electrons can freely move around.

UNDERSTANDING/KEY IDEA 4.5.B The strength of a metallic bond depends upon the charge of the ions and the radius of the metal ion.

The strength of the metallic bond depends upon three things: The number of delocalized electrons The size of the cation The charge of the cation

The number of delocalized electrons are the number of electrons in the outer shell. Sodium has one delocalized electron Magnesium has two delocalized electrons The greater the number of delocalized electrons and the smaller the cation, the greater the strength of the metallic bond.

The higher the charge, the greater the metallic strength. The strength of metallic bonding tends to decrease down a group as cations increase in size. Transition metals have very strong metallic bonds due to the large number of electrons that can become delocalized from both the 3d and 4s sub-levels.

APPLICATION/SKILLS Be able to explain electrical conductivity and malleability in metals.

PROPERTIES OF METALS Metals are good conductors of electricity and heat (thermal conductivity) because the delocalized electrons are highly mobile and move through the metal structure. They are also good conductors of heat due to the same electron mobility. Metals are malleable which means they can be shaped under pressure. This is due to the fact that the movement of electrons is non-directional.

Metals can also be ductile which means to be drawn into threads. Metals have high melting points because lots of energy is required to break the strong metallic bonds. Metals are shiny and lustrous because the delocalized electrons in the metallic structure reflect light.

APPLICATION/SKILLS Be able to explain trends in melting points of metals.

MELTING POINTS Melting points tend to decrease down a group due to the reduced attraction of the delocalized electrons and the positively charged cations. The stronger the metallic bond (due to size of cation and charge), the stronger the melting point.

UNDERSTANDING/KEY IDEA 4.5.C Alloys usually contain more than one metal and have enhanced properties.

ALLOYS Alloys are solid solutions. Remember that a solution is a homogeneous mixture of a solute with a solvent. In alloys, the solute and the solvent are both solids. Two metals are mixed together in the molten state. When they solidify, ions of the different metals are scattered throughout the lattice and are bound by the delocalized electrons.

APPLICATION/SKILLS Be able to explain the properties of alloys in terms of non-directional bonding.

PROPERTIES OF ALLOYS Alloys are possible because of the non-directional nature of the delocalized electrons and the fact that the lattice can accommodate ions of different sizes. Alloys have properties that are distinct from their component elements due to the different packing of the cations in the lattice. The alloy is often more chemically stable, stronger and resistant to corrosion.

GUIDANCE Trends are limited to “s” and “p” block elements only. Various alloys should be covered.

COMMON ALLOYS Steel – iron with carbon and other elements High tensile strength but tends to corrode Stainless steel – iron with nickel or chromium High strength and corrosion resistant Brass – copper and zinc Plumbing Bronze – copper and tin Coins and tools Pewter – tin, antimony and copper Decorative objects Sterling silver – silver and copper Jewelry and art objects