5b. The metallic bonding model - giant structure - giant lattice of metal ions

Doc Brown's Chemistry: Chemical Bonding and structure GCSE level, IGCSE, O, IB, AS, A2 A advanced level US grade 9-12 level Revision Notes

Metals have giant structures of atoms with strong metallic bonding leading to them having high melting and boiling points.

In pure metals, atoms are arranged in layers, which allows metals to be bent and shaped without breaking the bonds.

However, pure metals are too soft for many uses and so are mixed with other metals to make alloys which are harder.

You should be able to explain why alloys are harder than pure metals in terms of distortion of the layers of atoms in the structure of a pure metal by adding other elements like carbon in steel alloys.

Most metals are good conductors of electricity because the delocalised electrons in the metal carry electrical charge through the metal.

Metals are also good conductors of thermal energy because energy is transferred by the kinetic energy of the delocalised electrons.

METALLIC BONDING isn't quite like ionic or covalent bonding, although the metal atoms form positive ions, no negative ion is formed from the same metal atoms, but the immobile positive metal ions/atoms in the lattice are attracted together by the free moving negative electrons between them. So, like ionic bonding, you do get attraction between positive and negative particles and this is the metallic bond.

The giant metallic lattice of metal ions

• METAL STRUCTURE - another 'giant' structure
• All metals have similar properties BUT, there can be wide variations in melting point, boiling point, density, electrical conductivity and physical strength.
• To explain the physical properties of metals like iron or sodium we need a more sophisticated picture than a simple particle model of atoms all lined up in close packed regular rows and layers, though this picture is correctly described as another example of a giant lattice held together by metallic bonding.
• A giant metallic lattice – the crystal lattice of metals consists of ions (NOT atoms) surrounded by a 'sea of electrons' that form the giant lattice (2D diagram above right).
• Some of the outer electrons (–) from the original metal atoms are free to move around between the positive metal ions formed (+) in the giant metallic structure.
• These 'free' or 'delocalised' electrons from the outer shell of the metal atoms are the 'electronic glue' holding the particles together.
• There is a strong electrical force of attraction between these free electrons (mobile electrons or 'sea' of delocalised electrons) (–) and the 'immobile' positive metal ions (+) that form the structure of the giant metallic lattice and this is the metallic bond. The attractive force acts in all directions.
• Metallic bonding is not directional like covalent bonding, it is like ionic bonding in the sense that the force of attraction between the positive metal ions and the mobile electrons acts in every direction about the fixed (immobile) metal ions of the metal crystal lattice, but in ionic lattices none of the ions are mobile. a big difference between a metal bond and an ionic bond.

• In general, the more electrons are delocalised to form the metal lattice of ions, the stronger the bond - which obviously will have an effect on physical properties such as melting point and thermal expansion.

  • It isn't a strict rule, but if you compare sodium Na, magnesium Mg and aluminium in groups 1 to 3 on period 3 of the periodic table, you find the melting point increases.

  • As you go from left to right from groups 1 to  3, the number of outer shell electrons increases, so potentially donating 1 to 3 outer delocalised electrons to the bonding between the ions and electrons in the giant metallic lattice of ions.

  • The positive charge on the average ion will also increase - so both the negative charge and positive charge between the particles will increase .

  • This increases the bond strength and more energy (from a higher temperature) is needed to give the particles sufficient kinetic energy to loosen the lattice bonds sufficiently to melt the metal.

  • The weaker the metal bonding in the giant lattice, the more the metal expands on heating per degree rise in temperature - a higher thermal expansion coefficient.

 Doc Brown's Chemistry 

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