Metal Atoms Joining
Metals consist of giant covalent structures in which each atom contributes one or more of its valence electrons to the formation of an omni-directional delocalised covalent bond that extends throughout the structure. This is often referred to as a sea of electrons in GCSE textbooks.
Metals have a regular crystalline structure. You can see watch crystals of lead form if you immerse a strip of zinc in a solution of lead (II) nitrate. The main properties of metals are:
· they have a lustrous appearance (shiny) because of the reflection of light from the regular arrangement of atoms,
· they are good conductors of heat too because of this structure. Sound waves are able to travel through the lattice and so metals are sonorous, (they ring when struck)
· within the metallic structure the atoms give up some of their outer electrons and become positive ions while the electrons released move freely in the solid as a whole. They are therefore very good conductors of electricity,
· They are ductile: they can be worked into different shapes and malleable: pulled into a wire It is possible using models to show how layers of copper atoms, for example, can slip over each other.
Linking the microstructure to the macro properties
The compounds formed through covalent bonding are different from compounds formed by ionic bonds. Molecular compounds are typically (but not invariably) discrete groupings of atoms rather than extended aggregations of lattices such as exists in ionic solids.
Ionic lattices are potentially infinite and invariably solid at normal room temperatures, molecular aggregations are often so small that they can form gases and liquids. Moreover when they form solids, the forces of attraction between the molecules in the solid are generally much less strong that between ions in an ionic solid. Water should be a gas at room temperature but is a liquid because of the hydrogen bonds between the water molecules. Many molecular species form softer solids than ions do, and are more easily split apart into their constituent molecules by gentle application of heat; they are more commonly characterised by low melting points and boiling points. For example solid carbon dioxide sublimes into gas on heating.
There are examples too of atoms forming bonds with each other or neighbouring elements in the periodic table. An example is carbon atoms joining together to form diamond. The atoms are joined in a repeating tetrahedral lattice that extends throughout the solid. Other forms of carbon also exist. In addition to diamond, carbon can exist as graphite or as Buckminster fullerene.
These allotropes are all forms of carbon but with different structures. Atoms are arranged in hexagonal sheets of carbon atoms. Graphite is the only non-metal substance which can conduct because only three carbons are bonded in the sheet allowing current to flow through the free non-bonding electron .The sheets can slide over each other and graphite is often used as a lubricant for metal parts.