Chapter 7- Covalent and Metallic Bonding

  • When non-metals react with one another, they share electrons instead of transferring them just as in the diatomic elements.
  • The bond formed by sharing of electrons between non-metals is the covalent bond.
  • Sharing of one pair of electrons, i.e. two electrons, forms one covalent bond. Hydrogen has one covalent bond while nitrogen has three.

 

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  • One covalent bond is represented by one line in the structural formula.
  • Compounds linked by covalent bonds are called covalent compounds or molecular compounds. Water, carbon dioxide, and methane are examples.
  • Within each molecule of a simple molecular compound, there are weak intermolecular forces of attraction so they have low densities, and melting and boiling points.
  • Most simple molecular compounds are soluble in organic solvents but not in water.
  • They do not conduct electricity in any state due to lack of mobile electrons/ions.

GIANT MOLECULAR STRUCTURES

  • Diamond and graphite, allotropes of carbon have giant covalent structures.
  • Giant molecular structures are usually solids at r.t.p, have high melting and boiling points, and high densities, and do not conduct electricity in any state (except graphite).
  • Graphite is slippery so it is used as a lubricant while diamond is hard and shiny so it is used for cutting purposes.
  • Sand (silicon dioxide) is another giant molecular structure.

 

Common prefixes to name covalent compounds are mono, di, tri, tetra, penta and hexa.

METALLIC BONDING

  • In metals, electrons break free of the atoms forming cations and become delocalised themselves. The electrons and cations attract each other due to opposite charges on them.
  • The force of attraction between positive metal ions and ‘sea’ of delocalised electrons is the metallic bond.
  • Since there are a lot of free electrons that move throughout the metallic structure, metals are good conductors of heat and electricity.
  • Metals are ductile and malleable because when force is applied, layers of atoms can slide over one another without disrupting the structure.
  • Due to strong forces of attraction between electrons and ions, metals have high melting and boiling points, and densities (exception: Group I)

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