chemical bonding and structures & properties

• Valency refers to the number of electrons that must be gained, lost or shared in order to attain noble gas electronic configuration.
• 1. Covalent bonding is the sharing of electrons between atoms to attain a full valence shell of electrons.
  2. Covalent bonds form between non-metals.
  3. Covalent bonding can occur as long as electrons can be shared between atoms.
  4. An atom can also share its electrons with more than one atom.
• A single covalent bond involves two electron pairs being shared by two atoms. A double covalent bond involves four electron pairs being shared by two atoms. A triple covalent bond involves six electron pairs being shared by two atoms.
• Different ways of representing molecules:

  1. Structural formula: O=O/ F-F/N≡N
  2. Molecular formula: O2, F2, N2
  3. Dot and cross diagram:

  
• 1. Simple covalent molecules have a countable number of atoms in a fixed ratio.
  2. Atoms in simple covalent molecules are held together by weak intermolecular forces of attraction.
• 1. Giant covalent molecules have a practically uncountable number of atoms in each molecule.
  2. Atoms in giant covalent molecules are connected to each other by strong covalent bonds.
• Structural properties of simple covalent molecules:

  1. Low melting/boiling point: weak intermolecular forces of attraction between molecules are easily overcome with small amounts of energy (larger molecules have stronger intermolecular forces of attraction)
  2. Solubility: Insoluble in water and soluble in organic solvents
  3. Electrical conductivity: Simple covalent substances typically exist as neutral atoms, thus they do not have mobile charge carriers to conduct electricity (some simple covalent substances disassociate in water and the resulting solution can conduct electricity)
• Structural properties of giant covalent structures:

  1. High melting/boiling point: Large number of strong covalent bonds in the network require a large amount of energy to break
  2. Solubility: Insoluble in water and in organic solvents
  3. Electrical conductivity: Giant covalent structures with no mobile electrons do not conduct electricity (diamond and silicon dioxide) In graphite, each carbon atom is bonded to three other carbon atoms, leaving 1 unbonded electron per atom which is free and mobile to conduct electricity.
• Allotropes are different forms of the same element that have different structural arrangement of atoms.
• Structure of diamond:

  1. Each carbon atom forms covalent bonds with four other carbon atoms.
  2. 3D network structure in a tetrahedral arrangement.
  3. Diamond is hard. Large amount of energy is required to break the strong covalent bonds apart.

  
• Structure of graphite:

  1. Each carbon atom forms covalent bonds with three other carbon atoms.
  2. Layered 2D structure consisting of hexagonal rings.
  3. Graphite is soft and slippery. Only small amount of energy is required to overcome the weak intermolecular forces of attraction between layers of carbon atoms.
  4. Layers of carbon atoms can slide over each other easily.