Bonding

• Types of chemical bonding

  • Ionic bonding
  • Metallic bonding
  • Covalent bonding
• Ionic bonding


  • Formed between metal and non-metal atoms
  • Involves electrons being transferred to gain full outer shells
  • Found in ionic compounds e.g. sodium chloride (NaCl) and aluminium oxide (Al2O3)
  • Results in giant ionic structures
• Metallic bonding


  • Formed between metal atoms
  • Involves electrons becoming delocalised
  • Found in metals e.g. copper (Cu), iron (Fe)
  • Results in giant metallic structures
• Covalent bonding


  • Formed between non-metal atoms
  • Involves electrons being shared to gain full outer shells
  • Found in covalent compounds
  • Results in simple molecular or giant covalent structures
• Metals, e.g. copper, consist of a regular arrangement of positive ions surrounded by a sea of delocalised electrons
• The outer shell electrons from each atom become delocalised in metallic bonding
• The strong electrostatic forces between the positive ions and the sea of delocalised electrons hold the metallic structure together
• The delocalised electrons are free to move throughout the metallic structure, making metals good conductors of electricity
• The layers of metal slide over each other when a force is applied but the structure does not break apart because it is held together by the delocalised electrons, making metals malleable
• Ionic compounds


  • Consist of a regular arrangement of positive and negative ions
  • Electrons from the metal atoms are transferred to the non-metal atoms so that all the atoms have full outer shells
  • The oppositely charged ions are attracted together, forming a regular arrangement called a lattice or giant ionic structure
• Ionic compounds conduct electricity when molten or in aqueous solution because the ions are free to move
• Ionic compounds have high melting points because of the strong electrostatic forces between the oppositely charged ions
• Drawing dot and cross diagrams to represent ionic compounds

  1. Draw the electron arrangement of the metal atom, representing the electrons as dots, and missing off the outer shell electrons which have been lost in ionic bonding

  2. Draw the electron arrangement of the non-metal atom separately and to the right of the ion you have drawn, representing the electrons as crosses. Fill in any spaces in the outer shell of the atom with dots (electrons from the metal atom that have been gained in bonding)

  3. Draw a set of square brackets around each of the ions

  4. Work out the charges on the ions. The metal is always positive (having lost negative electrons). The size of the positive charge is the amount (the no. of outer shell electrons that have been lost). The non-metal is always negative, and the size of the charge is the no. of dots (electrons it has gained from the metal) in its outer shell

  5. Work out the formula of the compound by balancing the charges on the ions - the compound must be neutral overall

  6. Write the number of metal atoms in the formula in front of the metal ion and outside the square brackets. Do the same for the non-metal ion.
• The vast majority of covalent substances have simple molecular structures - they exist as molecules, or groups of atoms chemically joined
• The atoms in covalent molecules are held together by the strong attraction between the positive nuclei of the atoms and the negatively charged shared electrons
• There are only weak intermolecular forces between the covalent molecules, so they are easily pulled apart
• Covalent substances made up of discrete molecules do not conduct electricity
• A very small number of covalent substances exist as giant covalent structures, such as diamond and graphite
• Giant covalent structures have strong covalent bonds throughout the structure, so lots of energy is needed to break the many covalent bonds between atoms, giving them high melting and boiling points
• Diamond


  A form (allotrope) of carbon with a giant covalent structure where each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement
• Graphite


  A form (allotrope) of carbon with a giant covalent structure where the carbon atoms are arranged in hexagonal layers, with weak intermolecular forces between the layers
• Drawing dot and cross diagrams to represent covalent substances

  1. Work out the formula of the covalent substance

  2. Use the group number in the periodic table to work out how many electrons each of the atoms has in its outer shell and therefore how many electrons each atom needs to gain for a full outer shell

  3. Work out how many electron pairs each atom needs to share - if an atom needs 1 element for a full outer shell it needs to share 1 pair, if it needs 2 it needs to share 2 pairs etc.

  4. Use this to work out how the atoms are arranged. Draw the electronic structures of all the atoms, interlocking the outer shells of the atoms that are bonded together but leaving them empty of electrons

  5. Draw in the shared pairs of electrons where the outer shells are interlocked, ensuring that there is one dot and one cross in every pair

  6. Fill in the rest of the outer shell electrons for each atom outside the interlocking area, ensuring that the total number of crosses or dots in the outer shell is the same as the group number
• The number of protons is the atomic number
• Isotopes are atoms with different masses but the same number of protons, so they have the same chemical properties.
• The mass number is equal to the sum of the proton and neutron numbers.
• The number of neutrons can be found by subtracting the mass number from the atomic number
• Electrostatic forces


  The attractive forces that exist between particles that have opposite charges (positive and negative)
• Positive ions


  Atoms or molecules that have lost one or more electrons, giving them a positive charge
• Sea of delocalised electrons


  In metals, there is a "sea" or "cloud" of electrons that are not associated with any particular atom or molecule. These electrons are free to move throughout the metal.
• Hold the metallic structure together

  The positive ions and the sea of delocalised electrons are attracted to each other due to their opposite charges. This creates a strong force that holds the metal together, allowing it to maintain its shape and structure.