Chemistry topic 2 & 3

• Covalent Bonding

  Covalent Bonding occurs between two non metals.

  In this type of bonding, electrons are shared rather than taken ( ionic) or delocalised ( metallic)

  Electrons can sill move between atoms, but they belong to distinct atoms and cannot move throughout the structure.
• Single bond


  When atoms share one electron
• double bond


  when atoms share two electrons in a bond
• Electronegativity


  is an elements affinity for an electron. This is influenced by its atomic radius and the amount of electron shielding in an atom.
• Polarity

  The more electronegative an atom, the more it wants an electron, therefore even when electrons are shared in a covalent bond it will give rise to unequal sharing. Therefore this will produce partial charges in the molecule, resulting in polar bonds.
• Non Polar Bonds

  Non polar bonds arise when the electrons shared in the molecule are shared equally. This occurs when bonds form between atoms of the same element ( as their electronegativity is the same)
• Polar Bonds

  When two different atoms share electrons, they will form a polar bond.

  The atom that is more electronegative will attract the shared electron more, making the electron cloud around the molecule distorted.
• What notation is used to represent partial positive and negative?

  delta δ
• C-H Bond

  The C-H bond is regarded as non polar ( important to remember for the organic chemistry section)
• Covalent molecules

  Covalent bonding can occur in molecular and non molecular substances.

  A molecular substance is one that exists in molecule form, ie separate molecules.
• Covalent networks

  Some covalent compounds exist in continuous structures or lattices.

  Examples include diamond graphite and silicates
• Steps for empirical formula

  1. Assume a 100g sample of the compound so that the given percentages can be directly converted into grams

  2. Use each element's molar mass to convert the grams of each element to moles. n=m/M

  3. In order to find a whole-number ratio, divide the moles of each element by whichever of the moles from step 2 is the smallest

  4. If all the moles at this point are whole numbers (or very close), the empirical formula can be written with the moles as the subscript of each element

  5. In some cases, one or more of the moles calculated in step 3 will not be whole numbers. Multiply each of the moles by the smallest whole number that will convert each into a whole number. Write the empirical formula
• Empirical Formula vs Molecular/Chemical formula

  1. To find molecular formula from empirical formula find the molecular mass of the empirical formula
  2. Then divide the molecular mass of the actual substance (MF) by the molecular mass of the empirical formula
  3. This gives you the number that you multiply the EF by to get the MF
• VSEPR theory


  Valence Shell Electron Pair Repulsion
• How do atoms within a molecule arrange themselves?

  atoms within molecules are arranged into positions which minimise repulsion around the central atom according to VSEPER theory
• Linear Shape

  • No lone electron pairs
  • Line shape
  • 2-3 atoms?
• V shaped

  • spare electron pair bends the bonds into a v to reduce repulsion
• Trigonal planar

  • no lone electron pairs
  • 4 atoms with one central
  • flat plane shape
• Trigonal pyramid

  • lone electron pair provides pyramid shape to reduce repulsion
  • 4 atoms
• Tetrahedral

  • 5 atoms
  • trigonal pyramid with an extra electron added to the top of the pyramid
• Polar bonds vs molecules

  Polar bonds can exist within a non-polar molecule. This is because the polarity can cancel out across the molecule depending on its shape.
• Are primary or secondary bonds stronger?

  secondary bonds are much weaker than primary metallic, ionic, and covalent bonds.
• Three types of intermolecular forces/ secondary interactions

  • dispersion forces
  • dipole dipole interactions
  • hydrogen bonding
• Dispersion forces

  Dispersion forces are weak forces which exist between most atoms and molecules. They arise due to temporary dipoles.

  They are the main force in NON POLAR MOLECULES
• Dipole Dipole interactions

  This force occurs between polar molecules. As polar molecules have a permanent dipole ( positive and negative end). As these positive and negative ends can attract each other ( between molecules) this force of attraction is known as a dipole dipole interaction.
• Hydrogen bonding

  Hydrogen Bonding is just a specific case of dipole-dipole interaction. However it is a significantly stronger form of this secondary interaction.

  It occurs in polar molecules that have a hydrogen atom directly attached to either a nitrogen, Oxygen or Florine atom (NOF).

  This is due to the fact that the difference in electronegativity between H and either N, O or F is large. Thus there is a stronger dipole formed in the molecule, thus a larger slight positive or slight negative charge.
• organic chemistry 


  the study of carbon and its compounds (excluding carbon dioxide and carbonates (CO3 2−)
• inorganic chemistry


  inorganic chemistry is the study of all of the elements, excluding carbon
• functional groups


  how organic compounds are grouped, related to their chemical and physical properties
• alkane


  single bonds
• alkene


  atleast one double bond between carbons
• alkyne


  atleat one triple bond between carbons
• Naming organic molecules

  1. Find the longest continuous carbon chain in the molecule

  2. The molecule is named for this chain and will be a derivative of whatever this longest chain is
• Alkyl groups


  Hydrocarbon groups attached to the main chain, such as methyl, ethyl, propyl, etc. The group name is determined by the number of carbons in the chain usually minus one hydrogen where it attaches to the main chain
• Naming organic molecules

  1. Number the carbons along the main chain identified in step 1

  2. Identify the attached groups

  3. The carbon number where the group is attached appears in the name of the molecule

  4. By convention, the numbering should start so as to give the shortest possible length of the main chain before the first group is encountered
• Prefixes

  di-, tri-, tetra- etc. for the number of times a given alkyl group appears
• Naming organic molecules


  If several groups appear, then list them in alphabetical order in the name of the molecule
• meth-

  1
• eth

  2
• prop


  3