Covalent Bonding

• What is a molecule?

  When two or more atoms bond together
• What are molecules held together by?

  • Strong covalent bonds
  • Can be single double or triple
• Single bonds

  In covalent bonding 2 atoms share electrons so they have both got a full outer shells
• What are simple covalent compounds?

  • Compounds made up of many individual molecules
  • Atoms in the molecule held together by weak intermolecular forces
  • Low melting and boiling points
• Types of Giant covalent structures

  • Graphite
  • Diamond
• Structure of Graphite

  • Sheets of flat hexagons covalently bonded with 3 bonds each
  • The fourth outer electron of each carbon atom is delocalised
  • The sheets of hexagons are bonded together by van der Waals forces
• Properties of Graphite

  • Weak bonds between layer SO easily broken as layers slide over each other
  • Delocalised electrons are free to move SO conduct electricity
  • Layers are far apart compared to the length of the covalent bonds SO has a low density and used to make strong lightweight sports equipment
  • Strong covalent bonds in hexagon sheets SO has a high melting point
  • Insoluble - Strong covalent bonds hard to break
• Structure of diamond

  • Carbon atoms
• Structure of diamond

  • Carbon atoms each covalently bonded to four other carbon atoms
  • Tetrahedral shape
  • Crystal lattice structure
• Properties of Diamond

  • High melting point - over 3800K
  • Hard
  • Good thermal conductor - vibrations can travel easily through the stiff lattice
  • Can't conduct electricity - all outer electrons are held in localised bonds
  • Insoluble
• What is dative covalent bonding?

  • One of the atoms provides both of the shared electrons
  • Form when one of the atoms in the bond has a lone pair of electrons, and the other doesn’t have any electrons available to share.
• Example of dative covalent bonding

  Ammonium ion

  • Forms when the nitrogen atom is an ammonia molecule donates a pair of electrons to the proton (H+)
• What do bonding and lone pairs of electrons exist as?

  Charge clouds
• What is a charge cloud?

  A region in an atom where an electron is likely to be found.
• What is the Valence Shell Electron Pair Repulsion Theory?

  • Electrons are all negatively charged so charge clouds repel each other
  • The shape of a charge cloud effects how much it repels other charge clouds
• Why are bond angles sometimes reduced?

  • Lone pair charge clouds repel more than bonding pair charge clouds
  • So bonding pairs are pushed together by lone pairs
• Strength of repulsion between electron

  1. Lone pair - lone pair
  2. lone pair - bonding pair
  3. bonding pair - bonding pair
• How to work number of electron pairs

  1. Find central atom
  2. Work out electrons in outer shell
  3. Add 1 electron for every atom the central atom is bonded to
  4. Add up all electrons and divide by 2 to find number of electron pairs
  5. Find lone pairs (if any)
• Shapes of molecules and ions:

  Linear

  • 2 atoms attached to center atom
  • 0 unshared pairs
  • 180
  • AB2
• Trigonal Planar

  • 3 atoms attached to central atom
  • 0 lone pairs
  • 120
  • AB3
• Tetrahedral

  • 4 atoms bonded to center atom
  • 0 lone pairs
  • 109.5
  • AB4
• Trigonal planar

  • 3 atoms attached to center atom
  • 0 lone pairs
  • 120
  • AB3
• Octahedral

  • 6 atoms attached to center atom
  • 0 lone pairs
  • 90
  • AB6
• Bent

  • 2 atoms attached to center atom
  • 2 lone pairs
  • 104.5
  • AB2 E2
• Trigonal pyramidal

  • 3 atoms attached to central atom
  • 1 lone pair
  • 107
  • AB3 E
• How many degrees does an angle decrease by for each lone pair?

  2.5 degrees
• Types of Intermolecular forces?

  • Van der Waals forces
  • Permanent dipole-dipole forces
  • Hydrogen bonding
• What are van der Waals forces?

  • Cause atoms to be attracted to each other
  • Electron charge clouds move quickly
  • A temporary dipole is created which creates more
  • But are create and destroyed due to the constantly moving electrons
• What happens to van der Waals forces when yoU do down group 0?

  Increase
• When are permanent dipole-dipole forces found ?

  Between polar molecules.
• When does hydrogen bonding occur?

  In molecules which contain

  • Hydrogen
  • Nitrogen
  • Oxygen
  • Fluorine
• Are hydrogen bonds strong or weak?

  Strongest Intermolecular force but weaker than covalent and ionic bonds
• Order of strength of intermolecular forces

  strongest -

  • Hydrogen bonding
  • Permanent dipole-dipole interaction
  • van der Waals forces
• Do covalent structures conduct electricity?

  No because there are no free ions or electrons to carry the charge.
• Melting point of covalent structures

  • Low melting points
  • because the weak forces between molecules are easily broken.
• Solubility of covalent structures

  Dissolve in water depending on how polarised the molecules are
• Define electronegativity

  The power of an atom to attract the shared pair of electrons in a covalent bond
• How is electronegativity affected as you go across a period?

  Increase

  • Nuclear charge increases across the period
  • Similar shielding across the period
  • So ability to attract shared pair of electrons in the covalent bond increases
• What is a polar molecule?

  A molecule with a positive and negative end.
• What type of dipole do polar bonds have?

  Permanent dipole