This is a summary of my notes on sigma & pi bonds , covalent bonding, iconic bonding , metallic bonding and giant covalent structures from IB HL Chemistry.
Covalent Bonding: Lewis Structures
- Can be Molecular or Giant Lattice
Octet = a full outer shell of electrons/maxed out bonds
Drawing Lewis Structures:
1. Find the total no. valence electrons, add an electron for every negative charge, subtract an
electron for every positive charge
2. Decide on your central atom (the least electronegative, except H) and draw the bonds,
subtract the electrons used from the total in step 1
3. Assign the leftover electrons to the terminal atoms, subtract the electrons from the total in
step 2
4. (if necessary) Assign any leftover electrons to the central atom.
If the central atom has an octet or exceeds an octet, you’re usually done
If the central atom doesn’t have an octet, create multiple bonds
*Period 3 and onwards can exceed the octet rule
Electronegativity:
- In a covalent bond between two different atoms, the atoms do not attract the electron pair
equally.
- The more electronegative an atom, the more able it is to pull an electron pair towards itself.
- This can be measured on a variety of scales, we use Pauling’s scale of electronegativity.
- Fluorine is the most electronegative atom
- Some molecules will be polar
E.g. water, due to the high electronegativity difference between H & O
- Electronegativity on the periodic table increases from left to right because of the amount of
electrons in the atom’s outer shell
- It decreases going down the periods as there is a greater nuclear charge due to the amount of
electron shells in the atom
Example: NO3- Example: PF5 Example: CO2
- Total number of
electrons?
- Least electronegativity
in the middle
- Leftovers to the
terminal atoms
- Double bonds?
- Covalent bonds are stronger than h-bonds
, Giant Covalent and Ionic Structures
Silicon & Silicon Dioxide
- SiO2 = quartz
- Macromolecular covalent structures (similar to diamonds)
- Regular arrangement of atoms
- Each silicon atom is bonded to four oxygen atoms in a tetrahedral
array
- Each oxygen is bonded to two silicon atoms
- Due to two lone pairs on each oxygen atom, the basic shape about
each Si–O–Si unit is based on a tetrahedron
- Solid at room temperature
- The oxide has high melting & boiling points as the covalent bonds
between the atoms must be broken in order to melt/boil the structure
and this requires a lot of energy
- Main compound found in sand
- Uses: concrete production, in hydraulic fracturing, glass production, in toothpaste production
(to remove plaque), as a sedative
Buckminster Fullerenes (C60) and Carbon Nanotubes
- Fullerene = an allotrope s carbon whose molecules are connected by single or doble covalent
bonds – creating a mesh-like structure
- Named after Buckminster Fuller who made a similar architectural structure
- A molecular covalent structure (not a giant covalent because it is
not an infinite structure – each molecule is a fixed size)
- Formed of 60 carbon atoms covalently bonded together
- Between each C60 molecule there are London intermolecular forces
- Insoluble in water but soluble in some organic solvents (such as
benzene)
- Does not conduct electricity – there are delocalised electrons within
each molecule but they cannot move between the molecules so
electricity cannot be conducted
- Not very good at transmitting light – can be used as goggles to
protect people’s eyesight when looking at laser beams/intense light sources
- One-layer thick sheets of carbon atoms arranged in a hexagonal mesh (layer of graphene), the
sheets are wrapped in a cylinder shape and bonded together forming a tube
- Each carbon atom is bonded to 3 other carbon atoms –
leaving each carbon atom with a delocalised electron
- High thermal/electrical conductivity – due to delocalised
electrons’ ability to move through the structure – used in
computer circuits as very thin wires
- High tensile strength – useful for reinforcing composite
materials (e.g. tennis rackets and golf clubs),
Very light, strong & durable
- Very flexible
- High melting/boiling point
- Hollow tube is useful for drug/medicine cases as the tube
structure can help transport molecules around the body
Diamond, Graphite & Graphene
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