Bonding notes

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  • Created on: 15-05-14 18:57
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Bonding
Ionic
Evidence for the existence of ions
- Physical properties: high mpt and high bpt, molten and
aqueous samples are conductive whereas solid samples are
not, soluble in polar compounds
- Electron density maps show areas with low/ no electron
density between ions
- Conductivity of aqueous solutions shown through electrolysis
Ionic compounds
- Cations are +ve, anions are -ve
- Ionic crystals are giant lattices of ions held together by
strong net electrostatic attraction between the ions
- Isoelectronic: same electronic structure
- Ionic radius increases down a group as there are
more shells of electrons
- Across a period, ionic radius decreases as there are
more protons in the nucleus to pull them in. The first
non-metal will have a larger ionic radius than the last
metal due to the extra layer of electrons, but then the
non-metals follow the same trend
- How to measure LE of NaCl with Born-Haber cycle:
Atomise Na and Cl
Remove electron from Na (1st IE)
Add electron to Cl (1st EA)
Using the Hf[NaCl], calculate LE
Shows why NaCl2 does not form, as 2nd IE of Na is far
more endothermic than 1st IE, so would be less stable
Though Hf [MgCl] is exothermic, Hf [MgCl2] is more so,
so more stable, as either Mg+ would be needed, or Cl2-
Testing the ionic model
-Values measured with B-H cycle are not the same as ones
calculated with electrostatic theory
-This is because all ionic compounds are covalent to a certain
degree
-Group 1 halides values differ very little, so they are mostly ionic
-BeF2 though, has very different values
-Explanation through polarisation:
- Polarisation: distortion of electron cloud
in ion/ molecule by nearby charge
- Cations with high charge density (high
charge, low radius) will draw electrons
towards themselves more, so will form
compounds with a higher degree of
covalent character (= high polarising
power)
- Large anions with more elctrons are more polarisable than
small ones (i.e. I- is more polarisable than F-)
- More covalent character = stronger lattice = higher LE
value than predicted

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Covalent
- Strong bond formed by electrostatic attraction between positive nucleus and negative electrons
Evidence
- Properties of simple molecular substances:
- Gases, liquids or soft solids at rtp due to weak
intermolecular forces (low mpt, bpt)
- Are not conductors as they dont contain ions or free
electrons to carry electric charge
- More soluble in non-polar solvents than water (E.g. if
methane were to dissolve, it would have to break
hydrogen bonds = a lot of energy.…read more

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Metallic
- Giant lattice of metal ions in a sea of delocalised electrons
- The strong attraction between the free electrons and the positive ions holds the structure together
- Properties:
- High bpt and mpt (much energy needed to break metallic bonds)
- High density
- Good conductors of heat and electricity (free electrons can carry charge, but also move
faster when heat energy is given to them, which they then carry throughout the metal)
- Malleable (bonds are not directional as the electrons…read more

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