chemical bonding Flashcards
(9 cards)
What is the octet rule?
“Atoms bond together in order to achieve a stable electron configuration, i.e. one that has a full outer shell”
What is lewis bonding?
G. N. Lewis suggested that the chemical bond consisted of a pair of electrons shared between two atoms. This makes a single bond between the two atoms.
What is the problem with lewis bonding?
Problem:
- This assumes that all valence electrons are equal, but this is not the case since bonds are formed by the overlap of atomic orbitals.
- It is difficult to deal with excited states, e.g. some bonds change length (get weaker) when molecules are excited.
- Some experimental observations are impossible to explain, e.g. O2 has unpaired electrons and is therefore blue, despite the Lewis structure suggesting that both oxygen atoms have a full octet.
Describe hybridisation with multiple bonds
Consider ethene, H2C=CH2.
Instead of sp3 hybrids, the C form three sp2 hybrids each, by mixing the 2s and two 2p orbitals.
The third 2p orbital is unaffected.
The three sp2 hybrids form three bonds, one to the other C and two to the H’s. These form a trigonal planar geometry, with the unaffected 2p orbital orthogonal to the plane.
For a triple bond the C is sp hybridised and the two unaffected 2p orbitals give two orthogonal π bond
What is the LCAO method?
atomic orbitals
The Linear Combination of Atomic Orbitals. The LCAO method is based upon the approximation that a molecular orbital wavefunction for two electrons can be considered as being the product of the two one-electron wavefunctions from which it is derived.
What are the two different types of wave interference?
constructive interference (in-phase combination, bonding MO)
destructive interference (out-of-phase, anti-bonding MO)
What MO has the most energy?
Since greater electron density between the nuclei leads to greater stabilisation, and lower energy is a sign of greater stability, we can write the order of MO energies as:
Anti-bonding (highest energy)
Non-interacting AOs (in the middle)
Bonding MO (lowest energy)
I.e. the bonding MO is more stable than the AOs before bonding, whilst the anti-bonding MO is less stable.
What is bond order?
This is fine for simple molecules, but some bonds are very weak, or have intermediate types of bond. It is therefore useful to re-define the ”number of bonds between two atoms” as the bond order.
𝑏𝑜𝑛𝑑 𝑜𝑟𝑑𝑒𝑟 = ((𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑏𝑜𝑛𝑑𝑖𝑛𝑔 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛) − (𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑎𝑛𝑡𝑖 − 𝑏𝑜𝑛𝑑𝑖𝑛𝑔 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠))/2
What does diamagnetic and paramagnetic mean?
Diamagnetic - all electrons paired: repelled by a magnetic field
Paramagnetic - one or more unpaired electrons: attracted to a magnetic field
