CH1012
L1 /
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L1 /
7
LCAO APPROXIMATION
Linear Combination of Atomic Orbitals
• Electrons are not localised on an atom they are delocalised in MOs which are
linear combinations of AOs eg. s1s = 1s + 1s
• The electrons belong to the molecule as a
whole (not to the atoms).
whole (not to the atoms).
• The combination of 2 AOs ® 2 MOs
bonding + antibonding
bonding + antibonding
• The bonding MO is always lower in E than
the antibonding MO and the starting AOs.
the antibonding MO and the starting AOs.
• Electrons fill the lowest MOs first (aufbau)
• Only 2 e- are allowed in a MO (Pauli)
