Core chemistry 1
Molecular Structure
Year I
,10110121 Summary of lecture 5
• Ground state electron configurations of multi-electron atoms
• Pauli exclusion principle
• Relativity - introduces a 4th dimension, and 4th quantum number, ms
• Lithium, 2s lower energy than 2p
– both 2s and 2p are screened from the 3+ nuclear charge
– 2s is more penetrating of the 1s than the 2p and so is less screened
• Carbon, Hund’s rule of maximum spin multiplicity
• Sodium, 3s lower energy than 3p
– same reasons as for lithium
6.2
Molecular Structure
6.3
, Molecular structure - VSEPR
• Before looking at bonding, we need to understand the shapes of
some simple main group species
• Then look at a bonding model (Valence Bond) which rationalises
those shapes
• Valence Shell Electron Pair Repulsion
– a simple theory which successfully predicts the shape of molecules
– assumes that shape is determined by electron-electron repulsions
• Sidgwick and Powell (1940)
• extended by Gillespie and Nyholm
6.4
VSEPR - assumptions
• Atoms in a molecule are bound together by electron pairs, called BONDING
PAIRS. Multiple bonds involve more than one bonding pair.
• Pairs of electrons not involved in bonding are called LONE PAIRS or NON-
BONDING PAIRS
• Bonding pairs and lone pairs in a molecule adopt positions in which their
mutual interactions are minimised. Two reasons:
– (i) Electrons are negative charges and repel by electrostatics
– (ii) Pauli principle - electrons seek to get away from each other
• Lone pairs occupy more space than bonding pairs
• Double bonds occupy more space than single bonds
6.5
Molecular Structure
Year I
,10110121 Summary of lecture 5
• Ground state electron configurations of multi-electron atoms
• Pauli exclusion principle
• Relativity - introduces a 4th dimension, and 4th quantum number, ms
• Lithium, 2s lower energy than 2p
– both 2s and 2p are screened from the 3+ nuclear charge
– 2s is more penetrating of the 1s than the 2p and so is less screened
• Carbon, Hund’s rule of maximum spin multiplicity
• Sodium, 3s lower energy than 3p
– same reasons as for lithium
6.2
Molecular Structure
6.3
, Molecular structure - VSEPR
• Before looking at bonding, we need to understand the shapes of
some simple main group species
• Then look at a bonding model (Valence Bond) which rationalises
those shapes
• Valence Shell Electron Pair Repulsion
– a simple theory which successfully predicts the shape of molecules
– assumes that shape is determined by electron-electron repulsions
• Sidgwick and Powell (1940)
• extended by Gillespie and Nyholm
6.4
VSEPR - assumptions
• Atoms in a molecule are bound together by electron pairs, called BONDING
PAIRS. Multiple bonds involve more than one bonding pair.
• Pairs of electrons not involved in bonding are called LONE PAIRS or NON-
BONDING PAIRS
• Bonding pairs and lone pairs in a molecule adopt positions in which their
mutual interactions are minimised. Two reasons:
– (i) Electrons are negative charges and repel by electrostatics
– (ii) Pauli principle - electrons seek to get away from each other
• Lone pairs occupy more space than bonding pairs
• Double bonds occupy more space than single bonds
6.5
