CHEM1011 Lecture Notes - Lecture 14: Trigonal Pyramidal Molecular Geometry, Trigonal Planar Molecular Geometry, Lone Pair
CHEM1011 – Chemistry 1A – Part 9
LEC 14: *Continued
Molecular shapes
• Valence Shell Electron Pair Repulsion (VSEPR) Theory: atoms in a molecule want
to be as far away as possible from each other in order to minimise repulsion
charges as much as possible.
o Double bonds are treated as one region of electron density for VSEPR
purposes.
o Lone pairs are more demanding of space and alter the shape of the molecule
to accommodate for its repulsive forces. Not a part of final description of
shape. Only drastically change structure if they are on the central atom.
o Multiple bonds (double, triple) have larger e- clouds than single bonds,
therefore repel neighbouring e- pairs more strongly.
▪ Single < Double < Triple < Lone pair
o In resonance structures, the pair of e-s that are different spend an equal
amount of time across all bonds.
• CH4 vs NH3 vs H2O
o Lone pairs cause the other hydrogen atoms to shift similar to as if there was
an atom in its place (but are more demanding of space). Lone pairs arent
taken into account in the description, hence the different names for the
molecular shapes of the molecules.
• AX2 : Linear. Two bonding pairs.
• AX3 : Trigonal planar. Three bonding pairs.
• AX2E : Bent or V-shaped. Two bonding pairs, one lone pair.
find more resources at oneclass.com
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Document Summary
Not a part of final description of shape. Only drastically change structure if they are on the central atom: multiple bonds (double, triple) have larger e- clouds than single bonds, therefore repel neighbouring e- pairs more strongly, single < double < triple < lone pair. Lone pairs aren(cid:859)t taken into account in the description, hence the different names for the molecular shapes of the molecules: ax2 : linear. Two bonding pairs: ax3 : trigonal planar. Three bonding pairs: ax2e : bent or v-shaped. Two bonding pairs, one lone pair: ax4 : tetrahedral. Four bonding pairs: ax3e : trigonal pyramidal. Three bonding pairs, one lone pair: ax2e2 : bent or v-shaped. Two bonding pairs, two lone pairs: ax5 : trigonal bi-pyramidal. Four bonding pairs, one lone pair: ax3e2 : t-shaped. Three bonding pairs, two lone pairs: ax2e3 : linear. Four bonding pairs, three lone pairs: ax6 : octahedral. Six bonding pairs: ax5e : square pyramidal.
