Chemical Geometry
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True or False: Lone pairs of electrons occupy more space than bonded pairs in a molecule.
True
Which molecular shape is associated with three bonded atoms and no lone pairs?
Trigonal planar.
What is the significance of bond angles in chemical geometry?
They influence the reactivity and properties of the molecule.
What does VSEPR stand for?
Valence Shell Electron Pair Repulsion – theory that electron pairs repel and spread out to minimize repulsion, determining molecular shape.
2 electron domains → Shape?
Linear Bond angle is 180 degrees Example: CO2,
3 electron pairs → Shape?
Trigonal planar Bond angle 120 degrees Example: BF3, NO3-, ethene, SO2
4 electron domains→ Shape?
Tetrahedral Bond angle: 109.5° Example: CH₄, Ammonia, Water
5 electron domains → Shape?
A: Trigonal pyramidal Bond angle: ~107° Example: NH₃
Lone pairs vs bonding pairs – who repels more? .
A: Lone pairs repel more → they squish bond angles smaller than ideal.
What’s the difference between electron geometry and molecular geometry?
Electron geometry = based on all electron domains (includes lone pairs) Molecular geometry = based on actual atom positions (ignores lone pairs when naming the shape)
What does it mean bond additivity breaks down??
It is an indicator that there are multiple bonds in the compound
Which has the larger radius atoms or anions?
Atomic radii are much smaller than anions due to anions containing more electrons. The additional electrons cause a greater repulsive force and the nucleus cannot hold onto the electrons as tightly due to electron shielding.
Why do atomic radii increase as you go down a group?
The number of electron shells increases and nuclear charge is not strong enough to counteract this expansion
Why does atomic radii decrease across a period?
Because as you move across a period, protons are added to the nucleus, increasing the nuclear charge. Electrons are added to the same energy level, so there’s no extra shielding. The stronger attraction between the nucleus and electrons pulls the electron cloud closer, making the atom smaller.
Why do cations have a smaller atomic radius than their neutral atoms?
Because cations lose electrons, often losing an entire energy level (shell). There are now more protons than electrons, so the nucleus pulls the remaining electrons closer, shrinking the atom.
Why do the bonding pairs have less repulsive forces?
They are less repulsive due the dual attraction to the positive centres which stabilise the bonding pair
What geometry is formed when a central atom has 10 valence electrons (5 electron domains)?
Trigonal bipyramidal 3 atoms in a triangle (equatorial, 120°) 2 atoms above and below (axial, 90°) Example: PCl5, SF4
What geometry forms with 12 valence electrons (6 electron domains)?
Octahedral 90° between all positions Example: SF6, IF5
What shape forms with 14 valence electrons (7 electron domains)?
Pentagonal bipyramidal 5 in a pentagon (equatorial) 1 above, 1 below Example: IF7
What is resonance?
Resonance occurs when a molecule can be represented by 2 or more valid Lewis structures. The actual structure is a hybrid of these forms.
Compare the bond angles in methane (CH₄), ammonia (NH₃), and water (H₂O), and explain why they differ.
All three have tetrahedral electron pair geometry (4 regions of electron density). CH₄: 4 bonding pairs, 0 lone pairs → 109.5° bond angle (even repulsion). NH₃: 3 bonding pairs, 1 lone pair → ~107° (lone pair repels more strongly, reducing angle). H₂O: 2 bonding pairs, 2 lone pairs → ~104.5° (2 lone pairs cause even greater repulsion). ✅ Lone pairs repel more strongly than bonding pairs, decreasing bond angles as their number increases.