First 277 words of the document:
Shapes of Molecules
The molecular shape depends on the number of pairs of electron pairs in the outer
shell around the central atom.
Electron pairs repel each other
1. Electrons are all negatively charged, so it's pretty obvious that electron pairs will repel
each other as much as they can.
2. This sounds straightforward, but the type of the electron pair affects how much it repels
other electron pairs. Lone pairs repel more than bonding pairs.
3. So, the greatest angles are between lone pairs of electrons, and bond angles between
bonding pairs are often reduced because they are pushed together by lone-pair
o Lone-pair/lone-pair bond angles are the biggest.
o Lone-pair/bonding-pair bond angles are the second biggest.
o Bonding-pair/bonding-pair bond angles are the smallest.
The central atoms in these molecules all have four pairs of electrons in their outer shells, but
they're all different shapes.
When there are no lone pairs the angle is 109.5°.
Then, with every lone pair that is added the angle decreases by
The lone pairs repel the bonding pairs.
Linear molecules have two electron pairs and have an angle of 180°.
Trigonal planar molecules have 3 electron pairs and have an angle of 120°.
Octahedral molecules have 6 electron pairs and have an angle of 90°.
Tetrahedral molecules have 4 electron pairs with no lone pairs and have an angle of 109.5°.
Trigonal pyramidal molecules have 4 electron pairs with 1 lone pair and have an angle of 107°.
Non-linear molecules have 4 electron pairs with 2 lone pairs and have an angle of 104°.