Note
Helium and hydrogen can only hold a maximum of two electrons!!! AND – some atoms (like Boron,
Beryllium and Aluminum) can be happy with less than an octet!!!
Some compounds (like nitric oxide) can contain one or more atoms with an odd number of electrons
(usually 1 or 7) – therefore they do not have a satisfied octet!!!
Any atom that contains a 3d orbital (3 rd period and beyond) occupied with electrons can actually use this orbital for bonding – the result is that the atom can actually hold more than eight electrons!!! This is called an extended or expanded octet.
Background
Many chemical reactions, especially those in living things, depend on the ability of two compounds to come into contact with each other. The shape of the molecule determines whether or not molecules can get close enough to react.
Once a Lewis structure is drawn, you can determine the molecular geometry , or shape, of the molecule. The model used to determine the molecular shape is referred to as the Valence Shell Electron
Pair Repulsion model or VSEPR model. This model is based on arrangement that minimizes the repulsion of shared and unshared pairs of electrons around the central atom.
Terminology
Bond angles: Angles created from the central atom from any two terminal atoms.
Lone Pairs: 2 electrons that are not used in bonding.
Electron Pair Geometry: Provides a guide for the bond angles in a molecule.
Molecular Geometry: Describes the shape of the molecule .
Electron Domains: Refers to the number of lone pairs or bonded electrons around a central atom in a molecule.
ABE Formula: The ABE formula is used to help predict the correct geometry of a Lewis structure.
A: The central atom.
B: Number of bonded atoms to the central atom.
E: Number of unshared pairs or lone pairs of electrons on the central atom.
BIG QUESTION: How can we arrange molecules so there is minimal repulsion and atoms are as far away from each other as possible?
There are five basic shapes that all molecular geometries can be derived. They are linear, trigonal planar, tetrahedral, trigonal bipyrimidal, and octahedral.
Linear: Linear is derived from the word line. This makes sense because the 3-D shape is a line.
Has two electron domains; therefore the formula is AB
2 .
Trigonal Planar : Trigonal is derived from triangle which it gets it shape from. Planar is used to indicate that all atoms are on the same plane.
Has three electron domains; therefore the formula is AB
3 .
Tetrahedral : Tetra derived from having four bonds.
Has four electron domains; therefore the formula is AB
4
Trigonal Bipyrimidal : Involves exception to the octet rule because the central atom contains more than a full octet.
Has five electron domains; therefore the formula is AB
5 .
Octahedral – Also contains exception to the octet rule because the central atom contains more than a full octet.
Has six electron domains; therefore the formula is AB
6 .

Double, Triple Bonds:
Double or triple bonds count as "one pair" of electrons for the purpose of establishing the electron pair geometry. In carbon dioxide, the two double bonds count as two pairs of electrons around the carbon atom, predicting linear geometry.
Alkanes and Alkenes
Naming Hydrocarbons:
Hydrocarbons are compounds which are composed of hydrogen and carbon units. The majority of hydrocarbons are found in crude oils in which organic matter has decomposed to form these compounds.
1. Alkanes
Alkanes are hydrocarbons that have only single bonds between atoms. a) All alkanes have a formula of C n
H
2n+2 b) All alkanes end with 'ane'. c) The number of carbons corresponds to the number of carbon atoms in the molecule.
Examples:
Ethane:
2. Alkenes a) All alkenes have the formula C n
H
2n
. b) All alkenes end with ‘ene’.
Propane: c) The number of carbons corresponds to the number of carbon atoms in the molecule.
Examples:
Decene: