CHEM 221: Bonding & Lewis Structures

image by Sean MacEntee

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Octet Rule
The "Octet Rule" is a principle used that describes the idea of electron configuration stability. In other words, atoms will tend to fill their electron shells in such a way as to create the most stable configuration.

Ionic Bonds, Ionization Energy, Electron Affinity
Although ionic bonds are, for the most part, a matter of inorganic chemistry, I discuss it briefly for reviewing purposes. Ionic bonds are bonds between two or more atoms that are created when an electron is transferred from one atom to another atom. This transfer of electron creates a separation of charge and it is this separation of charge that holds the atoms together as per Coulomb's law.

Atomic Orbitals
In basic terms, atomic orbitals are regions of space in which an electron, or a pair of electrons, are found. It is important to understand that these regions are only probabilities and that in reality an electron can be found as far away from the nucleus as possible. So for example, following a mathematical calculation, we can say that there is a 95% probability that an electron is found within some orbital shape. This "uncertainty" about the location of an electron comes from "Heisenberg's Uncertainty Principle", which states that it is impossible to determine both the position of an electron and its momentum at the same time.

Polar and Nonpolar Covalent Bonds
Covalent bonds, unlike ionic bonds, are formed through the sharing of electrons between two or more atoms. The distribution of electrons in the bond depends on what types of atoms are present. Polar bonds contain atoms that have different electronegativity values while non-polar bonds are those bonds that have identical or very similar electronegativity values.

Introduction to Lewis Structure
​Lewis dot structures are essentially shortcut representations of electrons around the atom, molecule or compound. Although Lewis structures do not actually represent realistic conditions, they allow us to model these representations on the two dimensional paper. Let us take the fluorine atom for instance in its non-diatomic form. The fluorine atom contains a total of nine electrons surrounding the nucleus. The nucleus is surrounded by regions of space called atomic orbitals, which each contain a maximum of two electrons. The first two electrons are placed into 1s orbital, the second two electrons are placed into the 2s orbital, the next three electrons are placed into the 2px, 2py and 2pz orbital respectively. The final two electrons are placed into the 2px and 2py. For the fluorine atom, the Lewis dot structure only shows the electrons found in the 2s and 2p orbitals. For simplification purposes, the 1s orbital and its electrons are omitted.

Drawing Lewis Structure Example #1
Lewis dot structures are essentially shortcut representations of electrons around the atom, molecule or compound. Although Lewis structures do not actually represent realistic conditions, they allow us to model these representations on the two dimensional paper. (a) We are combining three hydrogen atoms with one carbon atom. Note that each neutral hydrogen atom has one electron in its 1s orbital and the carbon atom has six electrons distributed among its three orbitals (1s,2s and 2p orbitals). (b) We are combining two hydrogen atoms that each has one electron in the 1s orbital with a carbon atom that has a total of 6 electrons. (c) In order to create an ammonia compound, we need to combine three neutral hydrogen atoms (three electrons total) and a nitrogen atom, which contains a total of seven electrons. (d) In this part, we are combining the compound that we formed in part (a) with an additional nitrogen atom. (e) In order to form the final compound, we need to combine a total of four carbon atoms with six hydrogen atoms.

Drawing Lewis Structure Example #2
Lewis dot structures are essentially shortcut representations of electrons around the atom, molecule or compound. Although Lewis structures do not actually represent realistic conditions, they allow us to model these representations on the two dimensional paper.

Introduction to Resonance Forms
More often than not, several different Lewis dot structures exist for any given molecule. So then the question is: which one is the correct Lewis structure? The answer to this question is that neither Lewis Structure completes the picture and a combination of Lewis structures should be used to describe the molecule. These combination of Lewis structures for any given molecule are known as resonance forms. Note that the arrow that is used to represent different resonance forms is a double headed arrow and is different from the arrows used to represent equilibrium between products and reactants.

Resonance Forms Example
More often than not, several different Lewis dot structures exist for any given molecule. So then the question is: which one is the correct Lewis structure? The answer to this question is that neither Lewis Structure completes the picture and a combination of Lewis structures should be used to describe the molecule. These combination of Lewis structures for any given molecule are known as resonance forms. Note that the arrow that is used to represent different resonance forms is a double headed arrow and is different from the arrows used to represent equilibrium between products and reactants.