CHM 123 Chapter 7: 7.5 – 7.6 and 7.8 – 7.9
Electron-Dot Structure: the Octet Rule
Electron-dot structures, or Lewis structures
shows pattern of valence electron distribution in the molecule
useful for understanding the bonding in many compounds
allows us to predict shapes of molecules
allows us to predict properties of molecules and how they will interact together
Lewis theory
the basis of Lewis Theory is that there are certain electron arrangements in the atom that are more stable
◦ octet rule
bonding occurs so atoms attain a more stable electron configuration
◦ more stable = lower potential energy
◦ no attempt to quantify the energy as the calculation is extremely complex
Bonding pair: two of which are shared with other atoms
Lone pair or nonbonding pair: those that are not used for bonding
The Octet Rule
when atoms bond, they tend to gain, lose, or share electrons to result in 8 valence electrons
ns2np6
◦ noble gas configuration
Duet Rule: sharing of 2 electrons
◦ E.g H2
H : H
Octet Rule: sharing of 8 electrons
◦ Carbon, oxygen, nitrogen and fluorine always obey this rule in a stable molecule
◦ E.g F2, O2
many exceptions
◦ H, Li, Be, B attain an electron configuration like He
◦ He = 2 valence electrons
Li loses its one valence electron
H shares or gains one electron
◦ though it commonly loses its one electron to become H+
Be loses 2 electrons to become Be2+
◦ though it commonly shares its two electrons in covalent bonds, resulting in 4 valence electrons
B loses 3 electrons to become B3+
◦ though it commonly shares its three electrons in covalent bonds, resulting in 6 valence electrons
◦ expanded octets for elements in Period 3 or below
using empty valence d orbitals
Rules for writing Dots Lewis structures
use common bonding patterns
structures which result in bonding patterns different from common have formal charges
Write the correct skeletal structure for molecule
◦ Least electronegative atom will be in the center
◦ Hydrogen will always be the terminal
Calculate the total number of electrons for the Lewis structure by summing the valence electrons of each atom in the molecule
◦ If polyatomic ions, charges must be considered when calculating the total valence electrons
Distribute the electrons among the atoms, giving octets (or duet for hydrogen) to as many atoms as possible
If any atoms lack an octet, form double or triple bonds as necessary to give them octets.
expanded octets
◦ elements with empty d orbitals can have more than 8 electrons
odd number electron species e.g., NO
◦ will have 1 unpaired electron
◦ free-radical
◦ very reactive
incomplete octets
◦ B, Al
Draw a Lewis formula then assign formal charge for the following molecules and/or ions
HBr OF2
NH4+ SO32-
Drawing electron-dot structures for molecules with one central atom and multiple bonds
CH2O HCN
PO43-
Electron-dot for Radical
NO ClO2
Expanded octet
XeF2
7.8 Electron-dot structures of compounds containing only hydrogen and second-row elements
CH2F2 CH3CH3
7.9 – Resonance
when there is more than one Lewis structure for a molecule that differ only in the position of the electrons, they are called resonance structures
the actual molecule is a combination of the resonance forms – a resonance hybrid
◦ it does not resonate between the two forms, though we often draw it that way
look for multiple bonds or lone pairs
Rules for drawing resonance
Resonance structures must have the same connectivity
◦ only electron positions can change
Resonance structures must have the same number of electrons
Second row elements have a maximum of 8 electrons
◦ bonding and nonbonding
◦ third row can have expanded octet
Formal charges must total same
Better structures have fewer formal charges
Better structures have smaller formal charges
Draw all possible resonance structures and a resonance hydrid
◦ NO3-
◦ SO2
Dang4