Ionic and Covalent Bonding
● Electronegativity Differences
○ If less than 0.4, then the bond is nonpolar covalent
○ If from 0.4-1.7, then the bond is polar covalent
■ Much weaker than ionic bonds
● Requires less energy to break
■ Much lower melting point than ionic bonds
■ Very poor conductivity / very poor solubility in water
○ If greater than 1.7, then the bond is ionic
■ Very strong bonds
■ Very high melting point
■ Low conductivity when solid
● Very high conductivity when dissolved in water or melted
○ This is due to the solid lattice structure causing electricity
to be unable to move when the molecule is solid. When
dissolved, however, the atoms have very high conductivity.
● Bond energy is the amount of energy needed to break a bond
Lewis Structures & Exceptions to the Octet Rule
● Exceptions
○ Boron tends to form 3 covalent bonds
○ Elements in period 3 and beyond can exceed the octet rule
■ This happens often for sulfur and phosphorus
■ Extra electrons get placed on the central atom
○ Atoms in molecules with an odd number of electrons cannot form stable octets
■ Octet priority goes towards more electronegative atoms
Resonance Formal Charges, and Chemical Nomenclature
● Resonance occurs when there are multiple valid Lewis structures for a single molecule
○ Represented by double headed arrows
● Formal charge is the charge assigned to an atom in a molecule
○ Sum of all formal charges must equal overall charge of molecule
■ FC = V (valence electrons) - N (non bonding electrons) - B (bonds)
● Atoms in molecules prefer formal charges close to zero
○ Any negative formal charges should reside on the more
electronegative atoms
■ Major contributor
● Most favorable lewis structure
○ No charge, all formal charges are
zero
■ Contributes most to what the
molecule looks like in reality
● Naming
○ Binary ionic compounds
■ Cation named first, then anion
■ Name of cation is unchanged
■ Anion contains suffix “-ide”
● E.g. Potassium Iodide
■ Some atoms can form different types of ions
● Identify the charge on the metal ion using roman numerals
● Exceptions: Zn2+, Ag+, Cd2+, Groups 1, 2, Al
○ Polyatomic Ions
■
Ion
Name
Hg22+
Mercury (I)
NH4+
Ammonium
NO2-
Nitrite
NO3-
Nitrate
SO32-
Sulfite
SO42-
Sulfate
HSO4-
Bisulfate/Hydrogen Sulfate
OH-
Hydroxide
CN-
Cyanide
PO43-
Phosphate
HPO42-
Hydrogen Phosphate
H2PO4-
Dihydrogen Phosphate
SCN- or NCS-
Thiocyanate
CO32-
Carbonate
○ Binary Covalent Compounds
■ First element is named
■ Second element contains the suffix “-ide”
■ Use prefixes to denote number of atoms present
● Mono is never used for the first element
○ Naming Acids
■ If the anion ends in -ide, the acid is named with the prefix hydro- and the
suffix -ic
● E.g. Hydrochloric Acid
■ If the anion ends in -ate, the suffix -ic is added to the root name
● E.g. Sulfuric Acid
■ If the anion ends in -ite, the -ite is replaced by -ous
● E.g. Nitrous Acid
VSEPR Model, Bond Polarity, Dipole Moments
● Thalidomide
○ Drug that helped with nausea, morning sickness, and a range of other conditions
in pregnant women but when consumed as the S-isomer, thalidomide would
hinder fetal growth
● VSEPR Model
○ Used to predict the structure of molecules
● Electron Arrangement vs. Molecule Structure
○ Electron arrangement describes the general structure of the molecule
○ Molecular structure is determined by the location of the atoms
■ Tetrahedral has 109.5 degree bond angle
■ Trigonal Pyramidal has 107 degree bond angle due to electron pair on
central atom
■ Bent has 104.5 degree bond angle due to 2 electron pairs on central atom
● AXnEm notation
○ 2-3 electron groups
■ AX2 - linear
■ AX3 - trigonal planar
■ AX2E1 - Bent
○ 4 electron groups
■ AX4 - tetrahedral
○
○
■ AX3E1 - trigonal pyramidal
■ AX2E2 - bent
5 electron groups
■ AX5 - Trigonal bipyramidal
■ AX4E1 - Seesaw
■ AX3E2 - T-shape
■ AX2E3 - Linear
6 electron groups
■ AX6 - Octahedral
●
●
●
■ AX5E1 - Square Pyramidal
■ AX4E2 - Square Planar
Determining Molecule Structure
○ Draw lewis structure for the molecule
○ Count number of electron groups around central atom
○ Arrange the atoms/groups of atoms around the central atom in a way that maximizes the
distance between them
○ Determine the name of the molecule structure
Bond Polarity and Dipole Moments
Covalent bonds are polar if electronegativity difference is between 0.4 and 1.7