Download Outline Ch 8 - Mead`s Fabulous Weebly

Mead Chemistry Lap 8 Covalent Bonding
8.1 Molecular Compounds
A. Molecules
 Matter takes many forms
 Noble gases exist alone, don’t bond = monatomic
 Ionic compounds form bonds because of attractions between positive and negative
charges
 Other compounds use different types of bonding
 Covalent molecules use covalent bonding
 Def: bond formed by the sharing of electrons
 Molecules: neutral group of atoms joined by covalent bonds or sharing of
electrons
 Some molecules are diatomic
 Oxygen has 2 atoms that bond together
 Properties of molecular compounds
 Lower melting and boiling points than ionic compounds
 Many are liquid or gas at room temp
 Ionic compounds are crystal solids
 Most are 2 or more nonmetals
 Ionic compounds are metal + nonmetal OR cation + anion
B. Molecular Formulas
 Gives the number of atoms of each element in one molecule
 Example: H2O
 2 atoms of Hydrogen, 1 atom of Oxygen
 Does not show you how the molecule is built or the shape (structure)
 Other formulas can give information about the structure
 Structural formula
 Space filling model
 Ball and stick model
8.2 Covalent Bonding
A. Octet Rule
 Electron sharing occurs so each atom has 8 electrons in outer level
 May need to bond to more than one atom to get 8 electrons
B. Single Covalent Bond
 A shared pair of electrons = covalent bond
 Use electron dot diagrams to show how electrons are shared
 Examples:
 H2
 F2
 NH3
 Structural formula:
 Replaces a shared pair with a line to represent a bond
 Unshared pairs of electrons are shown as dots
 Examples:
 H2
 NH3
 CH4
C. Double and Triple Bonds
 Some atoms bond with more than one pair of shared electrons
 Double bond = 2 shared pairs of electrons
 Usually involves oxygen
 Triple bond = 3 shared pairs of electrons
 Usually involves nitrogen
 More examples
1. OF2
2. SCl2
3. CCl4
4. CHCl3
5. N2H4
6. C2H6
8.3 VSEPR Theory
A. 3D Structure of molecules
 Electron dot diagrams only give a flat view of the molecule (in 2D)
 Real molecules are not flat but 3D
 VSEPR theory explains and predicts the 3D shape of molecules
 Valence Shell Electron Pair Repulsion Theory
 Electron pairs repulse each other
 Shape will be based on the electron pairs trying to stay as far apart as possible
 Unshared electron pairs have a bigger effect than shared pairs
 Unshared electrons have stronger repulsion
 3D shape is based on the number of shared and unshared electron pairs
 Shape also determines the angle between each atom
 Ranging from 180o to less than 90o
B. Steps to finding VSEPR shape
1.
2.
3.
4.
5.
Draw electron dot diagram for compound
Determine number of electron pairs around central atom
Find number of shared pairs and unshared pairs
Use chart to determine shape
Use shape to determine angle between atoms
C. VSEPR Shapes
 Linear:
 2 electron pairs around central atom
 No unshared pairs
 Angle = 180o
 Trigonal planar:
 3 electron pairs around central atom
 No unshared pairs
 Angle = 120o
 Bent triatomic:
 3 electron pairs around central atom
 1 unshared pair
 Angle = less than 120o
 Tetrahedral
 4 electron pairs around central atom
 No unshared pairs
 Angle = 109o
 Pyramidal
 4 electron pairs around central atom
 1 unshared pair
 Angle = less than 109o
 Bent
 4 electron pairs around central atom
 2 unshared pairs
 Angle = less than 109o
 Trigonal bipyramidal
 5 electron pairs around central atom
 No unshared pairs
 Angle = 120o or 90o
 Octahedral
 6 electron pairs around central atom
 No unshared pairs
 Angle = 90o
8.4 Polar Bonds and Molecules
A. Bond Polarity
 Covalent bonds = atoms sharing electrons
 But atoms do not always share equally
 If equal sharing = non-polar covalent bond
 Diatomic molecules like O2, N2, Cl2
 If unequal sharing = polar covalent bond
 Use electronegativities to determine which atom will have the greater pull on the
electrons (page177)
 Higher electronegativity = stronger pull on electrons
 Electronegativities pg.177
 Stronger pull gives that atom a slightly negative charge
 Lesser pull gives that atom a slightly positive charge
 Example: H20
 Oxygen electronegativity = 3.5
 Hydrogen electronegativity = 2.1
 Oxygen is higher EN, stronger pull
 Slightly negative
 Hydrogen is lower EN, lesser pull
 Slightly positive
B. Use difference in 2 electronegativity numbers to determine what type of bond will form:
Difference in
Electronegativities
0.0-0.4
Most probable
Bond Type
Non-polar covalent
0.4-1.0
Moderately polar covalent
1.0-2.0
Very polar covalent
2.0 +
Ionic
 Examples: What type of bond will form between…
1. H and N
 H = 2.1 and N = 3.0
 Difference is 0.9
 Bond is moderately polar covalent
2. F and F
 F =4.0 and F = 4.0
 Difference is 0.0
 Bond is non-polar covalent
3. Ca and Cl
 Ca = 1.0 and Cl = 3.0
 Difference is 2.0
 Bond is ionic
4. Al and Cl
 Al = 1.5 and Cl = 3.0
 Difference is 1.5
 Bond is very polar covalent
C. Attractions between molecules: Hydrogen Bonds
 Bonds form between the slightly positive hydrogen in a polar molecule and other slightly
negative atoms
 Important to characteristics of water and other living things
 Hydrogen bonds in DNA