Chapter 9: Models of Chemical Bonding 1.Lewis Dot Symbols 2.Ionic Bonding 3.Metallic Bonding 4.Covalent Bonding 5.Polar Bonds 6.Electronegativity

Lewis symbols

Practice Give Lewis dot symbols for: magnesium nitrogen fluorine argon boron Oxide ion Sodium ion

Types of Bonding and Properties Ionic Metallic Covalent

Ionic Bonding Ionic compounds form The attraction Forming ions takes energy

Metallic Bonding Outer electrons Large numbers of metal atoms The electrons are ‘delocalized’

Covalent Bonds

Network Covalent Solids

Polar Covalent Bonds When two identical atoms form a covalent bond When different kinds of atoms combine,

(a) (b)

Electronegativity In general, electronegativity increases

The difference in electronegativity There is no sharp dividing line between ionic and covalent bonding: A bond is mostly ionic when

The degree of polarity, or ionic character, varies continuously with the electronegativity difference Each atom in a bond has a partial charge of about +0.5 or –0.5 units when the electronegativity difference is 1.7. Electronegativity diff.Type of bonding >1.7Mostly ionic Polar covalent <0.4Mostly covalent 0Nonpolar covalent

Practice Questions Identify the following bonds as A.Mostly ionic B.Polar Covalent C.Mostly Covalent D.Nonpolar Covalent C-Cl Re-H Li-Cl P-H Sn-Br Sr-O

Lewis symbols can be used to represent the covalent or electron pair bond Formulas drawn with Lewis symbols are called Lewis formulas or Lewis structures

The term structural formula Many molecules obey the octet rule: The number of bonds an atom (second-row) forms is determined by:

Number of Bonds C Has 4 e - Needs 4 e - Forms 4 bonds N O F B

single bond Double and triple bonds The bond order A single bond has bond order of a double bond a bond order of a triple bond a bond order of Bond length depends on bond order:

Drawing Lewis Structures The least electronegative atom is usually in the middle. (Or the atom which can form the most bonds.) Count total number of valence electrons in molecule/ion. Place them around the atoms to satisfy the octet rule:

Phosphorus Trichloride Carbon Dioxide

Not all structures obey the octet rule

Sulfur tetrafluoride Bromine pentafluoride

The preferred Lewis structure is the one that best fits the experimental data The structure of sulfuric acid in the vapor state. There are two different sulfur-oxygen bond lengths. The preferred Lewis structure needs different bond orders for these atoms.

Structure I Structure II

formal charge The formal charge on an atom is calculated

Consider the sulfur atoms in the two structures for sulfuric acid: Structure I: formal charge on S = Structure II: formal charge on S = When several Lewis structures are possible,

Iodate ion Nitrate ion Formal charge on O = Formal charge on I = Formal charge on O = Formal charge on I =

Some molecules and ions are not well represented by a single Lewis structure Consider the case of the formate ion, HCO 2 - Experiment gives These are called resonance structures

Draw the resonance forms for nitrate. Draw the resonance forms for dinitrogen monoxide. Draw resonance forms for thiosulfate.