Chapter 4

Chemical Bonding:Chemical Bonding:Ionic Bond ModelIonic Bond Model

Sec 4.1 Chemical BondsFree atoms are generally not encountered Atoms form molecules and aggregates Two broad types of compounds, ionic

compounds and molecular compounds

Ionic compounds are the subject of Chap 4 Molecular compounds are the subject of

Chap 5

Sec 4.1 Chemical BondsIn general ionic compounds share some similar properties: High melting points (500 – 2000 oC) Conduct electricity when in liquid or

solution form Solids (often crystals) at room

temperature Made up by ionic bonds Ionic bond – a chemical bond formed

through the transfer of electrons from one atom to another

Sec 4.1 Chemical BondsFundamentals of chemical bonds

1. Not all electrons in an atom participate in bonding. Generally only the outermost level are available

These electrons are known as valence (Sec 4.2)

2. Certain arrangements of electrons are very stable. This is referred to by the octet rule (Sec 4.3)

Sec 4.2 Valence ElectronsValence shell: the outermost incomplete shellValence electron: an electron in the valence shell For these definitions we generally refer to

the main group elements. Transition metals behave differently

For main group elements, valence electrons are always found in the s and p subshells

Sec 4.2 Valence ElectronsWe write a shorthand called a Lewis Structure to show the symbol and the valence electrons for an element

Lewis Structure is merely the symbol of an element surrounded by dots equal to the number of valence electrons

Sec 4.2 Valence Electrons

Figure 4.1 Page 77 Showing Lewis Structures

Sec 4.2 Valence ElectronsGeneral Patterns and Rules: Elements in the same group (column) have

the same number of valence electrons The number of valence electrons is the

same as the Roman Numeral numbering system IA has 1, VIIA has 7, etc

The maximum number of valence electrons for any element (with a few exceptions) is eight

Sec 4.3 The Octet RuleStable, in the context of electrons and atoms, means that the atom does not easily undergo spontaneous change The most stable configuration of electrons is the

noble gas configuration (s and p filled) For all but He, this is a filled valence shell with 8

electrons (hence octet). He is filled with 2

Octet Rule: in forming compounds, elements gain, lose, or share electrons in order to produce a stable noble gas configuration for all atoms involved

Sec 4.4 The Ionic Bond ModelIsotopes – change the neutrons and mass but not the charge for an atom

Ions – an atom with the normal number of protons and neutrons but a change in electrons that changes the charge

Sec 4.4 The Ionic Bond ModelIons have two possibilities, a positive charge or a negative charge

Anion – atom gains electrons, therefore negative charge (example F-1)Cation – atom loses electrons, therefore positive charge (example Mg+2)Why not a change of protons for a change in charge? Disagree with book as far as notation

Sec 4.5 Sign and Magnitude of Ions

What determines the normal ion charge for an element? Elements tend to gain or lose

electrons to reach a full valence shell (octet rule)

Many elements have two pathways to reach a full valence shell. Choose the path of less resistance

For example, losing 2 electrons is easier than gaining 6

Sec 4.5 Sign and Magnitude of Ions1. Metals containing 1-3 valence electrons

(Groups IA, IIA, and IIIA) tend to lose electrons (positive charge)

2. Nonmetals containing 5-7 valance electrons (Groups VA, VIA, and VIIA) tend to gain electrons (negative charge)

3. Elements in Group IV would be expected to gain or lose 4 but instead these elements form covalent bonds (Chap 5)

Sec 4.5 Sign and Magnitude of Ions

Example, predict Oxygen, Phosphorus, Argon, Potassium, Aluminum, Carbon

Transition metals and series elements are harder to predict, some can form several ions with different charges In general the charges on transition

metals will be given or can be determined by context Iron (II) ion means Fe+2

Iron (III) ion means Fe+3

Sec 4.6 Ionic Compound Formation

Elements don’t lose or gain electrons in a vacuum. Where does a lost electron go? Ionic bonds involve transfer, one element

loses an electron and another gains it Lewis dot structures often make it easy

to visualize the electron transfer process

Examples and how to write the compound out

Sec 4.7 Chemical Formulas“Criss-cross” method for balancing ionic compoundsThe overall compound should be neutralReduce the subscripts to the lowest ratio of whole numbers Examples: BaI2 (note charges not

written) Fe(III)2O3

MgO (note reduction of ratio)

Sec 4.8 Structure of Ionic Compounds

Ionic compounds in a solid state form a lattice of alternating positive and negative ions

Figure 4.4 (a) through (c) Page 85

Sec 4.8 Structure of Ionic Compounds

The lattice extends in 3 dimensions so each positive is surrounded by negative and vice versa Because of the larger structure, the

formula generally refers to a formula unit

Formula unit is the smallest ratio neutral unit that repeats itself in the overall compound

Sec 4.9 Naming Ionic CompoundsBinary Ionic Compounds contains only two elements, a metal and a nonmetal The metal always comes first Named by using the name of the metal

element followed by the stem of the nonmetal with the suffix –ide

Fluoride, Oxide, Bromide

Sec 4.9 Naming Ionic CompoundsTable of Nonmetal names and ion charges found Table 4.2 page 88 Examples of naming: Na2S, Al2O3 , CuBr3 , CaCl2 Examples of formulas: Iron (III) Oxide, Potassium Iodide,

Magnesium Sulfide

Fixed Charge aside on page 90 Green

Sec 4.9 Naming Ionic Compounds

Figure 4.7 Page 89 Copper (II) oxide is black, whereas copper (I) oxide is reddish brown. Iron (II) chloride is green, whereas iron (III) chloride is bright yellow.

Sec 4.10 Polyatomic IonsSometimes ions can be formed by several atoms acting together as one unit Polyatomic ions behave in the same way as

monoatomic ions, treat them as a single unit Polyatomic ions are not molecules

The vast majority of polyatomic ions are negatively charged (exception being NH4

+)

Sec 4.10 Polyatomic Ions

Table 4.3Page 91CommonPolyatomic ions

Sec 4.11 Naming Ionic Compounds II

Naming ionic compounds containing polyatomic ions is similar to naming binary ionic compounds

Metal named first, polyatomic ion named second

Sec 4.11 Naming Ionic Compounds II

Two things arise when writing chemical formulas

1. You MUST use parentheses when there is more than one polyatomic ion is required

Examples: Fe(OH)3

2. Keep polyatomic units together, even if the same element appears elsewhere in the formula

Examples: NH4NO3

Sec 4.11 Summary of Naming

Chemistry at a Glance Page 94

ProblemsAssigned problems pages 95 - 98 4.1, 4.7, 4.8 4.11, 4.13, 4.19, 4.25, 4.27 4.29, 4.33, 4.39, 4.41, 4.43, 4.49 4.55, 4.57, 4.61

Practice Test page 98