Chapter 4 Chemical Bonding: Ionic Bond Model

Sec 4.1 Chemical Bonds Free 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 Bonds In 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 Bonds Fundamentals 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 Electrons Valence shell: the outermost incomplete shell Valence 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 Electrons We 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 Electrons General 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 Rule Stable, 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 Model Isotopes – 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 Model Ions 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 Ions Metals containing 1-3 valence electrons (Groups IA, IIA, and IIIA) tend to lose electrons (positive charge) Nonmetals containing 5-7 valance electrons (Groups VA, VIA, and VIIA) tend to gain electrons (negative charge) 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 compounds The overall compound should be neutral Reduce 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 Compounds Binary 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 Compounds Table 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 Ions Sometimes 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 4.3 Page 91 Common Polyatomic 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

Problems Assigned 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