Presentation on theme: "Chapter 8: Ionic Compounds. Forming Chemical Bonds The atoms in compounds are held together by a force called a chemical bond. These bonds form because."— Presentation transcript:
Forming Chemical Bonds The atoms in compounds are held together by a force called a chemical bond. These bonds form because of an attraction between oppositely charged atoms, ions, or between electrons and the nuclei. The valence electrons are the ones mainly involved in bonding. Remember that elements in the same group have the same number of valence electrons.
Ions Elements react so that they can achieve the stable electron configuration of a noble gas, usually an octet of electrons. Remember, a cation is a positive ion formed when electrons are lost. Anions are negative ions formed when electrons are gained. The periodic table can be used to predict charges.
Practice For each of the following atoms, write the e- configuration. Then write the formula of the ion most likely to form and identify as a cation or anion. Finally, write the e- configuration of the ion. 1. Bromine, element 35 2. Gallium, element 31 3. Sulfur, element 16 4. Rubidium, element 37
Formation and Nature of Ionic Bonds To bond ionically, atoms must transfer valence e-. An atom that loses one or more e-, becomes a positive ion. An atom the gains one or more e-, becomes a negative ion. An ionic bond is an electrostatic force holding oppositely charged ions together.
Problems Write the e- configuration, in abbreviated form for the atoms in each pair. Then determine the ratio of the atoms in the ionic compound formed in each case. 5. Aluminum and fluorine 6. Lithium and oxygen 7. Beryllium and selenium 8. Gallium and sulfur
Properties of Ionic Compounds Ionic compounds are always nonconductors of electricity when solid, but good conductors when melted. They also act as electrolytes, substances that conduct electric current when dissolved in water. These characteristics are ways that ionic compounds can be identified, although each one individually is not reliable.
Lattice Energy In a solid ionic compound, the positive ions are surrounded by negative ions and the negative ions by positive ions. The resulting structure is called a crystal lattice and contains a regular, repeating 3-D arrangement of ion. This arrangement involves strong attraction between oppositely charged ions, and produces certain properties, such as mp, bp and brittleness.
Lattice Energy The amount of energy required to separate one mole of ions of an ionic compound is called the lattice energy. This value is expressed as a negative number. The more negative the number, the stronger the force of attraction between the ions. Lattice energy is greater for more highly charged ions and small for ions of lower charge or size.
Problem On the basis of the properties of the following “unknowns”, classify each as either ionic or not ionic. a. Conducts electricity when solid b. Conducts electricity when liquid and has a low mp c. Has a high bp and shatters when hammered d. High mp and conducts electricity when dissolved in water
Problem For each of the following pairs of ionic compounds, state which would be expected to have the higher (more negative) lattice energy. a. LiF or KBr b. NaCl or MgS c. MgO or RbI
Names and Formulas for Ionic Compounds Section 8.3
Why Formal Names Remember that scientists come from all over the world. The chemicals they referr to in conversation must be easily understood. Therefore a set of rules is used in the naming of compounds. This allows everyone to write a chemical formula when given the name and the name the compound when given the formula.
Formulas for Ionic Compounds Remember that ionic compounds form crystal structures by the way the ions arrange in the 3-D structure. The smallest ratio of the ions represented in an ionic compound is called the formula unit. Because the total # of e- gained by nonmetals is equal to the # lost by metals, the overall charge of a formula unit is zero.
Determining Charge Binary ionic compounds are composed of a positively charged monatomic ions and negatively charged monatomic ions of a nonmetal. A monatomic ion is a one-atom ion, such as Mg 2+ The charge of a monatomic ion is its oxidation number.
Oxidation Numbers Transition metals are named that because many of them can have more than one oxidation number. The oxidation state of an element in an ionic compound is the number of e- transferred from an element to form the ion. The oxidation # is used to determine the formulas of the ionic compounds they form.
Writing Formulas When writing the formula of a compound remember that the oppositely charged ions must have a sum of zero. When writing formulas, the cation symbol comes first, followed by the anion symbol. Subscripts are used to represent the number of ions of each element in an ionic compound. If no subscript, it is assumed to be one.
Example Let’s write the correct formula for the ionic compounds composed of the following elements. 1. Potassium and iodine 2. Magnesium and chlorine 3. Aluminum and bromine 4. Cesium and nitrogen 5. Barium and sulfur
Polyatomic Ions Some ionic compounds contain polyatomic ions. Polyatomic ions are ions made up of more than one atom. The charge given to a polyatomic ion applies to the entire group of atoms. Since polyatomic ions exist as a unit, do not change the subscripts of the atoms in the ions. Instead use parentheses.
Common Polyatomic Ions Learn the list of polyatomic ions and their charges as listed on page 224 in table 8-6.
Naming Ions and Ionic Compounds Most polyatomic ions are oxyanions. An oxyanion is a polyatomic ion composed of an element bonded to one or more oxygen atoms. Rules. The ion with more oxygen atoms is named using the root of the nonmetal plus the suffix –ate. The ion with fewer oxygen atoms is named using the root of the nonmetal plus the suffix –ite. For example: NO 3 - NO 2 - SO 4 2- SO 3 2-
Other Oxyanions Some elements form more than two oxyanions. These are named according to the number of oxygen atoms present. Examples, ClO 4 - ClO 3 - ClO 2 - ClO -
Rules for Naming Oxyanions The rules for naming oxyanions are as follows: 1. The oxyanion with the greatest number of oxygen atoms is named using the prefix per-, the root of the nonmetal and the suffix –ate. 2. The oxyanion with one less oxygen atom is named with the root of the nonmetal and the suffix –ate. 3. The oxyanion with two fewer oxygen atoms is named using the root of the nonmetal plus the suffix –ite. 4. The oxyanion with three fewer oxygen atoms is name using the prefix hypo-, the root of the nonmetal, and the suffix – ite.
Naming Ionic Compounds 1. Name the cation first and the anion second. 2. Monatomic cations use the element name. 3. Monatomic anions take their name from the root of the element name plus the suffix –ide. 4. If an element has more than one oxidation number, the oxidation # of the element in that compound must be indicated with a Roman numeral in parentheses following the cation. 5. If the compound contains a polyatomic ion, simply name the ion.
Examples Name the following compounds. 1. NaBr 2. CaCl 2 3. KOH 4. Cu(NO 3 ) 2 5. Ag 2 CrO 4
Metallic Bonds and Properties of Metals Section 8.4
Metallic Bonds Although they are not ionic, they share several properties. Metallic bonds form lattices in the solid state. In these lattices, the valence e- of the metals overlap, to create the electron sea model. The e- are not held by any specific atom but rather move easily from one atom to the next. They are often called delocalized electrons.
Metallic Bonds The moving of the electrons causes the formation of the metallic cation. Each cation is bonded to the next and they are surrounded in a sea of e-. A metallic bond is the attraction of a metallic cation for delocalized electrons.
Properties of Metals The properties of metals can be explained by metallic bonding. Melting/Boiling points– In general, moderately high melting and boiling points Maleability– They can be hammered into sheets Ductile They can be drawn into wire
Properties (cont’d) Durability– The cations are not removed from the metal easily. Conductivity— Sea of electrons allows them to conduct electricity. Hardness/Strength— Increased with an increase in the # of delocalized e-
Alloys An alloy is a mixture of elements that has metallic properties. Alloys most commonly form when elements have similar size or one is significantly smaller than the other. Two basic types, substitutional and interstitial.
Examples of Alloys Examples of alloys are: Brass Bronze Cast Iron Gold, 10 carat Pewter Stainless steel Sterling silver