4.1 Representing Ionic Compounds

Forming Ionic Compounds Ionic compounds are formed when one or more valence electrons (electrons in the outer energy level) are transferred from a metal atom to a non-metal atom. Example: sodium chloride (NaCl) they are made up of positive ions (cations) and negative ions (anions) that are attracted to each other. the two oppositely charged ions are attracted to each other by a force called an ionic bond.

The Periodic Table and Valence Electrons Group 8 = 8 valence electrons Except for He, it has 2 electrons Group 1 = 1 valence electron Group 2 = 2 valence electrons Each column is called a “group” 13, 14, 15, 16, 17 Octet Rule: The loss and gain of electrons allows each atom to form a full outer energy level of electrons and, therefore, become stable (with the same number of electrons as the closest noble gas) Scientists often refer to this arrangement as a stable octet. www.chem4kids.com

Ionic structure / properties Ionic compounds are solids at SATP. In their solid form they form ionic crystals. These are more commonly known as salts. high melting and boiling points many are brittle when ionic compounds are dissolved in water, they dissociate into their ions and the solutions conduct electricity. NaCl(s)  Na+(aq) + Cl-(aq)

Types of Ionic Compounds… Binary Ionic Compounds: composed of only 2 different elements (a metal cation and a non-metal anion) some binary ionic compounds contain a multivalent metal – these have more than one ion charge (listed in the periodic table) Ternary Ionic Compound: composed of 3 different elements (one of the ions is polyatomic – made up of more than one element)

Naming Binary Ionic Compounds… The metal ion is always written first and keeps its whole name The non-metal is written second and has a slight change, the ending (suffix) is changed to –ide -example: Na2S – sodium sulfide

Writing Formulas for Binary Ionic Compounds… -example: magnesium chloride Step 1: Write the symbols with the metal first Mg Cl Step 2: Write the ionic charge above each symbol to indicate the stable ion that each element forms 2+ 1- Step 3: Determine how many ions of each type you need so that the total ionic charge is zero One Mg2+ ion will balance the charge of two Cl- ions (2+) + 2(1-) = 0 Step 4: Write the formula using subscripts to indicate the number of ions of each type MgCl2

Writing Formulas for Binary Ionic Compounds… -example: lithium oxide Step 1: Write the symbols with the metal first Li O Step 2: Write the ionic charge above each symbol to indicate the stable ion that each element forms 1+ 2- Step 3: Determine how many ions of each type you need so that the total ionic charge is zero Two Li1+ ions will balance the charge of one O- ion 2(1+) + (2-) = 0 Step 4: Write the formula using subscripts to indicate the number of ions of each type Li2O

The Cross-Over Method -example: lithium oxide Step 1: Write the symbols with the metal first Li O Step 2: Cross-Over Rule – Write the ionic charge above each symbol and cross over the amount of charge. 1+ 2- The formula is Li2O

The Cross-Over Method -example: calcium oxide Step 1: Write the symbols with the metal first Ca O Step 2: Cross-Over Rule – Write the ionic charge above each symbol and cross over the amount of charge 2+ 2- Ca O The formula is Ca2O2 Step 3: Reduce the Chemical Formula - Dividing by common factor of 2, we get: CaO

The Zero-Sum Rule The sum of all the charges in the chemical formula of the compound MUST EQUAL ZERO. Example: Al2S3 Ratio of 2 Al atoms : 3 S atoms Al cation  +3 charge S anion  -2 charge 2(+3) + 3(-2) = ZERO! Al S

Naming Binary Ionic Compounds having Multivalent Metals… -example: FeS Step 1: Identify the metal Iron (Fe) Step 2: Verify that the metal can form more than one kind of ion by checking the periodic table. 2+ 3+ Fe and Fe Step 3: Determine the ratio of the ions in the chemical formula 1 iron : 1 sulfide Step 4: Note the charge of the anion 2-

-example: FeS Step 5: The positive and negative charges must balance out so that the net charge is 0. Total negative charge: 2- Total positive charge: 2+ Step 6: Determine what charge the metal ion must have to balance the anion. 1(Fe ?) = 2+ Therefore, the charge on the iron must be 2+. Step 7: Write the name of the metal ion – include a roman numeral to show which charge the ion has in brackets, after the name. See Table 4.5 on pg. 147. The name of the metal ion is iron(II) Step 8: Write the name of the compound. iron(II) sulfide

Naming Ionic Compounds having Multivalent Metals… Binary Ionic Compounds Naming Ionic Compounds having Multivalent Metals… -example: Fe2S3 Step 1: Identify the Metal Iron (Fe) Step 2: Verify that the metal can form more than one kind of ion by checking the periodic table 2+ 3+ Fe and Fe Step 3: Determine the ratio of the ions in the chemical formula 2 iron : 3 sulfide Step 4: Note the charge of the anion 2-

-example: Fe2S3 Step 5: The positive and negative charges must balance out so that the net charge is 0. Total negative charge: 6- (3 X 2-) Total positive charge: 6+ Step 6: Determine what charge the metal ion must have to balance the anion. 2(Fe ?) = 6+ Therefore, the charge on the iron must be 3+. Step 7: Write the name of the metal ion – include a roman numeral to show which charge the ion has in brackets after the name. The name of the metal ion is iron(III) Step 8: Write the name of the compound. iron(III) sulfide

Writing Formulas for Binary Ionic Compounds having Multivalent Metals… -example: tin(IV) sulfide Step 1: Write the symbols with the metal first Sn S Step 2: Cross-Over Rule – Write the ionic charge above each symbol and cross over the amount of charge 4+ 2- Sn S The formula is Sn2S4 Step 3: Reduce the Chemical Formula - Dividing by common factor of 2, we get: SnS2

Ternary Ionic Compounds: Ionic Compounds with Polyatomic Ions naming compounds with polyatomic ions is similar to naming other ionic compounds You should note that compounds with polyatomic ions have names ending in -ate or –ite, not -ide Note that most are negative, except ammonium, NH4 + (see table 4.7 on pg. 148) Name Ca(OH)2, CuSO4, NH4NO3, Co2(CO3)3

Ionic Compounds with Polyatomic Ions Answers Ca(OH)2 CuSO4 NH4NO3 Co2(CO3)3 - calcium hydroxide - copper(II) sulfate - ammonium nitrate - cobalt(III) carbonate

Most Common Polyatomic Ions One extra oxygen "per-____-ate" Common/Normal Just memorize this one and you’ll know the rest One less oxygen ”-ite" Two less oxygens "hypo-____-ite" NO41- pernitrate NO31- nitrate NO21- nitrite NO1- hyponitrite CO42- percarbonate CO32- carbonate CO22- carbonite CO2- hypocarbonite ClO41- perchlorate ClO31- chlorate ClO21- chlorite ClO1- hypochlorite SO52- persulfate SO42- sulfate SO32- sulfite SO22- hyposulfite PO53- perphosphate PO43- phosphate PO33- phosphite PO23- hypophosphite

Ionic Compounds with Polyatomic ions Writing Formulas … -example: aluminum carbonate Step 1: Using the periodic table and a table of common polyatomic ions, identify each ion and its charge. Aluminum: Al3+ Carbonate: CO32- Step 2: Cross-Over Rule – Cross over the amount of charge. 3+ 2- Al CO3 Step 3: Use subscripts to show the ratio of ions. Place the polyatomic ion in brackets, if it needs a subscript. Al2(CO3)3

Ionic Compounds with Polyatomic ions Writing Formulas … -example: ammonium sulfate Step 1: Using the periodic table and a table of common polyatomic ions, identify each ion and its charge. Ammonium: NH4 + Sulfate: SO4 2- Step 2: Cross-Over Rule – Cross over the amount of charge. 1+ 2- NH4 SO4 Step 3: Use subscripts to show the ratio of ions. Place the polyatomic ion in brackets, if it needs a subscript. (NH4)2SO4