Molecules and Compounds Compounds Display Constant Composition Elements combine in fixed Proportions Law of Constant Composition H2O CO2

Molecules and Compounds Compounds Display Constant Composition If we decompose water, we find 16.0 grams of oxygen to every 2.00 grams of hydrogen. This produces an Oxygen to Hydrogen mass ratio of 8.0. Mass ratio = 16.0 grams = 8.0 2.00 grams Water has a constant Mass Ratio of Oxygen to Hydrogen of 8.0.

Molecules and Compounds Chemical Formulas: How to Represent Compounds Elements present Relative numbers Subscripts Part of the definition Changing subscript changes compound Metals – Left-hand side of Periodic Table Listed first Nonmetals – Right-hand side of the Periodic Table Those to the left- More Metal-like

Significance of Chemical Formulas A chemical formula indicates the relative number of atoms of each kind in a chemical compound. For a molecular compound, the chemical formula reveals the number of atoms of each element contained in a single molecule of the compound. example: octane — C8H18

Molecules and Compounds Writing Chemical Formulas Compound containing 2 aluminum atoms to every 3 oxygen atoms Al2O3 Compound containing 3 oxygen atoms to every 1 sulfur atom SO3

Molecules and Compounds Chemical Formulas: How to Represent Compounds Water – H2O Hydrogen and oxygen atoms 2:1 ratio Symbol and subscript Dihydrogen monoxide

Chemical Formulas and Compounds Chemical Formulas: How to Represent Compounds Carbon dioxide – CO2 Carbon and oxygen atoms 1:2 ratio Symbol and subscript Carbon dioxide Two nonmetals Left-most listed first One carbon – not mono

The chemical formula for an ionic compound represents one formula unit—the simplest ratio of the compound’s positive ions (cations) and its negative ions (anions). example: aluminum sulfate — Al2(SO4)3 Parentheses surround the polyatomic ion SO42- to identify it as a unit. The subscript 3 refers to the unit.

Molecules and Compounds Writing Chemical Formulas Polyatomic Ions Contain several identical groupings of atoms Polyatomic Ions Parenthesis Mg(NO3)2 Magnesium nitrate

Molecules and Compounds Writing Chemical Formulas Polyatomic Ions Mg(NO3)2 1 Mg 2 N 6 O

Molecules and Compounds Molecular View of Elements and Compounds Atomic Elements Single atoms Most Elements Molecular Elements Diatomic atoms Two atoms bonded together H2, N2, O2, F2, Cl2, Br2, I2 Molecular Compounds Two or more Nonmetals Ionic Compounds Metal and nonmetal

Molecules and Compounds Molecular View of Elements and Compounds

Molecules and Compounds Writing Formulas for Ionic Compounds Charge Neutral Most elements form only one type of ion Predictable charge Sodium chloride Na  +1 Cl  –1 NaCl Magnesium chloride Mg  +2 MgCl2

Common Monatomic Ions (pg.221 in your textbook)

Common Monatomic Ions…

Molecules and Compounds Writing Formulas for Ionic Compounds Write the symbol for the metal and its charge Write the symbol for the nonmetal and its charge Charge (without sign) becomes subscript for other ion Reduce subscripts to smallest whole number ratio Check that the sum of the charges of the cation cancels the sum of the anions

Molecules and Compounds Writing Formulas for Ionic Compounds Ionic compound between aluminum and oxygen Al3+ O2– Al2O3 2 (+3) = +6 3 (–2) = –6

Molecules and Compounds Writing Formulas for Ionic Compounds Ionic compound between magnesium and oxygen Mg2+ O2– MgO 1 (+2) = +2 1 (–2) = –2 Smallest whole number ratio is 1:1

Molecules and Compounds Naming Ionic Compounds Metal and Nonmetal Two Types Type I Metal has constant predictable charge Inferred from group number in Periodic Table Type II Charge NOT always the same Transition Metals

Molecules and Compounds Naming Ionic Compounds Type I Binary Ionic Compounds Binary compounds Two different kinds of elements Cation + anion with IDE Metal Predictable constant charge Name of metal Nonmetal Name of nonmetal Ending  IDE

Molecules and Compounds Naming Ionic Compounds Type I Binary Ionic Compounds Binary compounds Two different kinds of elements Cation + anion with IDE Metal Predictable constant charge Name of metal Nonmetal Name of nonmetal Ending  IDE

Molecules and Compounds Naming Ionic Compounds Type I Binary Ionic Compounds MgF2 Metal  Magnesium Nonmetal  Fluoride Magnesium fluoride KBr Metal  Potassium Nonmetal  Bromine Potassium bromide

Molecules and Compounds Naming Ionic Compounds Type I Binary Ionic Compounds CaCl2 Metal  Calcium Nonmetal  Chlorine Calcium chloride Na2O Metal  Sodium Nonmetal  Oxygen Sodium oxide

Molecules and Compounds Naming Ionic Compounds Type II Binary Ionic Compounds Binary compounds Two different kinds of elements Cation + anion with IDE Metal Charge NOT always the same Transition metals and roman numeral for charge Name of metal Nonmetal Name of nonmetal Ending  IDE

Molecules and Compounds Naming Ionic Compounds Type II Binary Ionic Compounds FeCl3 Metal  Iron Nonmetal  Chlorine Charge on Iron must be +3 Iron(III) chloride CrO Metal  Chromium Nonmetal  Oxygen Charge on Chromium must be +2 Chromium(II) oxide

Molecules and Compounds Naming Ionic Compounds Type II Binary Ionic Compounds PbCl4 Metal  Lead Nonmetal  Chlorine Charge on Lead must be +4 Lead(IV) chloride Must determine charge of Cation from the Formula

Molecules and Compounds Naming Ionic Compounds Polyatomic Ions Group of atoms with an overall charge Common Polyatomic Ions on pg. 226 in textbook. MEMORIZE (flashcards?)

Molecules and Compounds Naming Ionic Compounds Polyatomic Ions KNO3 Metal  Potassium Nonmetal  Nitrate Potassium nitrate FeSO4 Metal  Iron Nonmetal  Sulfate Iron(II) sulfate

Ionic Compounds Containing Polyatomic Ions…. Write the formula for tin(IV) sulfate Sn(SO4)2

Molecules and Compounds Naming Molecular Compounds Two or more nonmetals Right of the Periodic Table Binary Molecular Compounds Two elements Most “metal-like” first Prefix [Element 1] Prefix [Element 2] Do not begin with “mono” and if prefix is two syllables (& element starts with vowel) then drop the ending vowel of prefix. Prefixes Mono – 1 Di – 2 Tri – 3 Tetra – 4 Penta – 5 Hexa – 6 Hepta – 7 Octa – 8 Nona – 9 Deca - 10

Molecules and Compounds Nomenclature: Naming Compounds Systematic ways to name compounds Common names Water H2O Common name  Water Systematic name  Dihydrogen monoxide

Molecules and Compounds Naming Molecular Compounds Most “metal-like” – Carbon Nonmetal – Oxygen Carbon dioxide N2O Most “metal-like” – Nitrogen Dinitrogen monoxide

Give name for As2O5: Write formula for oxygen difluoride:

Molecules and Compounds Naming Acids Molecular Compounds Form H+ Sour Taste Binary Acids- acids that consist of two elements (usually hydrogen and a halogen) Oxyacids – acids that contain hydrogen, oxygen and a third element (usually a nonmetal)

Binary Acids… If anion ends in “ide” then acid name is “hydro—ic” HCl hydrochloric acid HBr hydrobromic acid HI hydroiodic acid

Naming Oxyacids No “hydro” in name If anion ends in “ate” then acid name is “—ic” If anion ends in “ite” then acid name is “—ous”

Oxyacids Anion Acid _____ sulfate SO42- sulfuric acid H2SO4 sulfite SO32- sufurous acid H2SO3 nitrate NO3- nitric acid HNO3 phosphate PO43- phosphoric acid H3PO4

Salts An ionic compound composed of a cation and the anion from an acid is often referred to as a salt. examples: Table salt, NaCl, contains the anion from hydrochloric acid, HCl. Calcium sulfate, CaSO4, is a salt containing the anion from sulfuric acid, H2SO4.

Oxidation Numbers The charges on the ions in an ionic compound reflect the electron distribution of the compound. In order to indicate the general distribution of electrons among the bonded atoms in a molecular compound or a polyatomic ion, oxidation numbers are assigned to the atoms composing the compound or ion. Unlike ionic charges, oxidation numbers do not have an exact physical meaning: rather, they serve as useful “bookkeeping” devices to help keep track of electrons.

Assigning Oxidation Numbers In general when assigning oxidation numbers, shared electrons are assumed to “belong” to the more electronegative atom in each bond. More-specific rules are provided by the following guidelines. The atoms in a pure element have an oxidation number of zero. examples: all atoms in sodium, Na, oxygen, O2, phosphorus, P4, and sulfur, S8, have oxidation numbers of zero.

Assigning Oxidation Numbers, continued.. The more-electronegative element in a binary compound is assigned a negative number equal to the charge it would have as an anion. Likewise for the less-electronegative element. Fluorine has an oxidation number of –1 in all of its compounds because it is the most electronegative element.

Assigning Oxidation Numbers, continued.. 4. Oxygen usually has an oxidation number of –2. Exceptions: In peroxides, such as H2O2, oxygen’s oxidation number is –1. In compounds with fluorine, such as OF2, oxygen’s oxidation number is +2. 5. Hydrogen has an oxidation number of +1 in all compounds containing elements that are more electronegative than it; it has an oxidation number of –1 with metals.

Assigning Oxidation Numbers, continued.. The algebraic sum of the oxidation numbers of all atoms in an neutral compound is equal to zero. The algebraic sum of the oxidation numbers of all atoms in a polyatomic ion is equal to the charge of the ion. Although rules 1 through 7 apply to covalently bonded atoms, oxidation numbers can also be applied to atoms in ionic compounds similarly.

Assigning Oxidation Numbers, continued.. Assign oxidation numbers to each atom in the following compounds or ions: a. UF6 b. H2SO4 c. ClO3-

Molecules and Compounds Nomenclature Summary

Molecules and Compounds Formula Mass & Molar Mass Formula Mass - Average mass of the Molecules that compose a compound (in a.m.u.’s) Molar Mass – Average mass of the molecules that compose a mole (6.02x1023) of the compound (in grams/mole) Same values different units!

Molar Mass Calculation What is the molar mass of barium nitrate, Ba(NO3)2? 262.35 g/mol