Molecules and Compounds Compounds Display Constant Composition

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Molecules and CompoundsCompounds Display Constant

Composition

• Elements combine in fixed Proportions• Law of Constant Composition

– H2O– CO2

Molecules and CompoundsCompounds Display Constant

CompositionIf 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 CompoundsChemical Formulas: How to

Represent Compounds• Chemical Formula

– 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 – Listed first

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 CompoundsWriting 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 CompoundsChemical Formulas: How to

Represent Compounds• Water – H2O

– Hydrogen and oxygen atoms

– 2:1 ratio– Symbol and

subscript– Dihydrogen

monoxide

Chemical Formulas and CompoundsChemical 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.

Polyatomic Ions• Contain several identical groupings of

atoms• Polyatomic Ions• Parenthesis• Mg(NO3)2

• Magnesium nitrate

Polyatomic Ions• Mg(NO3)2

• 1 Mg• 2 N• 6 O

Molecules and CompoundsMolecular 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 CompoundsMolecular View of

Elements and Compounds

Molecules and CompoundsWriting 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– Cl –1– MgCl2

Common Monatomic Ions(pg.221 in your textbook)

Common Monatomic Ions…

Ionic Compounds

1. Write the symbol for the metal and its charge2. Write the symbol for the nonmetal and its

charge3. Charge (without sign) becomes subscript for

other ion4. Reduce subscripts to smallest whole number

ratio5. Check that the sum of the charges of the cation

cancels the sum of the anions

Ionic Compounds

Ionic compound between aluminum and oxygen

Al3+ O2–

2 (+3) = +63 (–2) = –6

Ionic Compounds

Ionic compound between magnesium and oxygen

Mg2+ O2–

MgO1 (+2) = +2

1 (–2) = –2Smallest 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

• Metal– Predictable constant charge– Name of metal

• MgF2

– Metal Magnesium – Nonmetal Fluoride– Magnesium fluoride

• KBr– Metal Potassium– Nonmetal Bromine– Potassium bromide

• CaCl2

– Metal Calcium – Nonmetal Chlorine– Calcium chloride

• Na2O– Metal Sodium– Nonmetal Oxygen– Sodium oxide

Type II Binary Ionic Compounds

• Metal– Charge NOT always the same– Transition metals and roman

numeral for charge– Name of metal

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

Polyatomic Ions

• Group of atoms with an overall charge• Common Polyatomic Ions on pg. 226 in

textbook. MEMORIZE (flashcards?)

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

• 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.

•PrefixesMono – 1Di – 2 Tri – 3Tetra – 4Penta – 5Hexa – 6Hepta – 7Octa – 8Nona – 9Deca - 10

Molecules and CompoundsNomenclature: Naming

Compounds

• Systematic ways to name compounds• Common names• Water

– H2O– Common name Water– Systematic name Dihydrogen monoxide

Molecules and Compounds Naming Molecular Compounds

• CO2

• Most “metal-like” – Carbon• Nonmetal – Oxygen• Carbon dioxide

• N2O• Most “metal-like” – Nitrogen• Nonmetal – Oxygen• Dinitrogen monoxide

• Give name for As2O5:• Write formula for oxygen difluoride:

Molecules and Compounds Naming Acids

•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 SO4

2- sulfuric acid H2SO4

sulfite SO32- sufurous acid H2SO3

nitrate NO3- nitric acid HNO3

phosphate PO43- phosphoric acid H3PO4

• 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.

1. 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..2. 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.

3. 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..6. The algebraic sum of the oxidation numbers of

all atoms in an neutral compound is equal to zero.

7. The algebraic sum of the oxidation numbers of all atoms in a polyatomic ion is equal to the charge of the ion.

8. 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

Molecules and Compounds Compounds Display Constant Composition

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