Chapter 5 Molecules and Compounds

2006, Prentice Hall

Sucrose molecule (sugar), contains C, H, and O atoms.

However, the properties of sucrose are very different

from those of C, H, and O alone. The properties of a

compound are, in general, different from the properties

of the elements that compose it.

molecular formula C12H22O11

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CHAPTER OUTLINE

Definitions of Molecules and Compounds Law of Constant Composition Types of Compounds Binary Ionic Compounds (Type I and Type II) Polyatomic Ions Binary Molecular Compounds Naming Acids and Oxyacids Formula Mass

• molecule - consist of at least two different atoms in a definite arrangement held together by very strong chemical bonds.

• compound is a substance consisting of two or more elements in fixed and definite proportions

Definition of Molecule and Compound

Molecules and Compounds

• Salt– Sodium – shiny, reactive,

poisonous– Chlorine – pale yellow gas,

reactive, poisonous– Sodium chloride – table salt

• Sugar– Carbon – pencil or diamonds– Hydrogen – flammable gas– Oxygen – a gas in air– Combine to form white crystalline

sugar

Law of Constant Composition• all pure substances have constant composition

– all samples of a pure substance contain the same elements in the same percentages (ratios)

– mixtures have variable composition

H2O molecules

always a 2 H to 1 O

ratio

H2 & O2 mixture;

we could vary the

amount of either

element

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LAW OF DEFINITECOMPOSITION

In a pure compound, the elements are always present in the same definite proportion by mass.

Mass of sample

Mass of N Mass of H

Sample 1 1.840 g 1.513 g 0.327 g

Sample 2 2.000 g 1.644 g 0.356 g

% N in Sample 1 =

% N in Sample 2 =

1.513 gx100 = 82.23%

1.840 g1.644 g

x100 = 82.20%2.000 g

Based on this law, the mass of an element can be determined from its mass percent in a compound.

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Barium iodide, BaI2, contains 35.1% barium by mass. How many grams of barium does an 8.50 g sample of barium iodide contain?

Example 1:

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g Ba8.50 g BaI x = g Ba

g BaI

35.1

1002.98

3 significant figures

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When 12.66 g of calcium are heated in air, 17.73 g of calcium oxide is formed. What is the percent of oxygen in this compound?

Example 2:

3 significant figures

Mass of oxygen = 17.73 g – 12.66 g = 5.07 g

Percent oxygen =5.07 g

x 100 = 28.6%17.73 g

Formulas Describe Compounds

• a compound is a distinct substance that is composed of atoms of two or more elements

• a compound can be described by the number and type of each atom in the simplest unit of the compound– molecules or ions

• each element is represented by its letter symbol• the number of atoms of each element is written

to the right of the element as a subscript

Formulas Describe Compounds

water = H2O two atoms of

hydrogen and 1 atom of oxygen

table sugar = C12H22O11 12

atoms of C, 22 atoms of H and 11

atoms O

Tro's Introductory Chemistry, Chapter 5

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Order of Elements in a Formula• metals written first

– NaCl

• nonmetals written in order from Table 5.1– CO2

– there are occasional exceptions for historical or informational reasons

• H2O fits rule, but does NaOH?

Table 5.1Order of Listing Nonmetalsin Chemical Formulas

C P N H S I Br Cl O F

Types of Chemical Formulas• An empirical formula gives the relative number of

atoms of each element in a compound. • A molecular formula gives the actual number of

atoms of each element in a molecule of the compound.

• For example, the molecular formula for hydrogen peroxide is H2O2, and its empirical formula is HO.

• The molecular formula is always a whole number multiple of the empirical formula.

• For many compounds, such as H2O, the molecular formula is the same as the empirical formula.

• A structural formula uses lines to represent chemical bonds and shows how the atoms in a molecule are connected to each other.

Comparison of Formulas and Models for Methane, CH4

• The molecular formula of methane indicates that methane has 1 carbon atom and 4 hydrogen atoms.

• The structural formula shows how the atoms are connected: each hydrogen atom is bonded to the central carbon atom.

• The ball-and-stick model and the space-filling model illustrate the geometry of the molecule: how the atoms are arranged in three dimensions.

Tro's Introductory Chemistry, Chapter 5

Classifying Materials• atomic elements = elements whose

particles are single atoms

• molecular elements = elements whose particles are multi-atom molecules

• molecular compounds = compounds whose particles are molecules made of only nonmetals

• ionic compounds = compounds whose particles are cations and anions

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Molecular Elements• Certain elements occur as 2 atom molecules• Rule of 7’s

– there are 7 common diatomic elements– find the element with atomic number 7, N– make a figure 7 by going over to Group 7A, then down– don’t forget to include H2

H2

Cl2

Br2

I2

7VIIA

N2 O2 F2

Molecular Compounds

• two or more nonmetals

• smallest unit is a molecule

Ionic Compounds

• metals + nonmetals

• no individual molecule, instead these have 3-dimensional array of cations and anions made of formula units (NaCl)

Classify each of the following as either an atomic element, molecular element,

molecular compound or ionic compound

• aluminum, Al

• aluminum chloride, AlCl3• chlorine, Cl2• acetone, C3H6O

• carbon monoxide, CO

• cobalt, Co

= atomic element

= ionic compound

= molecular element

= molecular compound

= molecular compound

= atomic element

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TYPES OF COMPOUNDS

Compounds are pure substances that contain 2 or more elements combined in a definite proportion by mass.

Compounds can be classified as one of two types:

Metals and non-metals

Two non-metals

Nomenclature of Compounds

Common Names – Are Exceptions(like nicknames)

H2O = water, steam, ice

NH3 = ammonia

CH4 = methane

NaCl = table salt

C12H22O11 = table sugar

Metal Cations• Type I

– metals whose ions can only have one possible charge

• IA, IIA, (Al, Ga, In)

– determine charge by position on the Periodic Table

• IA = +1, IIA = +2, (Al, Ga, In = +3)

• Type II– metals whose ions can have more

than one possible charge– determine charge by charge on

anion

How do you know a metal cation is Type II?

its not Type I !!!

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BINARY IONICCOMPOUNDS (TYPE I)

Binary compounds contain only two elements. Ionic compounds are formed by combination of a

metal and a non-metal. Type I ions are those cations that form only one

ion. In these compounds, charges of the cations must

equal the charges of the anions since the net charge is zero.

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BINARY IONICCOMPOUNDS (TYPE I)

Subscripts are used to balance the charges between cations and anions.

sodium bromide

Na+ Br

+1 -1 = 0

NaBr

No subscripts

needed

potassium sulfide

K+ S2

+1 -2 0

K2+

+2 -2 = 0

K2S

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Write formulas for the following ionic compounds:

calcium chloride

Ca2+ Cl

+2 -1 0

CaCl2

sodium sulfide

Na+ S2

+1 -2 0+2 -2 = 0

Na2S

Example 1:

Cl2

+2 -2 = 0

Na2+

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Type I Binary Ionic Compounds

• Contain Metal Cation + Nonmetal Anion

• Metal listed first in formula & name

1. name metal cation first, name nonmetal anion second

2. cation name is the metal name

3. nonmetal anion named by changing the ending on the nonmetal name to -ide

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BINARY IONICCOMPOUNDS (TYPE I)

MgCl2 magnesium chloride

NaI sodium iodide

AlF3 aluminum fluoride

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Name the following ionic compounds:

barium chloride

Na3P sodium phosphide

BaCl2

Example 2:

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BINARY IONICCOMPOUNDS (TYPE II)

Type II ions are those cations that form more than one ion.

When naming compounds formed from these ions, include the ionic charge as Roman numeral, in parentheses, after the metal’s name.

This method of nomenclature is called the “stock” system.

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1. name metal cation first, name nonmetal anion second

2. metal cation name is the metal name followed by a Roman Numeral in parentheses to indicate its charge (only difference from type II!)– determine charge from anion charge

3. nonmetal anion named by changing the ending on the nonmetal name to -ide

BINARY IONICCOMPOUNDS (TYPE II)

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BINARY IONICCOMPOUNDS (TYPE II)

FeCl2

? -1

? -2 = 0

+2 -1iron (II) chloride

FeCl3

? -1

? -3 = 0

+3 -1iron (III) chloride

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BINARY IONICCOMPOUNDS (TYPE II)

Cu2O

? -2

2? -2 = 0

+1 -2copper (I) oxide

Roman numeral DOES NOT

represent the subscriptCuO

? -2

? -2 = 0

+2 -2copper (II) oxide

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BINARY IONICCOMPOUNDS (TYPE II)

Type II cations can also be named by an older method (classical).

In this system, cations with the higher charge end in –ic, while cations with the lower charge end in –ous.

In this system, some cations are named based on their Latin roots.

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CLASSICAL SYSTEM (DERIVED FROM LATIN)

gold (aurum for Aurora the Roman goddess of the dawn)

silver (argentum for 'bright')

copper (cuprum for 'Cyprus' where the Romans first obtained copper)

tin (stannum for alloys containing lead)

lead (plumbum for 'lead')

mercury (hydrargyrum for 'liquid silver' or quick silver)

antimony (stibium for 'not alone')

iron (ferrum for 'firmness‘)

potassium (kalium via the Arabic qali for alkali)

sodium (natrium for soda)

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BINARY IONICCOMPOUNDS (TYPE II)

FeCl2

+2 -1ferrous chloride

FeCl3

+3 -1ferric chloride

Lower charge

Higher charge

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BINARY IONICCOMPOUNDS (TYPE II)

Cu2O+1 -2

cuprous oxide

CuO+2 -2

cupric oxide

Lower charge

Higher charge

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SnCl2

? -1

? -2 = 0

+2 -1tin (II) chloride

Example 1:

Name each of the following compounds using the stock and classical nomenclature system:

Stock

ClassicalLower chargestannous chloride

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Cu2S

? -2

2? -2 = 0

+1 -2copper (I) sulfide

Example 1:

Name each of the following compounds using the stock and classical nomenclature system:

Stock

ClassicalLower chargecuprous sulfide

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Sn

Example 2:

Write formulas for each of the following compounds:

Tin (II) bromide Br

+2 -1

+2 -1 0

Br2

SnBr2

+2 -2 = 0

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Sn

Example 2:

Write formulas for each of the following compounds:

Stannic oxide O

+4 -2

+4 -2 0

SnO2

+4 - 4 = 0

O2

ExamplesWhat, if anything, should go into the parenthesis

• LiCl = lithium ( ) chloride

• AlCl3 = aluminum ( ) chloride

• PbO = lead ( ) oxide

• PbO2 = lead ( ) oxide

• Mn2O3 = manganese ( ) oxide

XX

II

IV

III

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POLYATOMIC IONS

Some ionic compounds contain polyatomic ions, an ion composed of several atoms bound together.

Patterns for Polyatomic Ions

1. elements in the same column form similar polyatomic ions

– same number of O’s and same charge

ClO3- = chlorate BrO3

- = bromate

2. if the polyatomic ion starts with H, the name adds hydrogen- prefix before name and add 1 to the chargeCO3

2- = carbonate HCO3-1 = hydrogencarbonate

Periodic Pattern of Polyatomic Ions

-ate groups

BO3-3 NO3

-1

SiO3-2

PO4-3

SO4-2

ClO3-1

AsO4-3

SeO4-2

BrO3-1

TeO4-2

IO3-1

CO3-2

IIIA IVA VA VIA VIIA

Patterns for Polyatomic Ions• -ate ion

– chlorate = ClO3-1

• -ate ion + 1 O same charge, per- prefix– perchlorate = ClO4

-1

• -ate ion – 1 O same charge, -ite suffix– chlorite = ClO2

-1

• -ate ion – 2 O same charge, hypo- prefix, -ite suffix– hypochlorite = ClO-1

© 2012 Pearson Education, Inc.

Naming Ionic Compounds Containing a Polyatomic Ion

• Some examples of more than two ions in the series.

ClO − hypochlorite BrO

− hypobromite IO −

hypoioditeClO2

− chlorite BrO2− bromite IO2

− ioditeClO3

− chlorate BrO3− bromate IO3

−

iodateClO4

− perchlorate BrO4− perbromate IO4

− periodate

When naming these ions in the homework, be sure to include the word ion in your answer, as in “perchlorate ion.”

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POLYATOMICCOMPOUNDS

When writing formulas for polyatomic compounds, treat the polyatomic ion as one group.

potassium nitrate

K+ NO3–

+1 -1 = 0

KNO3

calcium hydroxide

Ca2+ OH–

+2 -1 0

(OH–)2

Ca(OH)2

+2 -2 = 0

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POLYATOMICCOMPOUNDS

ammonium acetate

NH4+ C2H3O2

–

+1 -1 = 0

NH4C2H3O2

sodium sulfate

Na+ SO42–

+1 -2 0

Na2+

Na2SO4

+2 -2 = 0

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POLYATOMICCOMPOUNDS

copper (II) nitrate

Cu2+ NO3–

+2 -1 0

Cu(NO3)2

(NO3–)2

+2 -2 = 0

Type IIRoman numeral

represents charge of ion

Alternate name =cupric nitrate

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Mg(OH)2magnesium hydroxide

NaCN

Example 2:

Name the following polyatomic compounds:

Type I ion(Does not require roman numeral)

sodium cyanide

Fe2(SO4)3

Type II ion(Requires roman

numeral)

+3 2

iron (III) sulfate

ferric sulfate

Subclasses

• Compounds containing a metal and a nonmetal = binary ionic– Type I and II

• Compounds containing a polyatomic ion = ionic with polyatomic ion

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BINARY MOLECULAR COMPOUNDS

Molecular compounds are formed by combination of 2 or more non-metals.

The smallest particles of molecular compounds are molecules.

These compounds are named similar to ionic compounds, with the second element named based on its root and suffix “-ide”.

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BINARY MOLECULAR COMPOUNDS

Greek prefixes are used to indicate the number of atoms in these compounds.

Number Prefix Number Prefix

1 mono- 6 hexa-

2 di- 7 hepta-

3 tri- 8 octa-

4 tetra- 9 nona-

5 penta- 10 deca-

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Examples:

Name the following binary molecular compounds:

CS2

carbon disulfide

First atom uses a prefix only when more than one

atom is present

Second atom always uses a

prefix

indicates 1 carbon atom

indicates 2 sulfur atoms

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Examples:

Name the following binary molecular compounds:

IF7

iodine heptafluoride

indicates 1 iodine atom

indicates 7 fluorine atoms

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SUMMARY OF BINARYNOMENCLATURE

Ionic Molecular

Type I

Type II

Naming Acids

• Contain H+ cation and anion– in aqueous solution

• Binary acids have H+ cation and nonmetal anion

• Oxyacids have H+ cation and polyatomic anion

Naming Acids• Binary Acids = hydro prefix + stem of the name of the

nonmetal + ic suffix

Example: HCl hydrochloric acid

• Oxyacids– if polyatomic ion ends in –ate = name of polyatomic ion with –ic

suffix

Example: HNO3 nitric acid– if polyatomic ion ends in –ite = name of polyatomic ion with –

ous suffix

Example: H2SO3 sulfurous acid

base name of oxyanion + -ic acid

base name of oxyanion + -ous acid

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ACIDS

Acids are molecular compounds that form ions when dissolved in water.

Binary Acids

Formulas are written similar to binary ionic compounds, assigning a +1 charge to hydrogen.

HCl+1 -1

H2S+1 -2

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NAMING BINARY ACIDS

When naming the acids, use hydro- prefix, followed by the name of the non-metal with an –ic ending, followed with the word acid.

HCl

H2S

acidhydrochloric

hydrosulfuric acid

HF hydrofluoric acid

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POLYATOMICACIDS

Several polyatomic acids are important in the study of chemistry, and their names must be learned.

These acids and the polyatomic ions that form from their ionization are as follows:

Most of these are oxyacids

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POLYATOMICACIDS

sulfuric acid H2SO4SO4

2

sulfate

phosphoric acid H3PO4PO4

3

phosphate

nitric acid HNO3NO3

nitrate

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POLYATOMICACIDS

carbonic acid H2CO3

CO32

carbonate

HCO3

bicarbonate

acetic acid HC2H3O2C2H3O2

acetate

The names of acids containing oxyanions ending with -ite take this form:

The names of acids containing oxyanions ending with -ate take this form:

base name of oxyanion + ous

base name of oxyanion + ic

Names of Some Common Oxyacids and Their Oxyanions

Formula Mass: The Mass of a Molecule or Formula Unit

• also known as molecular mass

• For any compound, the formula mass is the sum of the atomic masses of all the atoms in its chemical formula:

Mass of 1 molecule of H2O = 2(1.01 amu H) + 16.00 amu O = 18.02 amu

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THE END