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Chapter 2

Atoms,

Molecules,

and Ions


Contents and Concepts

Atomic Theory and Atomic Structure

The key concept in chemistry is that all matter is

composed of very small particles called atoms. We

look at atomic theory, discuss atomic structure,

and finally describe the periodic table, which

organizes the elements.

1. Atomic Theory of Matter

2. The Structure of the Atom

3. Nuclear Structure; Isotopes

4. Atomic Masses

5. Periodic Table of the Elements


Chemical Substances: Formulas and Names

We explore how atoms combine in various ways to

yield the millions of known substances.

6. Chemical Formulas; Molecules and Ionic

Substances

7. Organic Compounds

8. Naming Simple compounds

Chemical Reactions: Equations

9. Writing Chemical Equations

10. Balancing Chemical Equations


Learning Objectives

Atomic Theory and Atomic Structure

1. Atomic Theory of Matter

a. List the postulates of the atomic theory.

b. Define element, compound, and chemical reaction in the context of these postulates.

c. Recognize the atomic symbols of the elements.

d. Explain the significance of the law of multiple proportions.


2. The Structure of the Atom

a. Describe Thomson’s experiment in which he discovered the electron.

b. Describe Rutherford’s experiment that led

to the nuclear model of the atom.


3. Nuclear Structure; Isotopes

a. Name and describe the nuclear particles

making up the nucleus of the atom.

b. Define atomic number, mass number, and

nuclide.

c. Write the nuclide symbol for a given

nucleus.

d. Define and provide examples of isotopes of

an element.

e. Write the nuclide symbol of an element.


4. Atomic Masses

a. Define atomic mass unit and atomic mass.

b. Describe how a mass spectrometer can be

used to determine the fractional abundance

of the isotopes of an element.

c. Determine the atomic mass of an element

from the isotopic masses and fractional

abundance.


5. Periodic Table of the Elements

a. Identify periods and groups on the periodic

table.

b. Find the main-group and transition

elements on the periodic table.

c. Locate the alkali metal and halogen groups

on the periodic table.

d. Recognize the portions of the periodic table

that contain the metals, nonmetals, and

metalloids (semimetals).


Chemical Substances: Formulas and Names

6. Chemical Formulas; Molecular and Ionic

Substances

a. Determine when the chemical formula of a

compound represents a molecule.

b. Determine whether a chemical formula is

also a molecular formula.

c. Define ion, cation, and anion.

d. Classify a compound as ionic or molecular.

e. Define and provide examples for the term

formula unit.


f. Specify the charge on all substances,

whether ionic and molecular.

g. Write an ionic formula, given the ions.

7. Organic Compounds

a. List the attributes of molecular substances

that make them organic compounds.

b. Explain what makes a molecule a

hydrocarbon.

c. Recognize some functional groups of

organic molecules.


8. Naming Simple Compounds

a. Recognize ionic compounds.

b. Learn the rules for predicting the charges of

monatomic ions in ionic compounds.

c. Apply the rules for naming monatomic ions.

d. Learn the names and charges of common

polyatomic ions.

e. Name an ionic compound from its formula.

f. Write the formula of binary compound from

its name.


g. Determine the order of elements in a binary

(molecular) compound.

h. Learn the rules for naming binary molecular

compounds.

i. Name a binary compound from its formula.

j. Write the formula of a binary compound

from its name.

k. Name a binary molecular compound from

its molecular model.

l. Recognize molecular compounds that are

acids.


m. Determine whether an acid is an oxoacid.

n. Learn the approach for naming binary acids

and oxoacids.

o. Write the name and formula of an anion

from the acid.

p. Recognize compounds that are hydrates.

q. Learn the rules for naming hydrates.

r. Name a hydrate from its formula.

s. Write the formula of a hydrate from its

name.


Chemical Reactions: Equations

9. Writing Chemical Equations

a. Identify reactants and products in a

chemical equation.

b. Write chemical equations using appropriate

phase labels, symbols of reaction

conditions, and the presence of a catalyst.


10. Balancing Chemical Equations

a. Determine if a chemical reaction is

balanced.

b. Master the technique for balancing

chemical equations.


Postulates of Dalton’s Atomic Theory

All matter is composed of indivisible atoms. An atom is an extremely small particle of matter that retains its identity during chemical reactions.

An element is a type of matter composed of only one kind of atom.


A compound is a type of matter composed of atoms of two or more elements chemically combined in fixed proportions.

A chemical reaction involves the rearrangement of the atoms present in the reacting substances to give new chemical combinations present in the substances formed by the reaction.


Atomic Symbol

A one- or two-letter notation used to represent an

atom corresponding to a particular element.

The first letter must be UPPERCASE.

The second letter, when present, must be

lowercase.

Symbols come from the element name;

several names come from Latin.


Law of Multiple Proportions

When two elements form more than one

compound, the masses of one element in these

compounds for a fixed mass of the other element

are in small, whole-number ratios.


As an example, compare the mass of oxygen in

the two oxygen compounds of carbon:

Carbon monoxide 12 g C 16 g O

Carbon dioxide 12 g C 32 g O

The ratio of oxygen in CO2 to CO:

216

322

��g

g

COinoxygen

COinoxygen


Like Dalton, today’s chemists model

atoms using spheres. Modern models

are often drawn using computer

programs and use different colors to

represent atoms of different elements.

Which of these models depicts CO2?

A B C D EE is CO2


Structure of the Atom

The atom is composed of two parts:Nucleus

Central core

Positively charged

Contains most of the atom’s mass

Electrons

Very light

Negatively charged

Exist in the region around the nucleus


Discovery of the Electron

J. J. Thomson used an apparatus similar to the

one shown on the next slide, a cathode ray tube.

He discovered that the particles that make up the

cathode ray are negative and are part of all matter.

This finding is illustrated on the following slides.

As a result, Thomson concluded that atoms are

not indivisible, as Dalton had postulated.


Cathode Ray Tube


Cathode rays are attracted to

the positive end of the

magnet and repelled by the

negative end.


Thomson’s experiments allowed him to calculate

the ratio of the electron’s mass to its charge.

Robert Millikan calculated the charge on the

electron. He determined the charge on various

drops of oil and found the smallest increment in

charge—that is, the charge on the electron. This is

illustrated on the next slide.


Nuclear Model of the Atom

Experiments done in Ernest Rutherford’s

laboratory used positively charged alpha particles

to bombard very thin gold (and other metals) foil.

Most alpha particles passed through the foil, but a

few were scattered at large angles, sometimes

almost straight backward.

Based on these findings, Rutherford proposed that

99.95% of the atom’s mass is in the positively

charged nucleus.


Proton

A nuclear particle having a positive charge equal

to that of the electron and a mass more than 1800

times that of the electron.

The number of protons in an atom is called the

atomic number, Z.

An element is a substance whose atoms have the

same number of protons and thus the same atomic

number, Z.


Neutron

A nuclear particle having a mass almost equal to

that of the proton but no electrical charge

The mass number, A, is the total number of

protons and neutrons in the nucleus.

Isotopes are atoms whose nuclei have the same

atomic number (number of protons) but different

numbers of neutrons (mass number).


Nuclide

An atom characterized by a certain atomic

number, Z, and mass number, A.

Nuclide symbol

Examples: Li73 C13

6

symbolChemicalAZ

Na2311


Write the nuclide symbol for the

atom that has 19 protons and 20

neutrons.

K39

19

Atomic number: Z = 19

The element is potassium, K.

Mass number: A = 19 + 20 = 39

The nuclide symbol is


Atomic Mass

The average atomic mass for the naturally

occurring element expressed in atomic mass units

(amu).

Atomic Mass Unit

Equal to exactly one-twelfth the mass of a carbon-

12 atom


An element has four naturally occurring

isotopes. The mass and percentage of

each isotope are as follows:

Percentage Abundance Mass (amu)

1.48 203.973

23.6 205.9745

22.6 206.9759

52.3 207.9766

What is the atomic weight and name of

the element?


To find the portion of the atomic weight due to

each isotope, multiply the fraction by the mass of

the isotope. The atomic weight is the sum of these

products.

Fractional

Abundance

Mass (amu) Mass From

Isotope

0.0148 203.973 3.01880040

0.236 205.9745 48.6099820

0.226 206.9759 46.7765534

0.523 207.9766 108.771762

207.177098

The atomic weight is 207 amu; the element is lead.


Periodic Table of the Elements

A tabular arrangement of elements in rows and

columns, highlighting the regular repetition of

properties of the elements.


A period consists of the elements in one

horizontal row.

A group consists of the elements in one vertical

column.

Groups are numbered using two systems:

IUPAC Numbers 1 through 18

Older system Roman numerals I–VIII and

the letters A and B


Metal

A substance or mixture that has a characteristic luster or shine and is generally a good conductor of heat and electricity.

Nonmetal

An element that does not exhibit the characteristics of a metal

Metalloid

An element having characteristics of both metals and nonmetals


Chemical Formula

A notation that uses atomic symbols with

numerical subscripts to convey the relative

proportions of atoms of the different elements in

the substance.


Molecule

A definite group of atoms that are chemically

bonded together (tightly connected by attractive

forces).

Molecular Formula

A formula giving the exact number of different

atoms of an element in a molecule.


Polymer

A very large molecule that is made up of a number

of smaller molecules repeatedly linked together.

Monomers

The small molecules that are linked together to

form a polymer.


An ion is an electrically charged particle obtained from an atom or chemically bonded group of atoms by adding or removing one or more electrons.

A cation is a positively charged ion formed by losing one or more electrons. A cation is named by its element name followed by the word ion. For example, Na+ is the sodium ion, etc. Under the definition of anion add the following: An anion is named by changing the last part of the element name to –ide followed by the word ion. For example, F- is the fluoride ion, O2- is the oxide ion, N3- is the nitride ion, etc.

An anion is a negatively charged ion formed by gaining one or more electrons.


An ionic compound is composed of cations and anions.

Ions are arranged in a repeating three-dimensional pattern, forming a crystal.

The formula of an ionic compound gives the smallest possible integer number of ions in the substance (without writing charges) so that the combination is electrically neutral.

The formula gives the formula unit of the compounds. A formula unit is not a molecule!


What is formula of the ionic

compound of Mg2+ and N3-?

The common multiple of the charges is 6, so we

need three Mg2+ and two N3-. The resulting formula

is

Mg3N2


What is the formula of the ionic

compound of Ca2+ and PO43-?

The common multiple of the charges is 6, so we

need three Ca2+ and two PO43-. The resulting

formula is

Ca3(PO4)2


Organic Compounds

An important class of molecular substances; they

contain carbon combined with other elements –

notably hydrogen, oxygen, and nitrogen.

Hydrocarbons contain only carbon and hydrogen.


A functional group is a reactive portion of a

molecule that undergoes predictable reactions.

ExamplesName of

Group

Functional

Group

Methyl alcoholAlcohol

Dimethyl etherEther

Acetic acidCarboxylic

acid


Chemical nomenclature is the systematic naming

of chemical compounds.

Compounds that are not organic are called

inorganic compounds.

Carbon monoxide, carbon dioxide,

carbonates, and cyanides are also classified

as inorganic compounds.


Naming Inorganic Compounds

1. The cation is always named first with the

name of the cation without the word ion.

2. The anion is named second with the name

of the anion without the word ion.

3. Examples:

Cation Anion Name

Na+ Br- Sodium bromide

Mg2- O2- Magnesium oxide

Al3+ Cl- Aluminum chloride


Monatomic ions

Ions formed from a single atom.

Common monatomic ions are listed on the next

slide by group and by period.


Common Monatomic Ions of the

Main-Group Elements

Bi3+Pb2+Tl3+,

Tl+Ba2+Cs+

I-Te2-Sn2+In3+Sr2+Rb+

Br-Se2-Ga3+Ca2+K+

Cl-S2-Al3+Mg2+Na+

F-O2-N3-Be2+Li+

H-

6

5

4

3

2

1

VIIAVIAVAIVAIIIAIIAIAPeriod


Rules for Predicting the Charge on a

Monatomic Ion

1. Most main-group metals have one monatomic

ions with a charge equal to the group number.

Al, in Group IIIA(3), has one ion, Al3+.

Sr, in Group IIA(2), has one ion, Sr2+.

K, in Group IA(1), has one ion, K+.


2. Some main-group metals with high atomic

number have more than one cation. One cation

will have the charge of the group number minus

2; the second cation will have a charge equal to

the group number

Pb in Group IVA(14) has two ions:

Pb2+ and Pb4+

Tl in Group IIIA(13) has two ions:

Tl+ and Tl3+


3. Most transition metals form more than one

cation, of which one is +2.

Zn and Cd form only the +2 ion.

Ag forms only the +1 ion.

4. Nonmetal main-group elements form one

monatomic anion with a charge equal to the

group number minus 8.

F in Group VIIA(17) forms the F- ion.

S in Group VIA(16) forms the S2- ion.

N in Group VA(15) forms the N3- ion.


Naming Monatomic Ions

Monatomic cations are named after the element if

the element forms only one cation.


If more than one cation forms:

a. In the Stock system, the charge is written using a Roman numeral and is enclosed in parentheses.

Cu2+ is copper(II).

Cu+ is copper(I).

b. In an older system, the suffic –ic (for the higher-charged cation) or –ous for the lower-charged cation) is added to the element’s stem.

Cu2+ is cupric.

Cu+ is cuprous.


Fe3+ is iron(III) or ferric ion.

Fe2+ is iron(II) or ferrous ion.

Hg2+ is mercury(II) or mercuric ion.

The second ion mercury forms is diatomic:

Hg22+ is mercury(I) or mercurous ion.


Cr3+ is chromium(III) or chromic ion.

Cr2+ is chromium(II) or chromous ion.

Mn2+ is manganese(II) or manganous ion.

Co2+ is cobalt(II) or cobaltous ion.

Zinc forms only Zn2+, so it is called zinc ion.

Cadmium forms only Cd2+, so it is called cadmium

ion.

Silver forms only Ag+, so it is called silver ion.


Polyatomic Ion

An ion consisting of two or more atoms chemically

bonded together and carrying an electrical charge.

Table 2.5 lists common polyatomic ions.


Cations

mercury(I) or mercurous Hg22+

ammonium NH4+

Anions

peroxide O2-

hydroxide OH-

cyanide CN-


phosphate PO43-

monohydrogen phosphate HPO42-

dihydrogen phosphate H2PO4-

carbonate CO32-

hydrogen carbonate (bicarbonate) HCO3-

sulfate SO42-

hydrogen sulfate (bisulfate) HSO4-

sulfite SO32-

hydrogen sulfite (bisulfite) HSO3-


acetate C2H3O2-

oxalate C2O42-

chromate CrO42-

dichromate Cr2O72-

permanganate MnO4-

nitrate NO3-

nitrite NO2-


hypochlorite ClO-

chlorite ClO2-

chlorate ClO3-

perchlorate ClO4-


What are the names of the following

ionic compounds?

BaO

Cr2(SO4)3

BaO is barium oxide.

Cr2(SO4)3 is chromium(III) sulfate or chromic

sulfate.


What are the chemical formulas for

the following ionic compounds?

potassium carbonate

manganese(II) sulfate

The ions K+ and CO32- form K2CO3

The ions Mn2+ and SO42- form MnSO4


Binary Molecular Compounds

A compound composed of only two elements.

Binary compound of a metal and a nonmetal are

generally named using ionic rules.


Naming Binary Molecular Compounds

We usually name the elements in the order given

in the formula.

Name the first element using the element name.

Name the second element using the element root

+ -ide suffix.


Add a prefix to each name to indicate the number

of atoms of that element. The prefix mono- is used

only when needed to distinguish two compounds

of the same two elements.

The final vowel of the prefix is often dropped when

followed by an element name that begins with a

vowel. Oxygen is the most common example.

N2O4 dinitrogen tetroxide (“a” is dropped)

NO nitrogen monoxide (only one “o”)

(also called nitric oxide)


Prefixes (Greek)

One (1) mono-

Two (2) di-

Three (3) tri-

Four (4) tetra-

Five (5) penta-

Six (6) hexa-

Seven (7) hepta-

Eight (8) octa-

Nine (9) nona-

Ten (10) deca-


Some compounds have common names that differ

from their systematic names:

H2S hydrogen sulfide (the “di” is omitted)

H2O water

NH3 ammonia

Common names need to be memorized.


Acids and Corresponding Anions

Oxoacids contain hydrogen, oxygen, and a third central atom.

To name an acid from its anion name:

1. Change an –ate suffix to –ic.

2. Change an –ite suffix to –ous.

3. Add the word “acid.””

For example:

HNO3 nitric acid

H2SO4 sulfuric acid


sulfuric

acid

H2SO4sulfate ionSO42-

sulfurous

acid

H2SO3sulfite ionSO32-

phosphoric

acid

H3PO4phosphate ionPO43-

nitric acidHNO3nitrate ionNO3-

nitrous

acid

HNO2nitrite ionNO2-

carbonic

acid

H2CO3carbonate

ion

CO32-

OxoacidOxoanion


hypochlorous

acid

HClOhypochlorite

ion

ClO-

chlorous

acid

HClO2chlorite

ion

ClO2-

chloric

acid

HClO3chlorate

ion

ClO3-

perchloric

acid

HClO4perchlorate

ion

ClO4-

OxoacidOxoanion


What are the names of the

following compounds?

OF2

S4N4

BCl3

OF2 is oxygen difluoride

S4N4 is tetrasulfur tetranitride

BCl3 is boron trichloride


What are the formulas for the

following binary molecular

compounds?

carbon disulfide

nitrogen tribromide

dinitrogen tetrafluoride

The formula for carbon disulfide is CS2.

The formula for dinitrogen tetrafluoride is N2F4.

The formula for nitrogen tribromide is NBr3.


Bromine has an oxoacid, HBrO2,

bromous acid (compare to HClO2,

chlorous acid). What are the name

and formula of the corresponding

anion?

The anion corresponding to HBrO2 is

bromite, BrO2-.


Hydrate

A compound that contains water molecules weakly

bound in the crystals.

The formula of a hydrate is written with a dot

before the water molecule(s) included.

For example:

CuSO4�5H2O


Hydrates are named using the anhydrous (without

water) compound name followed by the prefix for

the number of water molecules included and the

word “hydrate.”

For example:

CuSO4�5H2O is named

copper(II) sulfate pentahydrate.


A compound whose common name

is green vitriol has the chemical

formula FeSO4�7H2O. What is the

chemical name of this compound?

FeSO4�7H2O is iron(II) sulfate heptahydrate.


Calcium chloride hexahydrate is

used to melt snow on roads. What

is the chemical formula of the

compound?

The chemical formula for calcium chloride

hexahydrate is CaCl2�6H2O.


A chemical equation is the symbolic

representation of a chemical reaction in terms of

chemical formulas.

For example: 2Na + Cl2 � 2NaCl

Reactants are the starting materials; they are

written on the left of the equation.

Products are the materials at the end of the

reaction; they are written on the right of the

equation.


Because a reaction must accurately describe the

chemical reaction, it must be consistent with the

law of conservation of mass.

When this is not the case, after correct formulas

are written for each reactant and product, the

coefficients are adjusted so that the same number

of each atom is present in both the reactants and

the products.

This is called balancing the equation.


For example, the reaction of sodium with chlorine

produced sodium chloride.

First, we determine the correct formula for each

compound.

Sodium is Na.

Chlorine is Cl2.

Sodium chloride is NaCl.


Second, we write the reaction.

Na + Cl2 � NaCl

Third, we check the number of each atom on each

side of the equation.

This equation shows two Cl atoms on the reactant

side and only one Cl atom on the product side. To

balance the Cl atoms, we insert a coefficient of “2”

before NaCl on the product side.

Na + Cl2 � 2NaCl


Na + Cl2 � 2NaCl

Now the Na are not balanced: there is one on the

reactant side and there are two on the product

side. To balance Na, we insert the coefficient “2”

before Na on the reactant side.

2Na + Cl2 � 2NaCl

The reaction is now balanced!


Balance the following equation:

CS2 + O2 � CO2 + SO2

Tally the number of each atom on each side:

C 1 on reactant side; 1 on product side

S 2 on reactant side; 1 on product side

O 2 on reactant side; 4 on product side

Begin by inserting the coefficient “2” before

SO2 on the product side. We leave O2 until

later because it is an element.


CS2 + O2 � CO2 + 2SO2

Tally the atoms again:




Insert a “3” before O2:

CS2 + 3O2 � CO2 + 2SO2


CS2 + 3O2 � CO2 + 2SO2








NH3 + O2 � NO + H2O


N 1 on reactant side; 1 on product side

H 3 on reactant side; 2 on product side


Begin by inserting the coefficient “2” before

NH3 on the reactant side and the coefficient “3”

before H2O on the product side. We leave O2

until later because it is an element.


2NH3 + O2 � NO + 3H2O





To balance N, insert a “2” before NO:

2NH3 + O2 � 2NO + 3H2O


2NH3 + O2 � 2NO + 3H2O





Since this gives us an odd number oxygens, we

double the coefficients on NH3, NO, and H2O

and to balance O, insert a “5” before O2.


Tally the atoms again to double check:

4NH3 + 5O2 � 4NO + 6H2O







C2H5OH + O2 � CO2 + H2O





Begin by balancing H. Insert the coefficient “3”

before H2O on the product side. We leave O2

until later because it is an element.


C2H5OH + O2 � CO2 + 3H2O





To balance C, insert a “2” before CO2.


C2H5OH + O2 � 2CO2 + 3H2O





To balance O, insert a “3” before O2.


C2H5OH + 3O2 � 2CO2 + 3H2O






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