Chapter 2 Atoms, Molecules, and Ions

Chapter 2Atoms,

Molecules, and Ions

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 C16 g O

Carbon dioxide 12 g C32 g O

The ratio of oxygen in CO2 to CO:2

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.

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Cathode Ray Tube

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Cathode rays are attracted to the positive end of the magnet and repelled by the negative end.

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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.

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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.

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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.

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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).

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Nuclide

An atom characterized by a certain atomic number, Z, and mass number, A.

Nuclide symbol

Examples: Li73 C13

symbolChemicalAZ

Na2311

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Write the nuclide symbol for the atom that has 19 protons and 20 neutrons.

Atomic number: Z = 19The element is potassium, K.

Mass number: A = 19 + 20 = 39

The nuclide symbol is

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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

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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?

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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

207.177098The atomic weight is 207 amu; the element is lead.

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Periodic Table of the Elements

A tabular arrangement of elements in rows and columns, highlighting the regular repetition of properties of the elements.

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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

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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

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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.

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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.

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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.

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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.

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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!

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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

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What is the formula of the ionic compound of Ca2+ and PO4

The common multiple of the charges is 6, so we need three Ca2+ and two PO4

3-. The resulting formula is

Ca3(PO4)2

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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.

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A functional group is a reactive portion of a molecule that undergoes predictable reactions.

ExamplesName of Group

Functional Group

Methyl alcoholAlcoholOH

Dimethyl etherEtherO

Acetic acidCarboxylic acid

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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.

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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

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Monatomic ions

Ions formed from a single atom.

Common monatomic ions are listed on the next slide by group and by period.

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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+

VIIAVIAVAIVAIIIAIIAIAPeriod

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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+.

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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+

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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.

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Naming Monatomic Ions

Monatomic cations are named after the element if the element forms only one cation.

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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.

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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.

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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.

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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.

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Cations

mercury(I) or mercurous Hg22+

ammonium NH4+

Anions

peroxide O2-

hydroxide OH-

cyanide CN-

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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-

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acetate C2H3O2-

oxalate C2O42-

chromate CrO42-

dichromate Cr2O72-

permanganate MnO4-

nitrate NO3-

nitrite NO2-

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hypochlorite ClO-

chlorite ClO2-

chlorate ClO3-

perchlorate ClO4-

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What are the names of the following ionic compounds?

Cr2(SO4)3

BaO is barium oxide.Cr2(SO4)3 is chromium(III) sulfate or chromic sulfate.

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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

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Binary Molecular Compounds

A compound composed of only two elements.

Binary compound of a metal and a nonmetal are generally named using ionic rules.

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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.

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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)

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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-

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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.

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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

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sulfuric acid

H2SO4sulfate ionSO42-

sulfurous acid

H2SO3sulfite ionSO32-

phosphoric acid

H3PO4phosphate ionPO43-

nitric acidHNO3nitrate ionNO3-

nitrous acid

HNO2nitrite ionNO2-

carbonic acid

H2CO3carbonateion

OxoacidOxoanion

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hypochlorous acid

HClOhypochlorite ion

chlorous acid

HClO2chlorite ion

chloric acid

HClO3chlorate ion

perchloric acid

HClO4perchlorate ion

OxoacidOxoanion

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What are the names of the following compounds?

OF2 is oxygen difluoride

S4N4 is tetrasulfur tetranitride

BCl3 is boron trichloride

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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.

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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 isbromite, BrO2

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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:

CuSO45H2O

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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:

CuSO45H2O is named

copper(II) sulfate pentahydrate.

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A compound whose common name is green vitriol has the chemical formula FeSO47H2O. What is the chemical name of this compound?

FeSO47H2O is iron(II) sulfate heptahydrate.

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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 CaCl26H2O.

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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.

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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.

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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.

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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

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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!

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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 sideS 2 on reactant side; 1 on product sideO 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.

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CS2 + O2 CO2 + 2SO2

Tally the atoms again:

C 1 on reactant side; 1 on product side

S 2 on reactant side; 2 on product side

O 2 on reactant side; 6 on product side

Insert a “3” before O2:

CS2 + 3O2 CO2 + 2SO2

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CS2 + 3O2 CO2 + 2SO2

S 2 on reactant side; 2 on product side

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NH3 + O2 NO + H2O

Tally the number of each atom on each side:N 1 on reactant side; 1 on product sideH 3 on reactant side; 2 on product sideO 2 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.

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2NH3 + O2 NO + 3H2O

N 2 on reactant side; 1 on product side

H 6 on reactant side; 6 on product side

To balance N, insert a “2” before NO:

2NH3 + O2 2NO + 3H2O

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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.

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Tally the atoms again to double check:

4NH3 + 5O2 4NO + 6H2O

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C2H5OH + O2 CO2 + H2O

Tally the number of each atom on each side:C 2 on reactant side; 1 on product sideH 6 on reactant side; 2 on product sideO 3 on reactant side; 3 on product side

Begin by balancing H. Insert the coefficient “3” before H2O on the product side. We leave O2 until later because it is an element.

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C2H5OH + O2 CO2 + 3H2O

Tally the number of each atom on each side:

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

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C2H5OH + O2 2CO2 + 3H2O

To balance O, insert a “3” before O2.

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C2H5OH + 3O2 2CO2 + 3H2O

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