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Chapter 2Atoms and Molecules

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Objectives

Atomic StructureThe Periodic TableAtomic Number, Mass Number, and Isotopes

Compounds

Ions

Writing Formulas of Compounds Writing Names of Compounds

Modern Atomic Theory

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All matter is made up of tiny particles called atoms.Atoms of the same element are chemically alike; atoms of different elements are chemically different.

Individual atoms of the same element may not all have the same mass. However, the atoms of an element, as it occurs naturally, have a definite average mass that is characteristic of that element.

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Atoms are not subdivided in chemical changes.

All atomsatoms of the same elementsame element have the same numbersame number of protonsprotons.

All atomsatoms are neutralneutral particles. The numbernumber of electronselectrons must equalequal to the numbernumber of protonsprotons.

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The symbolsymbol of an element is used to represent one atomone atom of the element.

An atomatom is the smallest particle of an element that can exist either alone or in combination with other such particles.

nucleus

energy levels

electrons

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TThe nucleusnucleus of an atom is the very small, centrally located, positively charged part of the atom. It is made up of subatomic particles called protonsprotons and neutronsneutrons.

ProtonsProtons are subatomic particles that have a positive charge and are found in the nucleus of atom. The chargecharge of a proton is +1+1.

Neutrons Neutrons are electrically neutral subatomic particles found in the nucleus of an atom. Since the neutron is electrically neutral, its chargecharge is 00.

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AAn energy levelenergy level, also called an electron electron shellshell, is a region around the nucleus of an atom in which electrons move about.

ElectronsElectrons are negatively charged subatomic particles that move around the nucleus in different regions called energy levels or electron shells. The chargecharge of an electron is -1-1.

1st energy level = K-shell = 2 1st energy level = K-shell = 2 electronselectrons

1st energy level = K-shell = 2 1st energy level = K-shell = 2 electronselectrons

2nd energy level = L-shell = 8 2nd energy level = L-shell = 8 electronselectrons

2nd energy level = L-shell = 8 2nd energy level = L-shell = 8 electronselectrons

3rd energy level = M-shell = 18 3rd energy level = M-shell = 18 electronselectrons

3rd energy level = M-shell = 18 3rd energy level = M-shell = 18 electronselectrons

4th energy level = N-shell = 32 4th energy level = N-shell = 32 electronselectrons

4th energy level = N-shell = 32 4th energy level = N-shell = 32 electronselectrons

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The Periodic TablePeriodic Table is an arrangement of the elements based on increasing atomic numbers such that elements with similar properties are in the same vertical column.

Dmitri MendeleevDmitri Mendeleev – Russian chemist credited for the first workable periodic table. (His table was based on increasing atomic masses.)

A groupgroup or familyfamily of elements consists of a vertical column of elements on the periodic table.

A periodperiod or seriesseries of elements consists of a horizontal row of elements on the periodic table.

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The alkali metalsalkali metals are the elements in column I of the periodic table.

The alkaline earth metalsalkaline earth metals are the elements in column II of the periodic table.

The halogenshalogens are the elements in column VII of the periodic table.

The noble gasesnoble gases are the elements in column VIII of the periodic table.

The transition metalstransition metals are the elements between groups II and III. (The middle part of the periodic table.)The rare earth elementsrare earth elements are the 28 elements at the

bottom of the periodic table and consist of the lanthanide lanthanide and actinideactinide series.

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NonmetalNonmetalss

MetalsMetals

Alk

alin

e E

art

h

Meta

ls

Alk

ali

Meta

ls

Halo

gen

s

Nob

l e

Gases

Rare Earth Elements

Transition Elements

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An element element is a substance all of whose atomsatoms have the same numbersame number of protonsprotons.The atomic numberatomic number, Z, is the number of protonsprotons in the nucleus of an atom. Atomic number is used to identify an element and to indicate its position on the Periodic Table.ElementElementElementElement Atomic Atomic

NumberNumberNumber of Number of

ProtonsProtonsNumber of Number of

ProtonsProtonsbarium 56 56

35bromine 35

92uranium 92

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AtomsAtoms of the same elementsame element but which differdiffer in the numbernumber of neutronsneutrons are called isotopesisotopes.Mass numberMass number, A, is the sum sum of the protonsprotons and neutronsneutrons in the nucleus of an atom.

Atomic number = Z = number of Atomic number = Z = number of protonsprotons

Atomic number = Z = number of Atomic number = Z = number of protonsprotons= number of = number of

electronselectrons= number of = number of electronselectrons

Mass number = A = protons + Mass number = A = protons + neutronsneutrons

Mass number = A = protons + Mass number = A = protons + neutronsneutrons

This means that:

neutronneutronss

== atomic numberatomic numbermass number mass number --

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Exercise #1.Exercise #1. An atom has 24 protons and 27 neutrons in its nucleus. Its atomic number is ________ and its mass number is ________. The atoms also contains ________ electrons and it is an atom of __________________.

2424 51512424

chromiuchromiumm

Exercise #2.Exercise #2. An isotope of barium contains 84 neutrons. Its atomic number is ________ and as such it contains ________ protons and ________ electrons. Its mass number ________.

56565656 5656

141400

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The notation ZZXXAA is called the nuclear symbol for some isotope. It is read X-AX-A, where X is symbol of some element, A is the mass number of the isotope, and Z is the atomic number of the element.

Exercise #3.Exercise #3. The notation 47Ag108 indicates an isotope of _____________. Since its atomic number is ________, the isotope contains ________ protons and ________ electrons. Its mass number is ________ and as such the isotope contains ________ neutrons.

silversilver47474747

4747108108 6161

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Exercise #4.Exercise #4. The notation Ra-238 represents an isotope of ________________. As such its atomic number is ________ and the atom must contain ________ protons and ________ electrons. Since the mass number is ________, the atom also contains ________ neutrons. Its nuclear symbol is _________.

radiumradium8888

8888 8888232388150150 8888RaRa238238

Exercise #5.Exercise #5. The notation P-31 indicates an isotope of _______________. The mass number of the isotope is _______ and since its atomic number is _______, the isotope contains _______ protons, _______ electrons, and _______ neutrons.

phosphoruphosphoruss 31311515 1515

1515 1616

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ElementsElements change chemicallychange chemically because their atoms lose, gain, or share electronsatoms lose, gain, or share electrons.

Metal atomsMetal atoms tend to lose electronslose electrons in chemical reactions.Nonmetal atomsNonmetal atoms tend to gain gain electronselectrons in reactionsreactions with metal metal atomsatoms. Nonmetal atomsNonmetal atoms tend to share share electronselectrons in reactionsreactions with other nonmetalsnonmetals.Metal Metal atoms do not reactdo not react with other metalsmetals.

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An ionion is a charged particle formed any time an atom loses or gains electrons.

A positively charged ionpositively charged ion is called a cationcation. It is formed as a metal atom metal atom loses electronsloses electrons.OxidationOxidation is the process whereby an atom losesatom loses electronselectrons to form a positive positive ionion.Metal elementsMetal elements tend to undergo oxidationoxidation. The namename of a positive ionpositive ion is usually the same as that of the elementelement.

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An anionanion is a negative ion formed as a nonmetal atom gains electrons.

ReductionReduction is the process whereby an atomatom gains electronsgains electrons to form a negative negative ionion.

Nonmetal atomsNonmetal atoms tend to gain electronsgain electrons in chemical reactions with metal atoms.

The namename of a monatomic negative ionnegative ion is the name of the elementelement with its ending changed to -ide-ide.

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Rules for Writing Names of Rules for Writing Names of IonsIonsRules for Writing Names of Rules for Writing Names of IonsIons

1.)1.) The name of a metal (positive) ion is the same as the element from which it is derived.

AlAl+3+3 = = aluminum aluminum ionion

CaCa+2+2 = = calcium calcium ionion

2.)2.) To name two different positive ions of the same element:

A.)A.) use the suffix -ous-ous to represent the smaller chargesmaller charge and the suffix -ic-ic to represent the larger chargelarger charge.

FeFe+2+2 = = ferrous ferrous ionion

FeFe+3+3 = = ferric ionferric ion

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B.)B.) use a Roman numeralRoman numeral in parenthesis next to the name of the element to identify the chargecharge of the ion (Stock system).

SnSn+2+2 = = tin(II) iontin(II) ion SnSn+4+4 = = tin(IV) tin(IV) ionion

3.)3.) To name monatomic negativenegative (nonmetal) ions, change the element’s name ending to --ideide.

SS-2-2 ==

sulfide ionsulfide ion ClCl-1-1 = = chloride ionchloride ion

4.)4.) To name the most commonmost common polyatomic ion (oxyanion) of a given element, change the element’s name ending to -ate-ate.

SOSO44-2-2 = = sulfate sulfate

ionionClOClO33

-1-1 ==

chlorate chlorate ionion

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5.)5.) Polyatomic ions that contain one lessone less oxygen atomoxygen atom than the -ate radical will have a name ending in -ite-ite.

SOSO33-2-2 = =sulfite sulfite

ionionClOClO22

-1-1 = = chlorite ionchlorite ion

6.)6.) The prefix per-per- is used to identify one one more oxygen atommore oxygen atom than in the ion’s name to which it is attached.

OO22-2-2 = = peroxide peroxide

ionionClOClO44

-1-1 = = perchlorate ionperchlorate ion

7.)7.) The prefix hypo-hypo- represents one lessone less oxygen atomoxygen atom than in the ion’s name to which it is attached.

ClOClO-1-1 = = hypochlorite hypochlorite ionion

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Identify the name, symbol, and charge of each ion.

Ion’s Name Symbol & Charge

1. calcium

2. Al+3+3

3. bromide

4. NO3-1-1

5. sulfate

CaCa+2+2

aluminumaluminum

BrBr -1-1

nitratenitrate

SOSO44-2-2

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A moleculemolecule is the smallest compound particle. It consists of two or more atoms held together by a force of attraction called a chemical bond.The chemical formulachemical formula of a compound is an expression which uses the symbols of elements to identify the composition of a compound.

ExamplesExamples ZnSO4 = zinc sulfate

BaCl2 = barium chloride

A compoundcompound is a substance which can by chemical action be decomposed into two or more simple substances.

FormulFormulaa

NameName

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A subscript is a small numeral placed to the right and below a chemical symbol of an element to represent that number of combined atoms in a molecule.

Ba3(PO4)2

3 atoms of barium2 atoms of phosphorus8 atoms of oxygen

formula barium phosphate

subscripts

indicate

represents

the compound

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A coefficient is a numeral placed in front of a chemical formula to represent that number of uncombined chemical particles.

3Ba3(PO4)2

coefficient 3

9 atoms of barium6 atoms of phosphorus

24 atoms of oxygen

indicates

3 formula units of barium phosphatewhich now means

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2NH3

2 atoms nitrogen6 atoms hydrogen

4Na2CO3

8 atoms sodium4 atoms carbon

12 atoms oxygen

Hg2(NO3)2

2 atoms mercury2 atoms nitrogen6 atoms oxygen

5Mg3(PO4)2

15 atoms magnesium10 atoms phosphorus40 atoms oxygen

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A compound made up of a metal and a nonmetal element is called an ionic ionic compoundcompound. A transfer of electrons from the metal atom to the nonmetal atom forms such a compound.A chemical bondchemical bond is a force of attraction that holds atoms together when they form a compound.

A compound compound is a substance, which consists of 2 or more elements so combined that only chemical action can separate them.

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A compound made of nonmetal elements only is called a covalent compoundcovalent compound. Such a compound is formed by a sharing of electrons between the nonmetal atoms.

A covalent bondcovalent bond is a chemical bond which is the result of a sharing of electrons between nonmetal atoms.

An ionic bondionic bond is a chemical bond which is the result of a transfer of electrons from a metal atom to a nonmetal atom.

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Rules for Writing Rules for Writing Formulas of CompoundsFormulas of Compounds

Rules for Writing Rules for Writing Formulas of CompoundsFormulas of Compounds

1.)1.) Identify the symbols and the charges of the ions listed in the name of the compound.

2.)2.) If the sum of the charges is zero, write the symbols next to each other.

Sodium Sodium chloridechlorideNaNa+1+1 ClCl-1-1 Since Since ++1 + 1 + --1 = 1 =

00

NameName

== NaClNaClFormulaFormula

Barium carbonateBarium carbonate

BaBa+2+2 COCO33-2-2 Since +2 + -2 = 0Since +2 + -2 = 0

== BaCOBaCO33

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Stannous Stannous bromidebromideSnSn++

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Br Br -1-1

Since Since ++2 + 2 + --

112211

SnBrSnBr22

3.)3.) If the sum of the charges is not zero, exchange the absolute value of the charges and record them as subscripts.

==

If a polyatomic ion is used more than once, enclose it in parenthesis and write the subscript outside the parenthesis.

Calcium Calcium phosphatephosphate

CaCa+2+2 POPO44-3-3 Since +2 + -Since +2 + -

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00

0023

== CaCa33(PO(PO44))22

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4.)4.) For binary compounds consisting of two nonmetal elements, the prefixes given in the name of the compound will be used as subscripts in the formula.

Numerical Numerical PrefixesPrefixesmon = 1mon = 1 di = 2di = 2 tri = 3tri = 3 tetra = 4tetra = 4

penta = 5penta = 5 hexa = 6hexa = 6 hepta = 7hepta = 7 octa = 8octa = 8

nona = 9nona = 9 deca = 10

deca = 10

Carbon Carbon dioxidedioxide

NameName FormulFormulaa COCO22

Trinitrogen Trinitrogen tetraoxidetetraoxide

NN33OO44

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5.) A.5.) A. If the name of an acid begins with the prefix hydrohydro-, identify the corresponding -ide-ide ion. Use the charge of the ion to determine the number of hydrogen ions in the acid formula.

hydrohydrobromic acid

bromidebromide ion BrBr--

Since charge is -1-1

Formula = HBHBrr

An acidacid is a compound that contains hydrogen ion, HH++ , as the positive ionpositive ion.

An acidacid is a compound that contains hydrogen ion, HH++ , as the positive ionpositive ion.

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5.) B.5.) B. If the name of an acid ends with the suffix -ic-ic, identify the corresponding -ate-ate ion. Use the charge of the ion to determine the number of hydrogen ions in the acid formula.

sulfuricic acid

sulfatesulfate ion

SOSO44-2-2

Since charge is -2-2

Formula =HH22SOSO44

An acidacid is a compound that contains hydrogen ion, HH++ , as the positive positive ionion.

An acidacid is a compound that contains hydrogen ion, HH++ , as the positive positive ionion.

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5.) C.5.) C. If the name of an acid ends with the suffix -ous-ous, identify the corresponding -ite-ite ion. Use the charge of the ion to determine the number of hydrogen ions in the acid formula.

nitrousous acid

nitritenitrite ion NONO22-1-1

Since charge is -1-1

Formula =HNOHNO22

An acidacid is a compound that contains hydrogen ion, HH++ , as the positive positive ionion.

An acidacid is a compound that contains hydrogen ion, HH++ , as the positive positive ionion.

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Acid Name Negative Ion

-ic acid -ate

-ous acid -ite

hydro- -ic acid -ide

chloratechloric acid

chlorous acid chlorite

hydrochloric acid chloride

ClO3-1HClO3

ClO2-1HClO2

Cl-1HCl

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Write the formula for each compound.

1. Zinc nitrate = Zn(NOZn(NO33))22

2. Carbon dioxide = COCO22

3. Iron(II) chromate = FeCrOFeCrO44

4. Cuprous sulfate = CuCu22SOSO44

5. Nitric acid = HNOHNO33

6. Sodium acetate = NaCNaC22HH33OO22