Module 2 - Atoms, Molecules, And Ions

8/3/2019 Module 2 - Atoms, Molecules, And Ions

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ATOMS, MOLECULES, AND IONS

Daltons Atomic Theory (1808)

Postulates:1. An element is composed of extremely

small,indivisible particles called atoms.

2. All atoms of a given element haveidentical properties that differ from

those of other elements.3. Atoms cannot be created, destroyed,

or transformed into atoms of anotherelement.

4. Compounds are formed when atoms ofdifferent elements combine with oneanother in small whole-number ratios.

5. The relative numbers and kinds ofatoms are constant in a givencompound.

Fundamental Laws of Matter1. Law of Definite Proportion

Different samples of the same compoundalways contain its constituent elements inthe same proportion by mass.2. Law of Multiple Proportion

If two elements can combine to formmore than one type of compound, the

masses of one element that combine with a

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Module 2: Atoms, Molecules and Ions Page 1 of 24


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fixed mass of the other element are in ratiosof small whole numbers.

3. Law of Conservation of MassMatter can be neither created nor

destroyed.

The Structure of the AtomFundamental Particles:

Particle Mass(amu)

Charge

Electron (e-) 0.00054858

-1

Proton ( p+) 1.0073 +1Neutron (no) 1.0087 0

The ElectronsHumphrey Davy in the early 1800s

passed electricity through compoundsand noted:

that the compounds decomposed intoelements.

Concluded that compounds are heldtogether by electrical forces.

Michael Faraday in 1832-1833 realizedthat the amount of reaction that occursduring electrolysis is proportional tothe electrical current passed through

the compounds

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Cathode Ray Tubes experiments performedin the late 1800s & early 1900s.

Consist of two electrodes sealed in a glass

tube containing a gas at very low pressure. When a voltage is applied to the cathodes a

glow discharge is emitted.

These rays are emitted from cathode (-end) and travel to anode (+ end).

Cathode Rays must be negatively charged!

J.J. Thomson modified the cathode raytube experiments in 1897 by adding twoadjustable voltage electrodes.

Studied the amount that the cathode raybeam was deflected by additional electricfield.

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- Thomson used his modification tomeasure the charge to mass ratio ofelectrons.

e/m = -1.75881 x 108 coulomb/g of e-

- Named the cathode rays electrons.

Robert A. Millikan won the 1st AmericanNobel

Prize in 1923 for his famous oil-dropexperiment.

- In 1909 Millikan determined thecharge and

mass of the electron.- Millikan determined that the chargeon asingle electron = -1.60218 x 10-19coulomb.

- Using Thomsons charge to massratio, weget that the mass of one electron is

9.10 x

10-28

g.

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e/m = -1.76 x 108 coulombe = -1.6022 x 10-19 coulomb

Thus m = 9.10 x 10-28 g

Radioactivity- 1895, Wilhelm Konrad Roentgen,

discoveredX-Rays

- Marie Curie, suggested the name

radioactivity,spontaneous emission of particles and/orradiation

Three types of rays:

1. Alpha rays ( ) positively chargedparticles

2. Beta rays ( ) are electrons3. Gamma rays ( ) high-energy rays;

no charge

The ProtonEugene Goldstein noted streams ofpositively

charged particles in cathode rays in1886.

Particles move in opposite direction ofcathode rays.

Called Canal Rays because they

passed through holes (channels or

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canals) drilled through the negativeelectrode.

Canal rays must be positive.

Goldstein postulated the existence ofa positive fundamental particle calledthe proton.

The NucleusErnest Rutherford directed Hans Geiger

andErnst Marsdens experiment in 1910.

- particle scattering from thin Aufoils

Gave us the basic picture of theatoms structure.

- In 1912 Rutherford decoded the

-particlescattering information.

Explanation involved a nuclear atomwith electrons surrounding the nucleus.

Rutherfords major conclusions from the -particle scattering experiment

The atom is mostly empty space. It contains a very small, dense centercalled the nucleus.

Nearly all of the atoms mass is in

the nucleus.

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The nuclear diameter is 1/10,000 to1/100,000 times less than atomsradius.

The Neutron James Chadwick in 1932 analyzed the

results of -particle scattering on thin Be films.

- Chadwick recognized existence of

massiveneutral particles which he called

neutrons.

Atomic NumberThe atomic number is equal to the

number of protons in the nucleus. Sometimes given the symbol Z. On the periodic chart Z is theuppermost number in each elementsbox.

- In 1913 H.G.J. Moseley realized thatthe

atomic number determines theelement .The elements differ from each otherby the number of protons in thenucleus.

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The number of electrons in a neutralatom is also equal to the atomicnumber.

Mass NumberMass number is given the symbol A.

A is the sum of the number of protonsand neutrons.

Z = proton number

N = neutron numberIn general, the mass number is given by

IsotopesIsotopes are atoms of the same element

but with different neutron numbers. Isotopeshave different masses and A values but arethe same element.

One example of an isotopic series is the

hydrogen isotopes.1H or protium is the most common

hydrogenisotope (one proton and no neutron). 2H or deuterium is the second mostabundant hydrogen isotope (one proton and

one neutron).

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Mass number = number of protons+ number of neutronsMass Number = atomic number+ number of neutrons


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3H or tritium is a radioactive hydrogenisotope (one proton and two neutrons).

A common symbolism used to show mass

and proton numbers isMass number

xAZAtomic Number

Thus for the isotopes of hydrogen, wewrite

H1

1 H21 H31protium deuterium tritium

The Periodic TableA chart in which elements having similar

chemical and physical properties aregrouped together.

The elements are arranged in increasingatomic number in horizontal rows called

periods and in vertical columns known asgroups or families based on similarities inchemical properties. Note that elementsnumber 111 to 115 have recently beensynthesized but not yet been named.

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Practice Exercise 2.1: Indicate the number of protons,

neutrons, and electrons in each of these species:

a. N15

7 b. S33

16 c. Cu65

29 d. Sr84

38


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IA 8A1 2

H2A

3A 4A 5A 6A 7A He

3 4 5 6 7 8 9 10

Li Be B C N O F Ne11

12 13 14 15 16 17 18

Na

Mg

3B

4B 5B 6B 7B 8B 1B 2B Al Si P S Cl Ar

19

20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se BR Kr37

38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

55

56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86

Cs

Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

87

88 89104

105

106

107

108

109

110

111

112

113

114

115

116

117

118

Fr Ra Ac Rf Db Sg Bb Hs Mt Ds Rg

58 59 60 61 62 63 64 65 66 67 68 69 70 71

Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

90 91 92 93 92 95 96 97 98 99100

101

102

103

Th Pa U Np PuAm

Cm

Bk Cf Es Fm Md Nn Lr

The elements can be divided into threecategories metal, nonmetals and

metalloids.1. A metal is a good conductor of heat

and electricity. (found on the left of thechart with)

2. A nonmetal is usually a poorconductor of heat and electricity. (are

found on the right side)

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3. A metalloid has properties that areintermediate between those of metalsand nonmetals. (found along the

zigzag line, separating the metals fromthe nonmetals)

Elements are often referred to

collectively by their periodic table groupnumber (Group 1A, Group 2A, etc.).

However some element groups have specialnames. Common examples are:

1. Group 1A elements are called alkalimetals (Li, Na, K, Rb, Cs, and Fr).

2. Group 2A elements are calledalkaline earth metals (Be, Mg, Ca, Sr,Ba, and Ra).

3. Elements in Group 7A are calledhalogens (F, Cl, Br, I, and At).

4. Elements in Group 8A are known asnoble or rare gases (He, Ne, Ar, Kr, Xeand Rn)

MoleculesA molecule is an aggregate of at least

two atoms in a definite arrangement heldtogether by chemical forces (also calledchemical bonds)Examples of molecules:

O2, H2, Cl2 diatomic molecules

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H2O, C12H22O11 polyatomicmolecules

IonsAn ion is an atom or a group of atoms

that hasa net positive or negative charge.Two basic types of ions:1. Positive ions or cations - one or more

electronsless than neutralNa+, Ca2+, Al3+ - monoatomic cationsNH4

+ - polyatomic cation2. Negative ions or anions - one or moreelectrons

more than neutralF-, O2- , N3- - monoatomic anionsSO4

2-, PO43- - polyatomic anions

Chemical FormulasChemical formula shows the chemical

composition of the substance. It is the ratio

of the elements present in the molecule orcompoundTypes of formulas:1. Molecular Formula shows the exactnumber of

atoms of each element in the smallest

unit

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of a substanceExamples:

Molecular

Formula

Name

H2 hydrogenO2 oxygenO3 ozone

H2O waterNote that oxygen and ozone are

allotropes of oxygen. Allotrope is one ortwo distinct forms of an element.

2. Empirical Formula tells us whichelements arepresent and the simplest ratio of their

atomsbut not necessary the actual member

ofatoms in a given molecule.

Example:EmpiricalFormula

MolecularFormula

Name

NH2 N2H4 hydrazine

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Practice Exercise 2.2: What are the empiricalformulas of the following compounds?

a)Al2Br6 b) Na2S2O4 c)K2Cr2O7 d)


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Formula of Ionic CompoundsFormula of ionic compounds is determined

by thecharges of the ions.

Charge on the cations must equal thecharge on the anions.

The compound must be neutral, that is,the sum of the charges on the cation andanion in each formula unit must be equalto zero.

If they are different, we apply thefollowing formula to make the formulaelectrically neutral: The subscript of thecation is numerically equal to the chargeon the anion and the subscript of theanion is numerically equal to the chargeon the cation. (We call this the criss-cross method). If the charges are

numerically equal, then no subscripts

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are necessary. This rule follows becausethe formulas of most ionic compoundsare empirical formulas, and as such, the

subscripts are reduced to the smallestwhole number ratios. Examples are asfollows.

Potassium bromide is formed by K+

and Br-.

The formula is KBr.

Zinc iodide is formed by Zn2+

and I-

.The

formula is ZnI2.Aluminum oxide is formed by Al3+ and

O2-. The formula is Al2O3.

Naming Ionic Compounds1. Binary compound is a compound that iscomposed of two elements, the cationderived from metal atom and a nonmetallicanion.

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Practice Exercise 2.3: Which of the followingcompounds are likely to be ionic? Which arelikely to be molecular?

a)CH4 b) NaBr c)BaF2 d)CCl4 e) ICl


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a. Cation with only one charge ( Alkalimetal cations, Alkaline earth metal cations,Ag+, Al3+, Cd2+, Zn2+)

Rule: Name the (+) ion + name the stemof the (-) ion + -ide

The -ide Nomenclature of SomeCommon Monatomic Anion Accordingto Their Position in the Periodic Table

Group4A

Group5A

Group6A

Group 7A

C4-

CarbideN3-Nitride

O2-Oxide

F1-

FluorideSi4-Silicide

P3-

Phosphid

e

S2-Sulfide

Cl1-

Chloride

Se2-

SelenideBr1-

BromideTe2-

TellurideI1-Iodide

Example:Formula Name

NaCl sodium chlorideAl2O3 aluminum oxideKBr potassium bromideZnI2 zinc iodide

b. Cation with more than one charge(Transition metals except Ag+, Al3+, Cd2+,

Zn2+)

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Rule: name the metal + (Roman numeralof the charge of the metal cation) + namethe stem of the (-) ion + -ide

Example:Formula Name

FeCl2 iron (II) chlorideFeCl3 iron (III) chlorideMnO manganese (II) oxideMn2O3 manganese (III) oxide

MnO2 manganese (IV) oxide

2. Ternary compound is a compoundconsisting of three elements.Rule: name the (+) ion + name the (-)ionExample:

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Formula NameLiOH Lithium hydroxideKCN Potassium cyanide

BaSO4 Barium sulfateCa3PO4 Calcium phosphateKMnO4 Potassium

permanganate


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TABLE 2.3 on page 27 of the GeneralChemistry: The Essential Concepts, 5th

edition by Raymond Chang displays theformulas, charges and names of somecommon cations and anion.

Naming Binary Molecular CompoundsMolecular compounds contain discrete

molecular units. They are usually composedof nonmetallic elements. Many molecularcompounds are binary compounds. Greekprefixes are used to indicate the number of

atoms in each element present in amolecule of a compound.

Numeric

Prefix Numeric

Prefix

1 mono 6 hexa

2 di 7 hepta

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Practice Exercise 2.4 : Name the following compounds.a) Cu(NO3)2 b) KH2PO4 c) NH4ClO3 d) PbO e)

Li2SO3

Practice Exercise 2.5: Write chemical formulas for thefollowing compounds.


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3 tri 8 octa4 tetra 9 nona5 penta 10 deca

Rule: Prefix name of the (+) ion + namethe cation + prefix name of the (-) ion +stem of (-) ion + -ide

Notes:The prefix mono may be omitted forthe first element. For oxides, the ending a in theprefix is sometimes omitted.

Exceptions to the use of Greekprefixes are molecular compoundscontaining hydrogen.Examples:

Formula

Name Formula

Name

B2H6 Diborane HCl Hydrogenchloride

CH4 Methane SiC Silicon

carbideSiH4 Silane CO Carbon

monoxide

NH3 Ammonia

CO2 Carbondioxide

PH3

Phosphine

NO2

Nitrogendioxide

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H2O Water N2O4 Dinitrogentetroxide

H2S Hydrogen sulfide

SO2 Sulfurdioxide

Naming AcidsAn acid can be described as a substance

that yields hydrogen ions (H+) whendissolved in water. Formulas for acids

contain one or more hydrogen atoms as wellas an anionic group.1. Binary acid is composed of hydrogenand nonmetal.

Rule 1: For dry acidHydrogen + stem of the non-metal +-ide

Rule 2: For aqueous acidHydro + stem of the nonmetal + -ic +acid

Examples:Formul

aName indry form

Name inaqueous

solution

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Practice Exercise 2.6: Name the following molecularformula.

Practice Exercise 2.7: Write chemical formulas for thefollowing molecular compounds.

a) sulfur tetrafluoride b) dinitrogen pentoxide


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HCl Hydrogenchloride

Hydrochloric acid

HF Hydrogen

fluoride

Hydrofluoric acid

H2S Hydrogensulfide

Hydrosulfuricacid

HCN Hydrogencyanide

Hydrocyanic acid

2. Ternary acid (oxoacid) containshydrogen, oxygen and another element (thecentral element).

Rule 1: For dry acidHydrogen + name of ion

Rule 2: For aqueous acid

Name stem of the middle element +-ous/-ic+ acid

The use of ous and -ic Addition of one O atom to the -ic acid:

The acid is called per...-ic acid. Thus,adding an O atom to HClO3 changes thechloric acid to perchloric acid, HClO4.

Removal of one O atom from the -icacid: The acid is called -ous acid. Thusnitric acid, HNO3 becomes nitrous acid,

HNO2.

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Removal of two O atoms from the -icacid: The acid is called hypo...ous acid.Thus when HBrO3 is converted to HBrO,

the acid is called hypobromous acid.

The rules for naming anions of oxoacids(oxoanions) are:

When all the H ions are removed fromthe -ic acid, the anions name endswith -ate. For example the anion CO3

2-

derived from H2CO3 is called carbonate. When all the H ions are removed from

the -ous acid, the anions name endswith -ite. Thus the anion ClO2

- derivedfrom HClO2 is called chlorite.

The names of anions in which one ormore but not all of the hydrogen ionshave been removed must indicate the

number of H+ ions present. For example,consider the anions derived from thephosphoric acid (H3PO4): dihydrogenphosphate (H2PO4

-), hydrogen phosphate(HPO4

2-).

Examples:

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Formula

Name in dryform

Name inaqueoussolution

H2CO3 Hydrogencarbonate

Carbonic acid

HNO3 Hydrogennitrate

Nitric acid

HClO4 Hydrogenperchlorate

Perchloricacid

HClO3 Hydrogenchlorate

Chloric acid

HClO2 Hydrogenchlorite

Chlorous acid

HClO Hydrogenhypochlorite

Hypochlorousacid

Naming BasesA base can be described as a substance

that yields hydroxide ions (OH-) whendissolved in water.Example: sodium hydroxide, NaOH

potassium hydroxide, KOHbarium hydroxide, Ba(OH)2

Ammonia (NH3), a molecular compoundin the gaseous or pure liquid state, is alsoclassified as a common base. This is

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because when dissolved in water it reactswith water and yield NH4

+ and OH- ions.

Assignment: 2.31, 2.33, 2.41, 2.45, 2.47,2.49, 2.59, 2.65, 2.73 (Pages. 54 56)

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Module 2 - Atoms, Molecules, And Ions