Phys_Chem-Chptr_2

8/7/2019 Phys_Chem-Chptr_2

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2010 Cosmopoint

HDF 111 : Physical ChemistryHDF 111 : Physical Chemistry

CHAPTER 2 :CHAPTER 2 :

Early Atomic Theory & StructureEarly Atomic Theory & Structure


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Slide 2 of 18 2010 Cosmopoint

Chapter 2: Early Atomic Theory and Structure

Learning Outcomes

After completing this chapter, you should

be able to do the following:

Define Daltons atom model Define atomic structure of the atom

Explain atomic number, isotopes, and atomic

mass concept


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2.1 Early Thoughts

2.2 Daltons Model of the Atom

2.3 Composition ofCompounds

2.4 The Nature of Electric Charge

2.5 Discovery of Ions

2.6 Subatomic Parts of the Atom

2.7 The Nuclear Atom

2.8 Isotopes of the Elements

2.9 Atomic Mass

Topic Outlines


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Introduction

Pure substances are classified aselements or compounds

What makes a substance possessits unique properties?

How small a piece of salt will stilltaste salty?

Substances are in their simplestidentifiable form at the atomic, ionic,or molecular level

Further division produces a loss of

characteristic properties What particles lie within an atom orion?

How are these tiny particles alike?How do they differ? How far can wecontinue to divide them?


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Early Thoughts

The earliest models of the atom weredeveloped by the ancient Greek philosophers

About 440B.C. Empedocles stated that allmatter was composed of four elements earth, air, water, and fire

Democritus (about 470 370B.C.) thoughtthat all forms of matter were composed of tinyindivisible particles which he called atoms(Greek word atomos, meaning indivisible)

He held that atoms were in constant motionand they combined with one another invarious ways (no scientific observations)

Aristotle (384 322B.C.) opposed the theoryof Democritus; he endorsed and advancedthe Empedoclean theory and dominated thethinking of scientists and philosophers untilthe beginning of the 17th century


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Daltons Model of the Atom

English schoolmaster JohnDalton (1766-1844) revivedthe concept of atoms

He proposed an atomic modelbased on facts andexperimental evidence

He described his theory in aseries of paper published from

1803 1810 The idea was each element

consist of different kind ofatom


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The essence of Daltons atomicmodel: Elements are composed of minute,

indivisible particle called atom

Atoms of the same element are alike

in mass and size Atoms of different elements have

different masses and sizes

Chemical compounds are formed bythe union of2 or more atoms ofdifferent elements

Atom combine to form compounds insimple numerical ratios, such as oneto one , one to two, two to three, andso on

Atoms of two elements may combinein different ratios to form more thanone compound


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Later investigationhowever have shown: Atoms are composed of

subatomic particles Not all the atoms of a

specific element have thesame mass

Atoms, under specialcircumstances, can bedecomposed


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Composition of Compounds

Water Hydrogen peroxide

11.2% H 5.9%

88.8% O 94.1%Atomic composition 2H + 1O 2H + 2O

Law of definite composition: a compound

always contains two or more elementschemically combined in a definite proportion by

mass


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Composition of Compounds

Law of multiple proportions: states atoms of twoor more elements may combine in different ratios to

produce more than one compound

The law of definite composition and law ofmultiple proportions state that

(1) the composition of a particular substance will always

be the same no matter what its origin or how it is

formed,(2) the composition of different compounds formed from

the same elements will always be unique


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Combined Elements Giving More Than One Compound

Compound Formula Percent composition

Copper (I) chloride CuCl 64.2%Cu, 35.8%Cl

Copper (II) chloride CuCl2

47.3%Cu, 52.7%Cl

Methane CH4 74.9%C, 25.1% H

Octane C8H18 85.6%C, 14.4% H

Methyl alcohol CH4O 37.5%C, 12.6% H, 49.9% O

Ethyl alcohol C2H6O 52.1%C, 13.1% H, 34.7% O

Glucose C6H12O6 40.0%C, 6.7% H, 53.3% O


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The Nature of Electric Charge

The properties of electric charge: Charge may be positive or negative

Unlike charges attract, like charges repel

Charges may be transferred from one object to another, by

contact or induction The less the distance between 2 charges, the greater the

force of attraction between unlike charges (or repulsionbetween identical charges)

The force of attraction (F) :

F = kq1

q2r2

q1 and q2 are the charges, r is the distance between thecharges and k is a constant


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Discovery of Ions

English scientist Michael Faraday (1791-1867) made thediscovery that certain substances when dissolved inwater conduct an electric current

He also noticed that certain compounds decompose into

their elements when an electric current is passedthrough the compound

Atoms of some elements are attracted to the positiveelectrode, while some other elements are attracted to thenegative electrode

He concluded that these atoms are electrically charged;he called them ions after the Greek word meaningwanderer

Any moving charge is an electric current

The electrical charge must travel through a substanceknown as a conducting medium


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Subatomic Parts of the Atom

Diameter of a singleatom ranges from 0.1to 0.5 nanometer

(1nm = 1 x 10-9

m) Hydrogen the

smallest atom diameter = 0.1nm

An atom containssubatomic particles:electrons, protons,and neutrons


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Electron

The electron is a particle

with a negative electrical

charge and a mass of

9.110 x 10-28g

This mass is 1/1837 the

mass of a hydrogen atom

Relative charge = -1

The size of electron has

not been determined

exactly; diameter is

believed to be < 10-12cm


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Proton

Proton firstobserved by Germanphysicist Eugen

Goldstein in 1886 Thompson calculated

its mass; approx.1837 times of themass of electron

Actual mass = 1.673 x10-24g; relative chargeof +1


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Thompson Model of The Atom

Electrons are negatively charged embedded in theatomic sphere

Atoms are electrically neutral, the sphere also containsan equal number of protons, or positive charges

A neutral atom could become an ion by gaining or losingelectrons

Positive ions: neutral atom loses electrons

Atom with net charge +1 has lost one electron

Atom with a net charge of +3 has lost 3 electrons

Negative ions gained additional electrons A net charge of-1 is produced by the addition of one

electron


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The Third Major Subatomic Particle

Discovered by James

Chadwick (1891-

1974)

Neutron actualmass 1.675 x 10-24g;

slightly greater than

that of proton


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Class Activity 1

The mass of a helium atom is 6.65x10-24g.

How many atoms are in a 4.0g sample of

helium?


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Answer

(4.0g) (1 atom He)

(6.65 x 10-24g)

= 6.0 x 1023 atoms He


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Atomic Numbers of the Elements

Atomic number of an

element is the number of

protons in the nucleus of

an atom of that element

The atomic number

determines the identity of

an atom

Hydrogen = atomic

number = 1; contains oneproton in its nucleus

Uranium = 92; 92 protons


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Isotopes of the Elements

Isotopes: atoms of anelement having the sameatomic number butdifferent atomic masses

called isotopes of thatelement

Hydrogen (1), deuterium(2), tritium (3)

Mass number: the sum ofthe number of protonsand the number ofneutrons in the nucleus


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Isotopes of the Elements

Isotopes: atoms of anelement having the sameatomic number butdifferent atomic masses

called isotopes of thatelement

Hydrogen (1), deuterium(2), tritium (3)

Mass number: the sum ofthe number of protonsand the number ofneutrons in the nucleus


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Atomic Mass

The mass of a single atom canbe determined precisely withmass spectrometer

The mass of a single hydrogen

atom = 1.673 x 10-24g. However, it is not

convenient/practical to comparethe actual masses of atoms ingrams => a table of relative

atomic masses using atomicmass units was devised

The term atomic weight issometimes used instead ofatomic mass


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Atomic Mass Unit

Carbon isotope having 6 protons & 6neutrons and designated carbon-12, or12C,was chosen as the standard for atomicmasses

This reference isotope was assigned avalue of exactly 12 atomic mass units (amu)

1 atomic mass unit is defined as equal toexactly 1/12 of the mass of a carbon-12atom

The actual mass of a carbon-12 atom =1.9927 x 10-23g => one atomic mass unit =1.6606 x 10-24g

6


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Average Atomic Mass

Hydrogen atoms, with a mass of about 1/12 that of acarbon atom, have an average atomic mass of1.00797amu.

Magnesium atoms, which are about twice as heavy as

carbon, have an average mass of24.305 amu. Most elements occurs as mixtures of isotopes withdifferent masses.

The atomic mass determined for an element representsthe average relative mass of all naturally occurringisotopes of that element.

The atomic masses of the individual isotopes areapproximately whole numbers because the relativemasses of the protons and neutrons are approximately1.0 amu each


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Average Atomic Mass

Isotope Isotopic mass Abundance Average atomic mass

(amu) (%) (amu)

63Cu 62.9298 69.0963.55

65Cu 64.9278 30.91

29

29

Average atomic mass:

(62.9298 amu)(0.6909) = 43.48 amu

(64.9278 amu)(0.3091) = 20.07 amu

63.55 amu


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Example 1

How many protons,

neutrons, and electrons arefound in an atom of14C ?

6


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Answer

The element is carbon.

Atomic number = 6

Number of protons orelectrons are equal to theatomic number = 6

The number of neutrons =14 6 = 8


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Class Activity 1

How many protons, neutrons, andelectrons are in each of theseisotopes

16O8 80Br35 235U92

64Cu29


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Class Activity 2

What is the atomic number and themass number of the elements thatcontains:

a)9 electrons

b)24 protons and 28 neutrons

c)197X79


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Class Activity 3

Chlorine is found in nature as twoisotopes, 37Cl17 (24.47%) and

35Cl17

(75.53%). The atomic masses are36.96590 and 34.96885 amurespectively. Determine the averageatomic mass of chlorine.


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