Atomos: Not to Be Cut Atomos: Not to Be Cut or “indivisible”or “indivisible”

The History of Atomic The History of Atomic TheoryTheory

ObjectivesObjectives Define “atom”Define “atom” Explain Democritus’ model of the atomExplain Democritus’ model of the atom List the postulates of Dalton’s atomic theoryList the postulates of Dalton’s atomic theory Explain what studies of cathode rays revealed Explain what studies of cathode rays revealed

about atomsabout atoms Explain the results of Rutherford’s gold foil Explain the results of Rutherford’s gold foil

experimentexperiment Evaluate the atomic theories presented by Evaluate the atomic theories presented by

Democritus, Dalton, Thomson, and RutherfordDemocritus, Dalton, Thomson, and Rutherford Construct a model based on each different Construct a model based on each different

theorytheory

Who are these men?Who are these men?

In this lesson, we’ll learn about the men whose quests for knowledge about the fundamental nature of the universe helped define our views.

DemocritusDemocritus

Greek Greek philosopherphilosopher

lived from 460 - lived from 460 - 370 B.C.370 B.C.

400 BC

AtomosAtomos

His theory: Matter His theory: Matter could not could not be divided into smaller and be divided into smaller and smaller pieces forever, smaller pieces forever, eventually the smallest eventually the smallest possible piece would be possible piece would be obtained.obtained.

This piece would be This piece would be indivisible.indivisible.

He named the smallest He named the smallest piece of matter “atomos,” piece of matter “atomos,” meaning “not to be cut.”meaning “not to be cut.”

Democritus’ IdeasDemocritus’ Ideas Atoms are small hard Atoms are small hard

particles.particles. Made of a single Made of a single

material that’s formed material that’s formed into different shapes into different shapes and sizes.and sizes.

They are always They are always movingmoving

They form different They form different materials by joining materials by joining togethertogether..

This theory was This theory was ignored and forgotten ignored and forgotten for more than for more than 2000 2000 years!years!

Why?Why?

The eminent The eminent philosophers of philosophers of the time, the time, AristotleAristotle and and Plato, had a Plato, had a more more respected, respected, (and ultimately (and ultimately wrongwrong) theory.) theory.

Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter. Their ideas held sway because of their eminence as philosophers. The atomos idea was buried for approximately 2000 years.

What is an What is an Atom?Atom?

An atom is the smallest particle that an element can be divided and still be that element.

For example the For example the smallest particle of smallest particle of carbon is a single carbon is a single atom of carbon. If you atom of carbon. If you divide it is no longer divide it is no longer carbon anymore.carbon anymore.

PIECES OF CARBON

CARBON ATOM

Carbon atoms as seen through a scanning tunneling microscope

Dalton’s Model 1776-1844Dalton’s Model 1776-1844

In the early 1800s, the In the early 1800s, the English Chemist and English Chemist and Schoolteacher John Schoolteacher John Dalton proposed a new Dalton proposed a new atomic theoryatomic theory

His theory was based His theory was based on on experiments!experiments!

Dalton’s TheoryDalton’s Theory

1. All 1. All elementselements are composed are composed of atoms.of atoms.

2. Atoms are indivisible and 2. Atoms are indivisible and indestructible particles.indestructible particles.

3. Atoms of the 3. Atoms of the samesame element element are exactly alike.are exactly alike.

4. Atoms of 4. Atoms of differentdifferent elements elements are are differentdifferent..

5. 5. CompoundsCompounds are formed by are formed by the joining of atoms of two the joining of atoms of two or more elements.or more elements.

Atoms and ElectricityAtoms and Electricity In 1839, English chemicst Michael

Faraday suggested that the structure of atoms was related to electricity

Experiments proved that he was right! Atoms contain particles that have electrical charge.

Atoms and ElectricityAtoms and Electricity The word electricity

comes from the Greek word “elektron” which meant “amber.”

The greeks knew that if you rubbed amber with a cloth, it would attract dust particles

Today, this is known as static electricity

Static electricity comes from electrical charges that are not in motion

Atoms and ElectricityAtoms and ElectricityBenjamin Franklin derived several facts

about electricityAn object could have one of two kinds of

charge, which he called (+) and (-). Like charges repel and opposite charges attractSome objects pick up excess negative charges

and these charges may jump in a discharge of static electricity.

(walk across a carpet and touch a doorknob)

Lightning is static electricity on a larger scaleWhere do these charges come from?

Thomson’s Plum Pudding ModelThomson’s Plum Pudding Model In 1897, the English

scientist J.J. Thomson conducted a series of experiments with cathode ray tubes.

Thomson discovered the electron

Thomson’s Cathode Ray Thomson’s Cathode Ray Experiment ConclusionsExperiment Conclusions

• Cathode ray is composed of negative particles and these particles come from the cathode

• This meant that atoms were not indivisible, but had negative particles which he called electrons

• He determined the ratio of an electron’s charge to its mass

Thomson ModelThomson ModelHe proposed the He proposed the

““PlumPlum PuddingPudding” ” model. model.

Atoms were made Atoms were made from a positively from a positively chargedcharged substancesubstance with negatively with negatively charged electrons charged electrons scatteredscattered about, about, like raisins in a like raisins in a pudding.pudding.

Rutherford’s Gold Foil Rutherford’s Gold Foil ExperimentExperiment

19081908Rutherford’s Rutherford’s

experiment experiment Involved firing a Involved firing a stream of tiny stream of tiny positively chargedpositively charged particles at a thin particles at a thin sheet of sheet of gold foilgold foil (2000 atoms thick)(2000 atoms thick)

Most of the positively Most of the positively charged “bullets” passed charged “bullets” passed right through the gold foilright through the gold foil

Some of the positively Some of the positively charged “bullets,” charged “bullets,” however, did bounce however, did bounce away from the gold sheet away from the gold sheet as if they had hit as if they had hit something solid. He something solid. He knew that positive knew that positive charges charges repelrepel positive positive charges.charges.

Rutherford’s ConclusionsRutherford’s Conclusions

The gold atoms in the sheet were The gold atoms in the sheet were mostly emptymostly empty spacespace. Atoms were . Atoms were notnot a pudding filled with a a pudding filled with a positively charged material.positively charged material.

Rutherford concluded that an atom had a Rutherford concluded that an atom had a small, small, dense, positively charged centerdense, positively charged center that that repelledrepelled his positively charged “bullets.”his positively charged “bullets.”

He called the center of the atom the “He called the center of the atom the “nucleusnucleus”” The nucleus is The nucleus is tiny tiny compared to the atom as a compared to the atom as a

whole. whole.

Rutherford’s ModelRutherford’s Model

All of an atom’s positively All of an atom’s positively charged particles were charged particles were inside inside the nucleus. the nucleus.

The negatively charged The negatively charged particles were particles were scatteredscattered outside the nucleus outside the nucleus around the atom’s around the atom’s edgeedge..

2424

Subatomic ParticlesSubatomic Particles

ParticleParticle Symbol Symbol Charge Charge RelativeRelativeMassMass

ElectronElectron e e-- 1-1- 0 0

ProtonProton p p++ + + 1 1

NeutronNeutron n n 0 0 1 1

2525

Atomic NumberAtomic Number

Counts the numberCounts the number

of of

protonsprotons

in an atomin an atom

2626

Atomic Number on the Periodic Atomic Number on the Periodic TableTable

1111

NaNa

Atomic Number

Symbol

2727

All atoms of an element have All atoms of an element have the same number of protonsthe same number of protons

1111

NaNa

11 protons

Sodium

2828

Number of ElectronsNumber of Electrons

An atom is neutral An atom is neutral The net charge is zeroThe net charge is zero Number of protons = Number of electronsNumber of protons = Number of electrons Atomic number = Atomic number = Number of electronsNumber of electrons

2929

Mass NumberMass Number

Counts the number Counts the number

of of

protons protons and and neutronsneutrons

in an atomin an atom

*Mass number is not on the periodic *Mass number is not on the periodic tabletable

3030

Atomic SymbolsAtomic Symbols

Show the Show the mass number and atomic numbermass number and atomic number

Give the symbol of the elementGive the symbol of the element

mass numbermass number

23 23 NaNa sodium-23sodium-23

atomic numberatomic number 1111

3131

More Atomic SymbolsMore Atomic Symbols

1616 3131 6565

OO P P Zn Zn

88 1515 3030

8 p8 p++ 15 p15 p++ 30 p30 p++

8 n8 n 16 n16 n 35 n35 n8 e8 e-- 1515 ee-- 30 e30 e--

3232

IsotopesIsotopes

Atoms with the same number of protons, Atoms with the same number of protons,

but different numbers of neutrons. but different numbers of neutrons.

Atoms of the same element (same atomic Atoms of the same element (same atomic

number) with different mass numbersnumber) with different mass numbers

Isotopes of chlorineIsotopes of chlorine

3535ClCl 3737ClCl1717 1717

chlorine - 35 chlorine - 37chlorine - 35 chlorine - 37

3333

Learning Check AT 5Learning Check AT 5

An atom has 14 protons and 20 neutrons.An atom has 14 protons and 20 neutrons.A.A. Its atomic number isIts atomic number is

1) 141) 14 2) 162) 16 3) 343) 34

B. Its mass number isB. Its mass number is1) 141) 14 2) 162) 16 3) 343) 34

C. The element isC. The element is1) Si1) Si 2) Ca2) Ca 3) Se3) Se

D.D. Another isotope of this element isAnother isotope of this element is

1)1) 3434X X 2)2) 3434X X 3)3) 3636XX 16 16 14 14 14 14

3434

Masses of AtomsMasses of Atoms

A scale designed for atoms gives their small A scale designed for atoms gives their small

atomic masses in atomic masses in atomic mass unitsatomic mass units (amu) (amu)

An atom of An atom of 1212C was assigned an exact mass of C was assigned an exact mass of

12.00 amu12.00 amu

Relative masses of all other atoms was Relative masses of all other atoms was

determined by comparing each to the mass of determined by comparing each to the mass of 1212C C

An atom twice as heavy has a mass of 24.00 An atom twice as heavy has a mass of 24.00

amu. An atom half as heavy is 6.00 amu.amu. An atom half as heavy is 6.00 amu.

3535

Calculating Average Atomic Calculating Average Atomic MassMass

You need to know…You need to know…

Percent(%) abundance of isotopesPercent(%) abundance of isotopes

Mass of each isotope Mass of each isotope

Weighted average =Weighted average =

(mass of isotope 1)(% abundance/100) +(mass (mass of isotope 1)(% abundance/100) +(mass

of isotope 2)(% abundance/100) + …of isotope 2)(% abundance/100) + …

3636

Atomic Mass of MagnesiumAtomic Mass of Magnesium

Isotopes Isotopes Mass of Isotope AbundanceMass of Isotope Abundance 2424Mg Mg == 24.0 amu 24.0 amu 78.70%78.70%

2525Mg Mg = = 25.0 amu 25.0 amu 10.13%10.13%

2626Mg Mg = = 26.0 amu26.0 amu 11.17%11.17%

(.7870 x 24.0) + (.1013 x 25.0) + (.1117 x 26.0)(.7870 x 24.0) + (.1013 x 25.0) + (.1117 x 26.0)Atomic mass (average mass) Mg = 24.3 amuAtomic mass (average mass) Mg = 24.3 amu

Mg24.3

3737

Learning Check AT7Learning Check AT7

Gallium is a metallic element found in Gallium is a metallic element found in

small lasers used in compact disc players. small lasers used in compact disc players.

In a sample of gallium, there is 60.2% of In a sample of gallium, there is 60.2% of

gallium-69 (68.9 amu) atoms and 39.8% of gallium-69 (68.9 amu) atoms and 39.8% of

gallium-71 (70.9 amu) atoms. What is the gallium-71 (70.9 amu) atoms. What is the

average atomic mass of galliumaverage atomic mass of gallium??

3838

Solution AT7Solution AT7

Ga-69 Ga-69

68.9 amu x 68.9 amu x 60.2 60.2 = = 41.5 amu for 41.5 amu for 6969GaGa

100100

Ga-71 (%/100)Ga-71 (%/100)

70.9 amu x 70.9 amu x 39.8 39.8 = = 28.2 amu for 28.2 amu for 7171GaGa

100100

Atomic mass Ga Atomic mass Ga = = 69.7 amu69.7 amu