History of Atomic History of Atomic TheoriesTheoriesDr. Chin ChuDr. Chin Chu

River Dell Regional High SchoolRiver Dell Regional High School

History of the Atomic History of the Atomic TheoryTheory

Remember: a scientific theory explains Remember: a scientific theory explains behaviors and the ‘nature’ of thingsbehaviors and the ‘nature’ of things

Theories can be revised when new Theories can be revised when new discoveries are madediscoveries are made

The theory describing the composition The theory describing the composition of matter has been revised many timesof matter has been revised many times

Ancient TimesAncient Times

Democritus Aristotle

VS.

Democritus (460-370 BCDemocritus (460-370 BC))

1.Matter is made up of “atoms” 1.Matter is made up of “atoms” that are solid, indivisible and that are solid, indivisible and indestructibleindestructible

2.Atoms constantly move in space2.Atoms constantly move in space

3.Different atoms have different 3.Different atoms have different size and shape size and shape

4.Changes in matter result from 4.Changes in matter result from changes in the grouping of changes in the grouping of atoms atoms

5. Properties of matter result from 5. Properties of matter result from size, shape and movementsize, shape and movement

AristotleAristotle (384-322 BC ) (384-322 BC ) 1. Four kinds of matter1. Four kinds of matter

a. Fire – Earth – Water – Aira. Fire – Earth – Water – Air

2. One kind of matter can transform 2. One kind of matter can transform

into anotherinto another

3. Rejected idea of the “atom” (idea 3. Rejected idea of the “atom” (idea then then

ignored for almost 2000 years ignored for almost 2000 years

4. This theory was more popular and 4. This theory was more popular and

it was easier to acceptit was easier to accept

Aristotle’s Theory of MatterAristotle’s Theory of Matter

Alchemy, a popular pursuit during the Middle Alchemy, a popular pursuit during the Middle Ages, was the search for a way to transform Ages, was the search for a way to transform common metals into gold.common metals into gold.

Though not successful to achieve their original Though not successful to achieve their original goals, alchemists did generate vast amount of goals, alchemists did generate vast amount of data relating to physical and chemical data relating to physical and chemical properties of matter, which in turn was properties of matter, which in turn was instrumental in the development of modern instrumental in the development of modern atomic theory later.atomic theory later.

Later on in the 17Later on in the 17thth century, some alchemists century, some alchemists began focusing on identifying new compounds began focusing on identifying new compounds and reactions.and reactions.

AlchemistsAlchemists

Important figures:Important figures:– Antoine Lavoisier (Law of Conservation Antoine Lavoisier (Law of Conservation

of Mass)of Mass)– Joseph Proust (Law of Definite Joseph Proust (Law of Definite

Proportions)Proportions)– John Dalton (Law of Multiple Proportions)John Dalton (Law of Multiple Proportions)

Near Modern TimeNear Modern Time

Antoine Lavoisier (1770s)Antoine Lavoisier (1770s) Experiment:Experiment:

2 Sn + O2 Sn + O22 2 SnO 2 SnO

tin oxygen tin (II) oxidetin oxygen tin (II) oxide

mass before reaction = mass after reactionmass before reaction = mass after reaction

Law of Conservation of MassLaw of Conservation of Mass

Matter cannot be created or destroyed Matter cannot be created or destroyed (in a chemical or physical change)(in a chemical or physical change)

1788-1799

Joseph Proust (1779)Joseph Proust (1779)

Develops Develops Law of Definite Composition-Law of Definite Composition- all all samples of a specific substance contain samples of a specific substance contain the same mass ratio of the same the same mass ratio of the same elementselements

a. ex: all samples of COa. ex: all samples of CO22 contains 27.3% contains 27.3%

carbon and 72.7% oxygencarbon and 72.7% oxygen

b. therefore ‘elements’ are combiningb. therefore ‘elements’ are combining

in a whole number ratioin a whole number ratio

John Dalton (1803)John Dalton (1803)

Dalton became a school teacher at the Dalton became a school teacher at the age of 12 (he left school at age 11)age of 12 (he left school at age 11)

Loved meteorology - pioneer in this fieldLoved meteorology - pioneer in this fieldStudied works of Democritus, Boyle and Studied works of Democritus, Boyle and

ProustProustWrote Wrote New System of ChemicalNew System of Chemical PhilosophyPhilosophy in in

18081808

Develops Develops Law of Multiple ProportionsLaw of Multiple Proportions

a. describes the ratio of elements a. describes the ratio of elements by by massmass in two different compounds in two different compounds composed of the same elementscomposed of the same elements

Example:Example:

carbon monoxide vs. carbon dioxidecarbon monoxide vs. carbon dioxide

1 part oxygen : 2 parts oxygen1 part oxygen : 2 parts oxygen

*when compared to the same amount of *when compared to the same amount of

carbon in each compoundcarbon in each compound

a. Matter is made of small particles-atomsa. Matter is made of small particles-atoms

b. Atoms of a given element are b. Atoms of a given element are identicalidentical in in size, mass, but differ from those of other size, mass, but differ from those of other elements*.elements*.

c. c. Atoms cannot be Atoms cannot be subdividedsubdivided or or destroyed*.destroyed*.

d. Atoms combine in small whole number d. Atoms combine in small whole number ratios to form compounds.ratios to form compounds.

e. Atoms combine, separate, or rearrange in e. Atoms combine, separate, or rearrange in chemical reactions.chemical reactions.

* Modified in Modern Atomic Theory* Modified in Modern Atomic Theory

Dalton collects data and Dalton collects data and developsdevelops Atomic Theory in Atomic Theory in

18031803

Subatomic ParticlesSubatomic Particles

A. J.J. Thomson (1887)A. J.J. Thomson (1887)

Experiments with cathode ray tubesExperiments with cathode ray tubes

Voltage source

+-

Vacuum tube

Metal Disks

Voltage source

+-

Voltage source

+-

Voltage source

+-

Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end

Voltage source

+-

Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end

Voltage source

+-

Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end

Voltage source

+-

Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end

Voltage source

+-

Voltage source

By adding an electric field By adding an electric field

Voltage source

By adding an electric field By adding an electric field

+

-

Voltage source

By adding an electric field By adding an electric field

+

-

Voltage source

By adding an electric field By adding an electric field

+

-

Voltage source

By adding an electric field By adding an electric field

+

-

Voltage source

By adding an electric field By adding an electric field

+

-

Voltage source

By adding an electric field he found that the By adding an electric field he found that the moving pieces were negative moving pieces were negative

+

-

Thomson’s Model of the AtomThomson’s Model of the Atom

a. electrons present (-)a. electrons present (-) b. atom is like plum b. atom is like plum

pudding - bunch of pudding - bunch of positive stuff (pudding), positive stuff (pudding), with the electrons with the electrons suspended (plums) suspended (plums)

Calculated the ratio Calculated the ratio between the charge of between the charge of the electron and its the electron and its mass: mass: e/me/m

“Chocolate Chip Cookie” or “Plum Pudding Model”

Millikan’s Oil Drop ExperimentsMillikan’s Oil Drop Experiments

Robert Milikan (1909)Robert Milikan (1909)– Oil Drop Experiment Oil Drop Experiment – Measured the electrical charge on the Measured the electrical charge on the

electronelectron– Mass can be calculated (Thomson Mass can be calculated (Thomson

determined the e/m ratio)determined the e/m ratio)– Mass is 1/1840 the mass of a Mass is 1/1840 the mass of a

hydrogen atomhydrogen atom– electronelectron has a mass of 9.11 x 10has a mass of 9.11 x 10-28-28 g g

Millikan’s Oil Drop Experiment Millikan’s Oil Drop Experiment (1909)(1909)

So, at this point we know:So, at this point we know:

- - Atoms are divisible particles Atoms are divisible particles– Electrons are negatively chargedElectrons are negatively charged– The mass of an electron is very smallThe mass of an electron is very small

HOWEVER HOWEVER – Atoms should have a (+) portion to Atoms should have a (+) portion to

balancebalance

the negative partthe negative part

- Electrons are so small that some other - Electrons are so small that some other particles must account for massparticles must account for mass

History of the Atomic Structure – History of the Atomic Structure – Summary thus farSummary thus far

Experiment (1909)Experiment (1909)

Gold Foil Experiment (Expectations)Gold Foil Experiment (Expectations)

a. Shot alpha particles at atoms of a. Shot alpha particles at atoms of goldgold

b. expected them to pass straight b. expected them to pass straight

throughthrough

Ernest Rutherford (1871-1937)Ernest Rutherford (1871-1937)

Lead block

Uranium

Gold Foil

Florescent Screen

He thought this would happen:

According to Thomson Model

He thought the mass of the positive charge was evenly distributed in the atom

Here is what he observed:

The positive region accounts for The positive region accounts for deflectiondeflection

Gold Foil Experiment ResultsGold Foil Experiment Resultsa. a. Most Most positive alpha particles pass right positive alpha particles pass right

throughthrough

b. However, a few were deflectedb. However, a few were deflected

c. Rutherford reasoned that the positivec. Rutherford reasoned that the positive

alpha particle was deflected or repelledalpha particle was deflected or repelled

by a concentration of positive charge by a concentration of positive charge

Gold Foil Experiment ConclusionsGold Foil Experiment Conclusions a. the atom is mostly empty spacea. the atom is mostly empty space

b. the atom has a small, dense positive b. the atom has a small, dense positive centercenter

surrounded by electronssurrounded by electrons

At this point in 1909, we know:At this point in 1909, we know:– pp++ = 1.67 x 10 = 1.67 x 10-24-24 g g– ee-- = 9.11 x 10 = 9.11 x 10-28-28 g g – The charges balance!The charges balance!

But,But,– How are the electrons arranged?How are the electrons arranged?– There is still mass that is unaccounted There is still mass that is unaccounted

forfor

History of the Atomic StructureHistory of the Atomic Structure

Electrons orbit nucleus in Electrons orbit nucleus in predictablepredictable paths paths

Niels Bohr (1913)Niels Bohr (1913)

In 1935In 19351. Discovers neutron in 1. Discovers neutron in

nucleusnucleus

2. Neutron is neutral - 2. Neutron is neutral - does does

not have a charge nnot have a charge n00

3. Mass is 1.67 x 103. Mass is 1.67 x 10-24-24 g g

slightly greater than slightly greater than the mass of a protonthe mass of a proton

Chadwick (1891 – 1974)Chadwick (1891 – 1974)

History of the Atomic History of the Atomic TheoryTheory

18031803 18971897 19091909 19131913 19351935 TodayTodaysolidsolid

particleparticleelectronelectron protonproton e- orbit e- orbit

nucleusnucleusneutronneutron Quantum Quantum

Atom Atom theorytheory

DaltonDalton ThomsonThomson RutherforRutherfordd

BohrBohr ChadwickChadwick SchrodingSchrodinger and er and othersothers

Charges balancedCharges balanced Mass accounted forMass accounted for However – However –

what about thewhat about the

behavior of thebehavior of the

electrons? electrons?

History of the Atomic StructureHistory of the Atomic Structure

1.1. The atom is mostly empty The atom is mostly empty

spacespace

2.2. Two regions:Two regions:aa. . NucleusNucleus- protons and - protons and

neutronsneutrons

b.b. Electron cloud-Electron cloud- region region where you have a 90% where you have a 90% chance of finding an chance of finding an electronelectron

The Quantum Atom TheoryThe Quantum Atom Theory

The Current Model for The Current Model for AtomsAtoms

1.1. All matter is made up of small particles All matter is made up of small particles called atoms.called atoms.

2.2. Atoms of the same element have the Atoms of the same element have the same chemical properties while atoms same chemical properties while atoms of different elements have different of different elements have different properties properties

3.3. Not all atoms of an element have the Not all atoms of an element have the same mass, but they all have a definite same mass, but they all have a definite average mass which is characteristic. average mass which is characteristic. (isotopes)(isotopes)

Modern Atomic TheoryModern Atomic Theory

4.4. Atoms of different elements Atoms of different elements combine to form compounds and combine to form compounds and each element in the compound each element in the compound loses its characteristic properties.loses its characteristic properties.

5.5. Atoms cannot be subdivided by Atoms cannot be subdivided by chemical or physical changes – only chemical or physical changes – only by nuclearby nuclear

changeschanges

Modern Atomic Theory (cont.)Modern Atomic Theory (cont.)