A History of Atomic Theory

Atomic Models

What is a model ?detailed, 3-D representation of an object - typically on smaller scale than original

model car

clothing model

model airplane

Let’s Take a Trip Through Time!

It Started with the Greeks

DEMOCRITUS

• Lived in Greece 2500 years ago (460-370 BC)

• “Father of modern science”

Demacritus said: “All matter is made of atoms that are tiny, indestructible and indivisible.”

Democritus

Greek banknote with Democritus and drawing of atom

Where did Democritus get his ideas forsmall, indestructible, indivisible atoms?

Atoms are small…

new rings old ring

EVIDENCE: old gold rings wear away slowly, getting thinner and thinner, but you never see gold atoms on your finger, so gold atoms must be very small!

Atoms are indestructible…

EVIDENCE: • Earth has been around long time• Mountains wash away but rocks continue

to exist • New plants grow where old plants die

Atoms are indivisible…

EVIDENCE: None

This was HYPOTHESIS made by Democritus to explain nature as he saw it.

What did Democritus think atoms looked like?

Appearance of atoms assumed based on behavior:

• Liquids pour • Solids are hard and rigid

Liquids pour. Why?

Liquids require a container

If liquid atoms were like little balls they would roll out when you tip the container

Greek idea of liquid atom

Solids are rigid. Why?

cockleburplant

cockleburs stick on clothing and each other

little hooks on cockleburs

Velcro® fastener Velcro hooks and loops Greek idea of solid atom

Aristotle had different ideas

ARISTOTLE: • famous Greek philosopher, born 384 BC• student of Plato (another famous philosopher)• teacher of Alexander the Great (who later conquered the

world)

Greek banknote and coin picturing Aristotle

Aristotle’s idea of matterAristotle did not believe Democritus’s idea of atoms was correct

Aristotle believed all matter made from four elements:

EarthAirFireWater

Aristotle’s idea of matter

Aristotle was more famous than Democritus, so people believed him, even though he was wrong!

Democritus’s idea of atomos (atoms) was lost for nearly 2000 years until John Dalton brought it back in 1803

Atom idea lost for ~2000 years

John Dalton, New Atom,1803

Born in England, 1766

Studied chemistry, physics, and color blindness

Brought back Democritus’s idea of an indivisible atom

Color blindness

Can you see a number in this box?If not, you may be color blind.

(More males are color blind than females)

Start writing -

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Dalton’s Billard Ball Model

Dalton’s Theory of Atoms

Five parts (postulates):

# 1: Elements are made of extremely small particles called atoms

So according to Dalton’s theory:atoms are considered to be like billiard balls

# 2: Atoms of given element are identical in size, mass, & other properties;

Atoms of different elements are different in size, mass, & other properties

(later found not to be exactly correct)

#3:Atoms cannot be subdivided, created, or destroyed(later proved wrong)

#4:

Atoms of different elements combine in simple whole-number ratios to form chemical compounds

(H2O 2:1 ratio H:O)

#5:

In chemical reactions, atoms are combined, separated and/or rearranged

Ex. of a chemical reaction:

+ +

2H2O + 2Na 2NaOH + H2

+

Dalton’s Chemical Symbols

We use different symbols today

Thomson’s Plum Pudding Model

J.J. Thomson English physicist (1856-1940) won Nobel Prize in 1906

1897:discovered ELECTRON - studied cathode rays using Crooke’s tube - showed atoms were divisible

Crooke’s tube

(stream of electrons) move from metal cathode (on left) to cross-shaped anode (on right)

cathode ↓

cross-shaped anode ↑

cross-shaped shadow ↓

cathode rays:

cross-shaped shadow is cast on glass (on right)

shows … electrons are particles smaller than an atom and have negative charge

Cathode Ray Tube Experiment

thought cathode rays were streams of particles smaller than atoms

magnetic fields deflect beam of charged particles

JJ Thomson’s discovery of (-) charged electrons proved that atoms were divisible!

Thomson’s “Plum pudding” atom

raisin

ENGLISH PLUM PUDDING

– electron

Thomson’s Plum Pudding Model: (+) charge evenly spread out while… (-) charge found in small bits – like raisins in plum pudding (or chocolate chips in cookies)

More About Charge…

opposite charges attract, like charges repel

RULE: Matter is NEUTRAL (no charge) so…

for every one (-) charged electron there must be one something of (+) charge

What do we mean by charge?

MATTER IS NEUTRAL (no charge):

• (+) charges equal (-) charges• charges cancel out each other

ELECTRONS are negatively charged (-1)

PROTONS are positively charged (+1)

ELECTRICAL CHARGE - property of matter: • # electrons greater or less than # protons

batteries have (+) and (-) ends

Rutherford’s Nuclear Model

Ernest Rutherford1871-1937

• 1908: won Nobel Prize (Chemistry)• 1920: discovered proton

discovered that most of mass of atom is found in the center (nucleus)

Rutherford’s Experiment – 1911

Rutherford’s exp’t: animation

Rutherford’s Gold Foil Experiment

Rutherford tested Thomson’s plum pudding model of atom: • sent tiny radioactive alpha (a) particles,

like bullets, towards thin sheet of gold foil

Rutherford gold foil experiment

expected a particles to pass straight through, like this…

most a particles went straight through without hitting anything, BUT a few were deflected, this means a particles must have hit something:

really heavy

OR (+) charged

gold foil experiment scattering results showed:

Plum pudding model of atom was INCORRECT, so…

Rutherford devised new model that fit his results…

proposed Nuclear ModelRutherford concluded:

• all atoms have dense, positive (+) center (center contains most of mass of atom)• atom is mostly empty space (except very tiny (–) electrons; but didn’t say where are)

nucleus/ [protons (+)]

electrons (–)/

Problems with Rutherford’s Modeldid not speculate

how electrons arranged around (+) center

to prevent (-) electrons from being attracted to (+) nucleus, electrons must orbit nucleus like Earth orbits sun

but…e- need energy to orbit

nucleus & Rutherford didn’t address this

name nucleus came later

So how BIG is the nucleus compared to the entire atom?

If atom is as big as football stadium… the nucleus is smaller than a

flea on 50-yard line!

If atom is as big as a period at the end of a sentence in textbook,

it would have the mass of 70 cars!

.

So how big is an atom?

most atoms are 1-2 angstroms across

1 Å = 1 X 10-10 m

6 Billion Cu atoms in a line = less than 1 meter!

Can we “see” an atom? NO!

James Chadwick1891-1974

worked with Rutherford 1932: discovered neutron 1935: Nobel Prize (Physics)

Bohr’s Planetary Model

Niels Bohr 1885 - 1962

created quantized atom model, 1915

worked on Manhattan Project

won Nobel Prize in 1922

Bohr’s model: e- only move from one energy level to another in atom: • e- absorb energy when jump from lower to higher

levels • e- emit energy when jump from higher to lower levels

Bohr’s new atomic model addressed issue of atom’s nucleus surrounded by electrons:

e- have definite orbits e- orbiting at different

energy levels

e- energy levels are quantized

e- only move by jumping levels (n = 1, n = 2, etc.)

:

electrons:• absorb energy when they jump away from nucleus

• emit energy when they jump towards nucleus

Bohr’s Planetary Model

• electrons travel only in specific orbits

• each orbit has definite energy

• inner most orbit (n=1): least energy

• outer most orbit (n=7): most energy

• atoms emit radiation when e- jumps from outer orbit to inner orbit (photons of light)

• e – in outermost orbits determine atom’s chemical properties

Modern Wave Mechanical Model

Erwin SchrödingerAustrian scientist (1887-1961)

• 1933: won Nobel Prize (Physics• calculated wave model of

hydrogen atom (1926)

• Today’s atomic model called:• modern model• quantum mechanical model• wave model• cloud model

Schrödinger’s wave model of atom:

• e- behaves as: energy wave AND as matter particle

(light also behaves as particle and wave)

Einstein had predicted that energy and matter were related in his equation E = mc2

If we could see an electron’s path it might look like this “cloud”

Modern/Wave/Cloud Model

electron’s energy is quantized (specific values)electrons in probability zones called “orbitals”,

not orbits

- location cannot be pinpointede- considered as both particles & waves

electrons move around nucleus at speed of light (3 x 108m/sec)

Orbitals

A Missing Particle – The Neutron1932: James Chadwick • discovered last major piece of

atom: neutron (“neutral”, no charge)

1935: won Nobel Prize (Physics) • neutron& proton weigh about

same while electron weighs almost nothing

(1891-1974)

The Modern Model

an atom is:• mostly empty space

• nucleus: most of atom’s mass

• nucleus contains protons & neutrons

• electrons in energy levels around nucleus

• electrons jump between levels, emitting & absorbing energy as jump

400 BC 1803 1904 1910 1913 1926

Niels Bohr

J J Thomson

Democratus

John Dalton

Schroedinger / Heisenberg

Ernest Rutherford

Atomic Theory

The development of atomic theory represents the work of many scientists over approximately 100 years

Next Atomic Theory ?

Which one of you will discover something new to add to the atomic model of the atom and win a Nobel Prize in the future?