400 BC 1803 1904 1910 1913 1926

Niels Bohr

J J Thomson

Democratus

John Dalton

Schroedinger / Heisenberg

Ernest Rutherford

Atomic Theory

TOPIC: Atomic Theory – how the model of the atom has evolved and

the scientists responsible

The Complete Modern Atom

An atom is:• mostly empty space • nucleus has most of

mass of an atom • Atom is divisible

• Protons (+1 charge)• Neutrons (no charge)• Electrons (-1 charge)

• nucleus contains protons & neutrons

• electrons are in energy levels around nucleus

Here’s how we got there…

It Started with the Greeks

DEMOCRITUS

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

• “Father of modern science”

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

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!

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

Democritus(Greek)

All matter is made of atoms that are tiny, indestructible, and indivisible (can’t be divided)Atomos

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

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

Aristotle believed all matter made from four elements:

Earth WaterAir (wind) Fire

Aristotle

Thought Democritus was wrong

Matter made of 4 elements – Earth Sun AirWater

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 gray box?If not, you may be color blind.

(More males are color blind than females)

Dalton’s Theory of Atoms

Five parts to Dalton’s modern atomic theory:

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

# 2:

Atoms of given element are identical in size, mass, and other properties;

atoms of different elements are different in size, mass, and other properties

(found out not exactly correct)

#3:

Atoms cannot be subdivided, created, or destroyed

(later proved wrong)

So Dalton’s atoms are kind of like billiard balls

#4: Atoms of different elements combine in simple whole-number ratios to form chemical compounds (H2O 2 Hydrogen :1 Oxygen)

#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 or rearranged

2H2O + 2Na 2NaOH + H2

#5:In chemical reactions, atoms are combined,

separated or rearranged

2H2O 2H2 + O2

+ +

+

Dalton’s Chemical Symbols

We use different symbols today

Dalton

Billard Ball

1) Atoms (tiny)2) Atoms of same

element are identical *3) Indestructible *4) Combine in whole #

ratio (H2O)5) In chem rxn. are

combined, separated, rearranged

*turned out to not be entirely be true

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

Discovered the ELECTRON in 1897:- studied cathode rays using

Crooke’s tube- showed atoms were divisible

Crooke’s tubeJJ Thomson’s discovery of the negatively charged electrons proved that atoms were divisible!

Thomson’s “Plum pudding” atom

– raisin

ENGLISH PLUM PUDDING– electron

JJ Thomson

Plum Pudding Model

Discovered the Electron (first subatomic particle to be discovered) using cathode ray tubes

Proved Dalton #3 to be wrong

the atom is divisible!

Ernest Rutherford1871-1937

• Discovered the proton in 1920• Won Nobel Prize in chemistry in 1908

Discovered that most of mass of atom is in nucleus And the rest of the atom is empty space – GOLD FOIL EXPERIMENT

Rutherford’s Experiment – 1911

Rutherford’s exp’t: animation

Rutherford gold foil experimentRutherford expected a particles to pass straight through, like this…

What he found is that most did go straight through (so atom is mostly empty space), BUT a few a particles bounced back (Concluded there is a nucleus)

Rutherford shot a particles, which have a positive

charge, since they bounced back he concluded

that the nucleus must have a _____________

charge

nucleus/ [protons (+)]

electrons (–)/

The NUCLEAR atom model

Positive

Rutherford

Nuclear Model-discovered the proton (+ charge)- Gold Foil Experiment

- Atoms have a positive nucleus

- Nucleus contain most of the mass

- Atom is mostly empty space

+

Chadwick

- worked with Rutherford- Discovered the neutron

Problem with Rutherford’s ModelOPPOSITES ATTRACT: What prevents (-) electrons from being attracted to (+) nucleus,

Niels Bohr 1885 - 1962

Bohr addressed the issue of electrons in the atom

Nucleus surrounded by electrons orbiting at different energy levels

Electrons have definite orbits

Bohr’s new atomic model had quantized energy levels, meaning the electrons could only move by jumping between levels (numbered n = 1, n = 2, n = 3, etc.)

Bohr

Planetary Model

- Electrons travel around the nucleus in orbits (shells)- Gain/Emit energy

when moving from level to level +

e- e-

e-

e-

Erwin Schrödinger Wave Model

Austrian scientist (1887-1961)• won the Nobel Prize in physics

in 1933• calculated wave model of

hydrogen atom in 1926 • also called cloud model,

quantum mechanical model, modern model of the atom

• atom model we still use today

Electrons in probability zones called “orbitals”, not orbits

- location cannot be pinpointedElectrons are particles & waves at same time

Electrons move around nucleus at speed of light

In Schrödinger’s wave model of atom, electron behaves as energy wave as well as a 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 it might look like this “cloud”

SchrodingerWave Mechanical (Cloud) Model = Modern Model

- Electrons behave like waves- Not located in an

exact orbit but rather a zone called an orbital

- Orbital = probable location of electron

The Complete Modern Atom

An atom is:• mostly empty space • nucleus has most of

mass of an atom • nucleus contains protons

& neutrons • electrons are in energy

levels around nucleus• electrons jump between

levels, emitting and absorbing energy as jump