The History of the Atom

History of the Atom Timeline

Democritus 460 BC and Dalton 1803 AD

Thomson

1897

Rutherford

1912

Bohr

1913

Modern Quantum Cloud Model post 1930

Marble ModelMarble Model Plum Pudding Plum Pudding ModelModel

The Nuclear The Nuclear ModelModel

The Planetary The Planetary ModelModel

Black Boxes

Black Boxes are anything that you cannot see inside

Like a cell phone…

Or a locked box…

Or an atom…

THE GREEK ATOM Democritus (around 460-370 BC.),

"a-tomos" and signifies "indivisible".

1.All matter is composed of indivisible atoms, which are too small to be seen.

2.There is a void, which is empty space between atoms.

3.Atoms are completely solid.

4.Atoms are homogeneous, with no internal structure.

5.Atoms are different in their size, their shape and their weight.

Early Greek PhilosophersThe ancient Greeks (~450 BC) believed there only four elements:

Earth

Air

Water

Fire

Aristotle

• The idea of the atom was strongly opposed by Aristotle and others.

• All substances were combinations of elements and elemental qualities. Fire, water, earth, and air. • The atom receded into the background.

John Dalton (1766-1804)

Democritus’ idea of the atom was largely ignored until an English schoolteacher did some experiments over 2000 years later, he was…

Leading to his atomic theory…

Dalton’s Atomic Theory (1803)1. All matter is made up of small indivisible atoms.

1. All atoms of one element are alike, but they are different from atoms of other elements

1. In compounds, atoms join in whole number ratios. (Law Definite Proportions- Proust)

1. Atoms are neither created nor destroyed in any chemical reaction. (Law of Conservation of Mass- Lavoisier)

1. In chemical reactions, atoms are combined, separated, or rearranged.

Dalton's Atomic Theory

1. All matter is made up of small indivisible atoms.

10

Dalton's Atomic Theory

2. All atoms of one element are alike, but they are different from atoms of other elements

11

Dalton's Atomic Theory

3. In compounds, atoms join in whole number ratios. (Law Definite Proportions- Proust)

12

Dalton's Atomic Theory

4. Atoms are neither created nor destroyed in any chemical reaction. (Law of Conservation of Mass- Lavoisier)

5. In chemical reactions, atoms are combined, separated, or rearranged.

13

John Dalton’s Atomic Theory

Almost right. A good start.

14

Structure of the atom after Dalton (ca. 1810)

very small

Thomson’s Cathode Ray Tube Experiments and the Discovery of the Electron

JJ Thomson (1856-1940) Thomson used thecathode ray tube to helpestablish the identity ofthe electron 1897.

Click here to listen to him talk about it

Gives off electrons Vacuum Magnet - redirected the electrons- nobody knows why the magnetic field bends

17

J.J. Thomson (1897): Cathode Rays

Atoms subjected to high voltages give off cathode rays.

• But, what was that green light?• Was it a light?• Was it a particle?• To test this, he brought a magnet close to the

cathode ray tube to see what would happen.

20

J.J. Thomson: Cathode Rays

Cathode rays can be deflected by a magnetic field.

Cathode rays are negatively charged particles (electrons).

Electrons are in atoms.

• So, the magnet caused the cathode ray to move. What does that tell us?

• Would a magnet affect a light from a flashlight? (you could try this at home)

• Probably not.• Therefore,! the cathode ray must be a particle

Thomson also noticed…That the cathode ray was coming out of the negative end (cathode) of the tube and going toward the positive end (anode)

Therefore, because opposites attract, he concluded that the cathode ray must be negative

He called these particles…

Negative end

Positive end

ELECTRONS!

The discovery would alter Dalton’s model of the atom because now there is something inside it

But, in addition to the negatively charged electrons, there must be something giving it a positive charge because the overall charge of the atom is neutral (not negative)

Let’s look at Thomson’s model of the atom…

Original Model of the Atom

Plum Pudding Model

These newly discovered electrons must have been balanced by some sort of positive charge. Thomson proposed a 'plum pudding' model, with positive and negative charge filling a sphere. Even Thomson's student Rutherford, who later proved model incorrect, believed it at the time.

Thomson’s Plum Pudding Model of the Atom

He believed the atom was made of positively charged stuff with negatively charged particles scattered throughout

Why the “plum pudding” model? What is “plum pudding”?

An American analogy to his atom would be…

Plum Pudding is an English dish sort of like bread pudding with raisins in it.

…Is like…

But we’ll still refer to it as the “Plum Pudding” model in class

28

J.J. Thomson – The Electron

Structure of the atom after Thomson (ca. 1900)

“Plum pudding” model: Negative

electrons are embedded in a

positively charged mass.

Positively charged mass

Electrons (-)

Unlike electrical charges attract, and

that is what holds the atom together.

Rutherford and the Nucleus

Ernest RutherfordNew Zealand physicist (1871-1937)

In 1909, Rutherford performed the Gold Foil Experiment.

In it, he shot alpha particles (very small, very dense, very fast particles) at a thin layer of gold foil.

Radioactivity• Three types of radiation were discovered by

Ernest Rutherford:– particles (positive, charge 2+, mass 7400 times of e-)– particles (negative, charge 1-)– rays (high energy light)

31

• He expected all of the alpha particles to go straight through

• It would be like if you were shooting bullets at a cake…all of the bullets (or alpha particles) would go straight through the cake (or gold foil atoms)

33

Ernest Rutherford (1909)

Scattering experiment: firing alpha particles at a gold foil

Rutherford’s Gold Foil Experiment

Expected- particles to go straight through the plum pudding with its diffuse positive charge

Actual- some particles bounced back! 1 in 8000,

Most of the alpha particles went straight through but a very tiny amount were deflected at odd angles?

That could only happen if there was something very tiny in the atom that was dense enough to deflect the alpha particles.

Like this…

Rutherford’s Reaction

• The amazed Rutherford commented that it was "as if you fired a 15-inch naval shell at a piece of tissue paper and the shell came right back and hit you."

The Nuclear Atom

Some alpha particles bounce off the gold foil. This means the mass of the atom must be concentrated in the center and is positively charged! Thomson’s model could not be correct.

38

DISCOVERY OF THE NUCLEUS

Rutherford proved that an atom was

mostly empty space

with a very small, very dense, positively charged nucleus in it.

Rutherford Video

40

Ernest Rutherford The Nucleus and the Proton

Structure of the atom after Rutherford

Planetary Model (published 1911)

The mass is not spread evenly throughout the atom, but is concentrated in the center, the nucleus.

Electrons (-) are now outside the nucleus.

The positively charged particles in

the nucleus are protons.

DISCOVERY OF NUCLEONS

Rutherford understood that the nucleus is itself composed of nucleons. These nucleons are of two types: positively charged, it's a proton. neutrally charged, it's a neutron The neutron was effectively discovered in 1932 by Chadwick.

42

James Chadwick – The Neutron

Structure of the atom after Chadwick (1932)

In the nucleus with the protons are particles of similar mass but no electrical charge called neutrons.

Electrons (-) are now outside the nucleus in quantized energy states called orbitals. (From Niels Bohr and quantum mechanics)

The positively charged particles in

the nucleus are protons.

nn+

BOHR ATOM In order to take account of atomicstability, Danish physicist, Niels Bohrcreated a new model of the atom in1913..

BOHR ATOM

The orbits of the electrons can't bejust anywhere but are "quantified”;only certain particular orbits arepermitted for the electron. It's not until one jumps from one orbit to another that it can emit (or absorb) light.

Modern Atomic Model• Also known as the quantum atomic model.

• Describes the atom as mostly empty space with a tiny massive nucleus with protons and neutrons at thecenter. Surrounding thenucleus is a cloudlike region of electrons moving toofast to describe their location in space and time.

46

Structure of the Atom

proton (+)

neutron

nucleus - responsible for the mass of the atom, positively charged

electrons - responsible for the volume and size of the atom, negatively charged

10-14 m

10-10 m

What is the relative size of the atom….?

• An atomic model the size of An atomic model the size of Busch Stadium would contain Busch Stadium would contain a pea sized nucleus containing a pea sized nucleus containing 95.95% of the atoms mass.95.95% of the atoms mass.

• The pea at the pitcher’s The pea at the pitcher’s mound would be the nucleus, mound would be the nucleus, and a firefly flying around and a firefly flying around outside would be an electron!outside would be an electron!

• The atom is mostly…..The atom is mostly…..• Empty space! Nothing!Empty space! Nothing!

Subatomic Particles• Protons and electrons are the only particles that have

a charge.• Protons and neutrons have essentially the same

mass.• The mass of an electron is so small we ignore it.

48

History of the Atom Timeline

Democritus 460 BC and Dalton 1803 AD

Thomson

1897

Rutherford

1912

Bohr

1913

Modern Quantum Cloud Model post 1930

Marble ModelMarble Model Plum Pudding Plum Pudding ModelModel

The Nuclear The Nuclear ModelModel

The Planetary The Planetary ModelModel