Nuclear History

Chemistry 332

Seven Important Eras Pre-atomists Early atomists Late atomists Plum pudding model Nuclear model Planetary model Quantum mechanical model

For a more complete listing check out the nuclear history page

1. Pre-Atomists c. 650 BC What was the world made of? Thales- water because it was everywhere Anaximenes- air because it was around and over

us Heraclitus- fire because it was an agent of change Empedocles- offered ideas of opposite pairs of

earth/air and fire/water Aristotle- combined all above ideas and added

the 5th element “ether”

2. Early Atomists c. 470 BC- 55BC

Leucippus- if you cut something in half enough times, you will reach a particle that can no longer be cut

Democritus Indivisible = atomos atoms were distinct in size

and shape each element was

therefore unique substances of the world

were made from combinations of different atoms

substances could be changed by altering the mixture of the composition

Atoms or Not? Aristotle’s followers

said how can you have a piece of finite matter so small it can’t be cut in half.

Atoms don’t make sense

Besides- look at the “proof” of the 5 element theory

Democritus’ followers said that you just can’t keep cutting something in half.

There must be some endpoint to the mess somewhere that is the basic building block of the universe

Who won? Aristotle’s theories

could be easily understood

The four (5) elements could be easily seen and understood

Democritus had no proof

fire

Smoke =air

ashes = earth

A little theory goes a long way

Because of this, the ideas of Aristotle prevailed “scientific” thinking for a long time

How long? The next major theory doesn’t arrive

until the 1800’s! Aristotle’s ideas lasted about 2000

years!!!

Were they Scientists? Were Aristotle and

Democritus scientists? What makes a

scientist? Performing controlled

experiments Since neither could

really did any experiments they were not scientists

So what were they? What do you call a

theorists who does not perform experiments?

A Philosopher

um…atoms

3. Late Atomists John Dalton (1766-1844) reinvents the ideas of atoms Dalton begins to perform experiments with matter

which chips away at Aristotle’s theories Multiple proportions- atoms can combine in different

whole number ratios to make different substances: CO -carbon monoxide is very different from CO2- carbon dioxide Dalton found that different masses of different

elements combined in simple whole number ratios of masses

So what? Two of Dalton’s contemporaries

were Nicholson and Carlisle If Aristotle was right, then none

of the four elements (earth, air, fire, or water) could be broken down into further elements

Right about this time, two Italians, Volta and Galvani each invented an electrical stack which could deliver a constant electrical current

Nicholson and Carlisle ran electricity through water and….

It decomposed!!!

What did it decompose into?

They thought: HO --> O + H

But it came out in a 2:1 ratio of H:O Therefore (after others’ experiments):

2 H2O --> 2 H2 + O2

And So?

Atoms came to be accepted in the general world

So what was the next question? What’s inside atoms?

4. Plum Pudding Model JJ Thomson (1856-

1940) Was using a Crooke’s

tube Found that running

electricity through a gaseous element produced a stream of particles

Experimented with magnets

-

Experiment #2

+

Conclusions? The stream of

particles was … Negative

Overall, atoms are...

Neutral

Therefore, atoms must also contain charges that are...

Positive

Plum Pudding Model Thomson figured out

that atoms contained positive and negative charges but had no idea how they were arranged.

He guessed there was a diffuse positive cloud with negative charges randomly distributed

Diffuse positive cloud

-

----

-

5. Nuclear Model Ernest Rutherford

(1910) Performed the

famous gold foil experiment

Set-up

Detection screen

Gold foil (0.00006 cm thick)

Uranium

Positive particles

Hypotheses If Thomson was

correct and atoms contained only diffuse positive charge then…

Most of these heavy positive particles should go right through, right?

Experiment

Detection screen

Gold foil

Uranium

Positive particles

What’s going on?

Most are going straight through But some are being deflected How much can they be deflected?

Continuation

HOLY ATOMS!

What conclusions can be drawn here? There is something inside an atom

that is heavy enough to deflect a particle straight back

That “something” must be positively charged

It must also be very small

Conclusions? Heavy, dense,

positively charged, small piece of the atom is….

The nucleus!!

Lets watch this animated

Nuclear Theory

Nucleus of atom contains @99.9% of the mass of the atom

Nucleus is positively charged Electrons orbit the nucleus randomly

and take up @99.9% of the space Electrons are negatively charged

Side note

Notice that Rutherford has no mention of the neutron

Neutron wasn’t discovered until 1933 by Chadwick

6. Planetary Model

Bohr proved that electrons don’t orbit randomly around the nucleus

He postulated that they orbit the nucleus like planets do around the sun

They circle in well-defined energy levels

Jumping to and from energy levels absorbs or releases energy

Atomic Line Spectra

Gas such as Neon or Hydrogen

Prism or diffraction grating

Expected to see a nice progression of color just like

normal BUT…..

Atomic Line Spectra

Gas such as Neon or Hydrogen

Prism or diffraction grating

Instead he got just a few discrete lines of color!

WHAT GIVES?

Why are only a few lines of color seen instead of the whole spectrum?

Bohr postulated that electrons exist at specific energy levels or quanta.

Electrons can only jump from specific energy level to specific energy level and not anywhere in between.

Planetary Model

Nucleus

e- e- e- e-

For Example:

Energy level 1

Energy level 2

OK

OK

NO!

For Example:

Energy level 1

Energy level 2

To go from 1 to 2 energy must be absorbed or put into the system of a specific amount…

Or…

Energy level 1

Energy level 2When you drop from a higher energy level to a lower one energy is released in a specific amount…

For Electrons

Nucleus

3 2

2 1

3 1

7. Quantum Mechanical Model

Heisenberg, Planck, Einstein, etc 1900’s to today The world of the atom is so small that

just by us looking for small pieces changes where they are

Thus we can’t be sure exactly about these energy levels so….

Electron Cloud Model

Nucleus

e- e- e- e-