Post on 18-Dec-2015
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
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
-
----
-
Hypotheses If Thomson was
correct and atoms contained only diffuse positive charge then…
Most of these heavy positive particles should go right through, right?
What’s going on?
Most are going straight through But some are being deflected How much can they be deflected?
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.
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…
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….