Download Atoms, Molecules and Ions 2

Starter Question
Draw a timeline showing all the important theories/ discoveries in
the evolution of Atomic Theory
Today we are:
•Looking at the Fundamental Laws of Chemistry
•Exploring the history of Atomic Theory
•What is the difference between an atom, molecule ion?
Important Dates
9/4- Test Corrections due
9/9- Chapter 2 Questions due
9/9- Atoms, Molecules and Ions Quiz
9/10- Nobel Prize Lecture
9/12- Atoms, Molecules and Ions Test
Dalton’s Atomic Theory
1. Each element is made up of small
particles called atoms
2. The atoms of a given element are identical.
The atoms of different elements are different.
3. Chemical compounds are formed when
atoms of different elements combine with each
other
4. Chemical reactions involve the reorganization of
the atoms
John Dalton
(1766- 1844)
The Electron
Thomson determined the charge to mass
ratio of an electron;
-1.76x108 Cg-1
Thomson reasoned that since electrons
could be produced from electrodes made
from various metals that all atoms must
contain electrons
Since atoms are known to be neutral,
Thomson assumed that all atoms must
contain a positive charge
JJ Thomson
(1856- 1940)
This resulted in the ‘Plum Pudding’
model of the atom.
In 1911 Rutherford decided to carry out an
experiment to test Thomson’s theory of the
atom
This experiment involved shooting alpha
particles at a thin sheet of gold foil.
If Thomson’s theory was correct then the
massive alpha particles would travel through
the foil with very few deflections.
Rutherford
1871- 1937
This was not the case…
In fact, Rutherford was so surprised by the results he said it was
like ‘firing a 15- inch shell at a piece of tissue paper and it coming
back to hit you’
This resulted in Rutherford coming up with a new model of the
atom.
In this model most of the atom was
composed of empty space, explaining
why most of the α particles passed
straight through the foil
The positive charge was concentrated
in the nucleus, explaining the large
deflections of a few α particles
The Neutron
Now Rutherford’s model explained almost everything that was
known about an atom, however it didn’t explain the discrepancy
in mass of atoms
It was known that hydrogen contained 1 proton and that helium
contained two. Therefore the mass of helium should be twice
that of hydrogen, however it was not. It was almost 4 times that
of hydrogen, so there must be another particle
Proof of this particle was provided by James Chadwick in 1932
Chadwick carried out an experiment, similar to
Rutherford's, where he bombarded a thin sheet
of metal (this t8ime beryllium) with alpha
particles.
When he did this high- energy radiation, similar
to γ radiation was emitted.
Further experimentation showed that this
radiation consisted of a third type of sub atomic
particle, named neutrons.
Chadwick
1891- 1974
Since neutrons had a mass slightly greater than a proton but no
electrical charge, the mass ratio mystery could be solved.