1. The Development of Atomic Theory
The scientists behind the Atom

2. John Dalton Sept 1776- Jul 1844 English chemist
Best known for his five- point atomic theory first published in 1805.

3. Five main points of Dalton's atomic theory:
Elements are made of extremely small particles called atoms.
Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.
Atoms cannot be subdivided, created, or destroyed.
Atoms of different elements combine in simple whole- number ratios to form chemical compounds.
In chemical reactions, atoms are combined, separated, or rearranged.

4. J.J. Thomson
Dec Aug 1940
Sir Joseph John "J. J." Thomson was a British physicist
He is credited with discovering electrons and isotopes

5. Discovery of the Electron
Around 1897 Thomson conducted experiments using a cathode ray tube.
A cathode ray
tube is a vacuum
with electrodes at
each end. A current
is passed through
the electrodes
which causes electrons to be emitted.

6. Discovery of Isotopes
Thomson is also credited with the discovery of the isotope in 1912.
An isotope has the same number of protons and electrons as the original element, but it has a different number of neutrons
Every element has isotopes
For example, Carbon has 3 main isotopes: Carbon-12, Carbon-13, and Carbon-14

7. Isotopes
The atomic masses on the periodic table are ‘isotopic averages’
This means they take all the isotopes’ mass and average them together.
To find an isotopic average:
First multiply the isotope’s mass by its percent abundance
Do this for all the isotopes
Then add your answers together

8. Isotopes Example: C-12 12 x 94% = 11.28 C-13 13 x 5% = 0.65
Total: = 12.07

9. Plum Pudding Model
Thomson believed the atom was composed of electrons surrounded by a soup of positive charge
This model is known as “the plum pudding model” - the idea being that the negatively charged electrons are like "plums" surrounded by positively charged "pudding".

10. Plum pudding model

11. Ernest Rutherford Aug 1871- Oct 1937
Ernest Rutherford was a New Zealand-born British chemist and physicist who became known as the father of nuclear physics

12. Gold foil experiment
1909
Rutherford believed Thomson’s view of the atom as ‘plum pudding’ was inaccurate based on his experiments with alpha particles and gold foil.
Alpha particle = 2 protons and 2 neutrons
They can be produced during some forms of radioactive decay.

13. Gold foil experiment
In his experiment Rutherford would aim a beam of alpha particles at a piece of gold foil
A screen was placed behind the foil as a backdrop for the alpha particles to appear upon.

15. Gold foil experiment
If Thomson’s plum pudding model was accurate the tiny alpha particles would pass through the gold atoms and fly straight into the screen.
What he noticed, however, was that the alpha particles did not fly straight, but were deflected, sometimes at extreme angles.

16. Gold foil experiment
Some of the alpha particles flew back straight at the alpha emitter.
Rutherford explained that to see an alpha particle reflected off of the gold foil was “as if you fired a 15-inch shell at a sheet of tissue paper and it came back to hit you.”

17. Gold foil experiment
This led to his conclusion that there was a positively charged center to atoms which contained most of the atom’s mass. He termed this region the nucleus.

18. Niels Bohr
Bohr believed that Rutherford’s model of the atom was incomplete
If the nucleus is positive and the electrons are just floating around then why don’t they cling to the nucleus?

19. Niels Bohr
Bohr believed the electrons in an atom traveled on specific rings around the nucleus.

20. Niels Bohr
He believed that for an electron to move from its ring it would need to gain energy

22. Erwin Schrodinger
Aug 1887 – Jan 1961
In 1926 Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position.

23. Erwin Schrodinger
Unlike the Bohr model, the quantum mechanical model does not define the exact path of an electron, but rather, predicts the odds of finding an electron in any particular location.

24. James Chadwick Oct 1891- Jul 1974
In 1932 he conducted experiments which proved the existence of a neutral subatomic particle similar in mass to a proton.
Chadwick named this particle a Neutron.