1. Models of the Atom

2. Review of Ancient Models of Matter
Leucippus and Democritus - the idea of the “atom” – the indivisible particle
Empedocles and Aristotle - the 4-element model
Aristotle, Aquinas and The Church – the 4-element model persists for over 2000 years.
The Alchemists – develop new chemical lab equipment and techniques but do not apply the Scientific Method

3. Enter the Scientists
The great breakthroughs made by Galileo, Copernicus and Isaac Newton made way for a revolution in pure science in Europe.
Great scientists such as Boyle, Lavoisier, Priestly and Scheele began to make strides in the field of Chemistry.
Scientists such as Francis Bacon and Rene Descartes developed the process we know as the Scientific Method.
The first scientist to apply scientific methodology to the study of matter and atoms was John Dalton

4. John Dalton
Dalton was a schoolteacher and later a professor of Mathematics and Natural philosophy (Science)
His strongest interests were in the field of Meteorology (Dalton’s Law of Partial Pressures) but he also made contributions in Chemistry
He began to make measurements of the masses of elements and created a simple table of elements.
He then went on to conduct experiments with chemical reactions and determined the masses and ratios of elements in the newly formed compounds.

5. Dalton’s Chemical Formulae

6. Dalton’s Atomic Theory
In 1803, Dalton suggested an atom is a solid featureless sphere.
Billiard Ball Model of the Atom
Dalton made two assertions about atoms:
atoms of each element are all identical to one another but different from the atoms of all other elements
atoms of different elements can combine to form more complex substances.

7. 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.

8. Problems with Dalton’s Model
Atoms exhibit electrical properties – charged atoms, static electricity, etc…
The true nature of atoms could not be determined since atoms were extremely small and no known methods of testing individual atoms was known

9. J.J. Thomson
In 1897, J.J. Thomson conducted experiments using a cathode ray tube.
Thomson’s was able to create a stream of particles that flowed through the tube and fluoresced when they hit a detectoe screen
Using magnets Thomson was able to bend the flow of particles.

10. JJ Thomson’s Model of the Atom
The particles were called “electrons” because of their electrical properties
Electrons were determined to have very small masses and a negative charge (they were repelled by the negative force of a magnet)

11. J.J. Thomson’s Model of the Atom
Atoms contain particles called electrons
Electrons have a small mass and a negative charge
The rest of the atom is a sphere of positive charge
Electrons are embedded in this sphere, so that the resulting atoms are neutral or uncharged
Thomson compared the (negative) electrons to plums embedded in a positively charged pudding

12. J.J. Thomson’s Model of the Atom
Hence the Plum Pudding Model or Raisin Bun Model

13. Ernest Rutherford
In 1895, the German scientist Roentgen discovered X-rays and started a rush of work to study the electromagnetic radiation emitted by radioactive elements.
In the early part of the 20th century, many scientists such as the Curies, Becquerel, Geiger, and Rutherford studied these invisible rays.
Scientists began using these minute waves to study the properties of atoms.

14. Ernest Rutherford
Rutherford decided to test the model of his mentor, Thomson.
In 1911, Ernest Rutherford conducted his famous gold foil experiment.
He aimed alpha particles (positively charged particles) at a thin sheet of gold foil
PREDICTION: Particles would pass straight through the gold foil

15. The Gold Foil Experiment
PREDICTED
OBSERVATION: A small number of alpha particles bounced almost straight back from the gold foil
OBSERVED

17. Ernest Rutherford’s Model of the Atom
An atom has a tiny, dense, positive core called the nucleus (which deflected the alpha particles and contains protons)
The nucleus is surrounded mostly by empty space, containing rapidly moving negative electrons (through which the alpha particles passed unhindered)
Ernest Rutherford

18. Ernest Rutherford’s Model of the Atom
Rutherford’s atom had a tiny, dense, positive nucleus orbited by electrons. Most of the atom’s volume is empty space

19. James Chadwick – the real Jimmy Neutron
In 1932, Chadwick conducted experiments that suggested that at least half of the mass of the nucleus was unaccounted for.
Chadwick’s research suggested the existence of a neutral particle in the nucleus with similar mass to the proton.
James Chadwick – the real Jimmy Neutron

20. Updated Model of the Atom

21. Modern Development of the Model of the Atom

22. A Problem with the Planetary Model
Classical Newtonian Physics suggest that any object travelling in a curved path (even if it is moving at a constant speed) is said to be accelerating.
An electron accelerating around the nucleus would continuously emit electromagnetic radiation and lose energy
Therefore, it would eventually fall into the nucleus and the atom would collapse
However, not consistent with real-world observations – atoms are stable

23. Assigned Questions p. 15 UC # 1, 2, 3, 4, 5, 6
Learning Checkpoint
Assigned Questions p. 15 UC # 1, 2, 3, 4, 5, 6