1. Ch 4: The Structure of the Atom
What is the fundamental unit of matter?

2. No Scientist Stands Alone
Determining atom’s exact nature is an on-going process
Set in motion more than 2000 years ago.
Each scientist fills in a piece of the puzzle.

3. 400 B.C. Democritus Greek philosopher
Physical world made of tiny particles (atomos)
Atomos = “indivisible”
No technology to prove

4. 300 B.C. Aristotle More influential
Matter can be infinitely divided  didn’t believe in atoms!!
Accepted opinion for 2000 years
The next Greek philosopher who considered the make-up of all things was Aristotle, who did not believe in atoms and who was a better known philosopher than Democritus. His ideas were closer to that of flux, and he maintained that all matter was made of the four fundamental substances of earth, air, fire, and water. You could say that Aristotle was the first Captain Planet (though he didn’t have heart as a substance)! So, unfortunately, the idea of atoms went by the wayside for about 2000 years.

5. 1642-1727 Isaac Newton First to reconsider atoms
No technology to prove!
Better known for physics theories
Isaac Newton was a famous physicist, even for his day. Though he is most famous for his physics theories, Newton actually spent more of his time as an alchemist. He often worked more than 20 hours a day, ate very little, and never married.

6. 1743-1794: Antoine Lavoisier Law of Conservation of Matter
Ideas based on experimentation
Known as “father of modern chemistry”
Beheaded in French Revolution.
Lavoisier discovered that diamond is another form of carbon, discovered oxygen and hydrogen as elements. His careful measurements in chemical reactions lead him to propose the idea that matter can’t be created or destroyed.

7. 1799: Joseph Proust
Law of definite proportions: a compound always contains the same elements in the same ratio.
Ex: Oxygen and hydrogen always react in an 8:1 ratio by mass to form water
16 grams of oxygen + 2 grams of hydrogen  18 grams water

8. 1766-1844: John Dalton Modern atomic theory:
All matter made of small particles called atoms
Atoms of an element are all identical
Atoms are indivisible
Atoms combine in whole-number ratios to form compounds
When reactions occur atoms are rearranged
Theory based on experimentation
By the time John Dalton came along technology and experimentation had improved to the point that he was able to propose a theory based upon scientifically gathered data. Dalton formulated the Law of Multiple Proportions which states that the same elements can combine in different ratios to form different compounds. Using his data and that of his contemporaries, he proposed the Modern Atomic Theory which is the basis of the Quantum Theory we follow today. Dalton’s theory was made of four basic postulates:
1. Elements are composed of tiny, separate, indivisible and indestructible particles. These particles, called atoms, maintain their identity when the element undergoes physical or chemical change.
2. All atoms of the same element are identical and different from the atoms of every other element.
3. Atoms combine in simple whole number ratios to form compounds.
4. Atoms of the same elements can combine in different ratios to form more than one compound.

9. Dalton’s atom Extremely small, dense sphere
Atoms of different elements look different
Oxygen Nitrogen

10. Cathode Ray Tube Electrical current through glass tube
Electricity travels from one end (cathode) to other (anode)
Used in “old” TV sets and computer monitors
Cathode ray tubes have phosphorous on the metal plates which glows when electricity runs through it.

11. 1856-1940: J.J. Thomson Cathode ray attracted/repelled by magnet
Cathode ray made of negatively-charged particles that are much smaller than an atom  electrons!
An atom is not “indivisible”!
He found that if he brought a strong magnet close to the tube he could cause the stream of particles to bend towards or away from the magnet, depending on which end of the magnet was used. His experiments prompted him to make a bold proposal: these mysterious rays are streams of negatively charged particles much smaller than atoms, which were, in fact, parts of the atoms, thereby proving the existence of the electron. This altered Dalton’s model of the atom, because we could no longer consider the atom to just be a solid dense sphere of matter.

12. Thomson’s Atom Looks like ball of cookie dough
Chocolate chip “electrons” randomly embedded in positively-charged sphere

13. 1871-1937: Ernest Rutherford Gold Foil Experiment:
Proved existence of nucleus
Nucleus is very dense and small compared to whole atom
Nucleus has (+) charge
No electrons in nucleus
Thomson knew the atoms as a whole were neutral, and since he had discovered that there were negatively charged particles within the atom, he proposed that there must also be a positively charged particle within the atom to balance out the charges. Thomson believed that these positive and negative charges were scattered randomly throughout the atom, kind of like a chocolate chip cookie. In 1908, however, Ernest Rutherford, a former student of Thomson's, proved Thomson's structure incorrect.  
Rutherford performed a series of experiments with radioactive alpha particles.  He placed the alpha particle source into a secure box and shot them towards a thin piece of gold foil, as illustrated in the diagram. Fluorescent screens were placed around the gold foil so that the path of the particle could be detected. The piece of gold foil was only about 2000 atoms thick, so the alpha particles, which are positively charged, should have been able to pass right through it.  While most of the alpha particles did pass straight through, a small number of them passed through at an angle as if they had bumped up against something, and some bounced straight back like a tennis ball hitting a wall.  This suggested that the charges were not evenly dispersed throughout the atom, but instead were concentrated in certain areas. Rutherford proposed that the positively charged particles, which were called protons, were concentrated in the center of the atom. Rutherford called this the nucleus. Also, since many of the particles passed straight through he suggested that the majority of the atom was made up of empty space. This is amazingly close to the theory that Democritus had proposed 2000 years earlier!

14. Rutherford’s atom Very dense nucleus with protons and neutrons
Electrons somewhere on outside

15. Niels Bohr - 1922 Atoms have energy levels
Bohr Planetary Model: electrons move in fixed orbits around nucleus

16. Modern Atomic Theory: Quantum Model
Continues to change as technology improves

17. The Pieces Come Together
Over 2000 Years after Democritus first suggested atoms
Technology made it possible!
“If I have seen further it is by standing upon the shoulders of giants.”
~ Isaac Newton
The study of the development of atomic theory is an excellent demonstration of how each scientist stands upon the shoulders of his or her predecessors. The development of technology is crucial so that ideas may be experimentally tested. Two thousand years after Democritus first thought of the idea of atoms, scientists have been able to demonstrate enough significant proof of their existence that the scientific community has accepted the theory.