1. History of Atomic Structure

2. Ancient Philosophy Who: Aristotle, Democritus When: More than 2000 years ago (400 B.C.) Where: Greece What: Aristotle believed in 4 elements: Earth, Air, Fire, and Water. Democritus believed that matter was made of small particles he named “atomos”. Two prevailing theories. Why: Aristotle and Democritus used observation and inference to explain the existence of everything.

3. Aristotle Democritus

4. Alchemists Who: European Scientists When: 800 – 900 years ago Where: Europe What: Their work developed into what is now modern chemistry. Why: Trying to change ordinary materials into gold.

5. Particle Theory Who: John Dalton When: 1803 Where: England What: Described atoms as tiny particles that could not be divided. Thought each element was made of its own kind of atom. Why: Building on the ideas of Democritus in ancient Greece.

6. Particle Theory 4 Parts of Dalton’s Theory All elements are composed of indivisible particles. Atoms of the same element are exactly alike. Atoms of different elements are different. Compounds are formed by joining atoms of two or more elements.

7. John Dalton

8. Discovery of Electrons Who: J. J. Thompson When: 1897 Where: England What: Thompson discovered that electrons were smaller particles of an atom and were negatively charged. Why: Thompson knew atoms were neutrally charged, but couldn’t find the positive particle. Therefore, the atom must be composed mostly of positively charged material. (Plum Pudding Model).

9. J. J. Thompson

10. Atomic Structure I Who: Ernest Rutherford When: 1908 Where: England What: Conducted an experiment to isolate the positive particles in an atom. Decided that the atoms were mostly empty space, but had a dense central core (nucleus). Why: He knew that atoms had positive and negative particles, so the nucleus must be positively charged.

11. Ernest Rutherford

12. Atomic Structure II Who: Niels Bohr When: 1913 Where: England What: Proposed that electrons traveled in fixed orbits around the nucleus. Scientists still use the Bohr model to show the number of electrons in each orbit around the nucleus. Why: Bohr was trying to show why the negative electrons were not sucked into the nucleus of the atom.

13. Niels Bohr

14. Atomic Structure III Who: James Chadwick When: 1935 Where: England What:.Discovered the neutral particle in the center of the atom (neutron). Why: Chadwick was trying to figure out the discrepancy of atomic mass not being equal to the number of protons plus electrons. He theorized there must be a massive particle that had no charge and was therefore hard to find.

15. Electron Cloud Model Electrons travel around the nucleus in random orbits (no definite paths). Scientists cannot predict where they will be at any given moment. Electrons travel so fast, they appear to form a “cloud” around the nucleus. The small, positively charged nucleus is surrounded by a large space called the electron cloud.

16. The Model of the Atom Through Time