The History of Atomic Theory

1. The History of Atomic Theory Mr Nelson

2. Democritus 400 BC • The Greek philosopher Democritus began the search for a description of matter more than 2400 years ago. • He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided?

3. Atomos • This piece would be indivisible. • He named the smallest piece of matter “atomos,” meaning “not to be cut.”

4. Why? • The eminent philosophers of the time, Aristotle and Plato, had a more respected, theory. Aristotle and Plato favored the earth,fire, air and waterapproach to the nature of matter.

5. Dalton’s Model • In the early 1800s, the English Chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms.

6. Dalton’s Model • 1803 • Dalton’s Model was that atoms are indivisible particles.

7. Dalton’s Theory • He deduced that all elements are composed of atoms. • Atoms of the same element are exactly alike. • Atoms of different elements are different. • Compounds are formed by the joining of atoms of two or more elements.

8. J. J. Thomson • In 1897, the English scientist J.J. Thomson provided the first hint that an atom is made of even smaller particles.

9. Thomson Model • Thomson studied the passageof an electric current through a gas. Using a CRT. • As the current passed through the gas, it gave off rays of negatively charged particles.

10. Thomson Model Where did they come from? • the atoms of the gas were uncharged. • Where had the negative charges come from?

11. Thomson • Thomson concluded that the negative charges came from within the atom. • Thomson called the negatively charged “corpuscles,” today known as electrons. • Since the gas was known to be neutral, he reasoned that there must be positively charged particles in the atom. • But he could never find them.

12. Thomson Model • “PlumPudding” model. • Atoms were made from a positively chargedsubstance with negatively charged electrons scattered around

13. Rutherford’s Gold Foil Experiment • In 1908 • English physicist Ernest Rutherford, began work on his gold foil experiment.

14. Rutherford’s Hypothesis • Rutherford was trying to verify Thomson’s model. • He expected positively charged alpha particles to go straight through a piece of very thin gold.

15. What Happened • Most alpha particles passed straight through the gold foil • A small percentage (1/8000) were deflected at large angles or returned to the source • http://micro.magnet.fsu.edu/electromag/java/rutherford/

17. Rutherford’s Experiment • There are 2 reasons alpha particles deflected • Density of the nucleus • Repelling charges

18. Rutherford’s Conclusion • An atom has • a small, dense, positively charged center that repelled the positively charged alpha particles. • Named the center of the atom the “nucleus” • The nucleus is tiny compared to the atom as a whole. • This could only mean that the gold atoms in the sheet were mostly open space.

19. Rutherford’s Nuclear Model • Rutherford reasoned that all of an atom’s positively charged particles were contained in the nucleus. The negatively charged particles were scattered outside the nucleus around the atom’s edge.

20. Neils Bohr • Was a Jewish Scientist in Copenhagen during the onset of WWII • Hitler was interested in his research of the atom. • He was moved to the US to protect his knowledge.

21. Bohr’s Explanation • Bohr thought that an electron travelled in a specific orbit at a certain distance from the nucleus called an energy level and had specific amounts of energy. • Worked well for Hydrogen and Helium

22. Nuclear symbols • In this unit we need to be familiar with this type of symbol A X Z

23. Hyphen Notation • Includes an element name a ‘-’ and a number • Example: Sulfur – 32 • This sulfur atom has an atomic mass of 32 • Since Sulfur has ______ protons & electrons • It also has ______ neutrons