Evolution of Atomic Theory

1. Evolution of Atomic Theory

2. Democritus/Leucippus (450 BC) Matter is made up of indivisible particles Called these particles “atomos”=indivisible The Early History of Atomic Theory

3. Aristotle’s Composition Theory (354 BC) The structure of DNA was determined in 1954. How many years after Aristotle's idea is that? The universe is made of matter and all matter is composed of four elements. Earth Air Fire Water Early thinkers looked for laws in science.

4. What is a “Law” in science? A “Law” in science is a concise statement or mathematical equation about a basic and repeatable relationship in nature.

5. Mass is neither created nor destroyed (but this did not always agree with experimental observations). 2 E mc = Antoine Lavoisier (1776)Law of Conservation of Mass What happen in American history in 1776? Later, Einstein showed that there is a connection between mass and energy such that matter can be converted to energy (and vise versa). Einstein’s idea is expressed in the equation: Where c is the speed of light in meters/second.

6. Antoine Lavoisier (1776) Law of Conservation of Mass Example Light bulb Before a bulb burns out, the bulb contains some inert gas molecules and a metal filament. After a bulb burns out, the bulb has a broken filament, small amount of metal atoms deposited on the glass inside the bulb, and the same amount of inert gas molecules it had before. The total mass of the system is not different.

7. Antoine Lavoisier (1776) Law of Conservation of Mass Example Copper carbonate can ultimately be decomposed into its elements. This illustrates the mass conservation law. CuCO3Cu + C + O2

8. The total mass of the system is unchanged: mass before reaction = mass after reaction 103.0 g = 40.2 g+ 52.5 g + 10.3 g CuCO3 Cu + C + O2 Antoine Lavoisier (1776) Law of Conservation of Mass Example copper carbonate decomposition reaction Before the reaction, you have 103 grams of copper carbonate. After the complete reaction: you still have 103 grams of material but all of the copper carbonate has disappeared.

9. Dalton’s Atomic Theory (1808) The first atomic theory Was Thomas Jefferson, James Monroe, or James Madison president of the United States in 1808? • Each element is made up of tiny, indivisible particles called atoms. • Atoms of the same element are identical. • (If the atoms are not the same, they are not from the same element.)

10. Dalton’s 1808 Atomic Theory (continued) Chemical compounds are formed when atoms combine with each other. A given compound always has the same relative numbers and types of atoms. Is this a law?

11. Dalton’s 1808 Atomic Theory (continued) Chemical reactions involve reorganization of atoms. Atoms are not changed in chemical reactions. How are the different atoms held together? Is this a law?

12. Dalton’s 1808 Atomic Theory (continued) Laws of chemical combination led to a theory of the atom as a solid indivisible sphere (i.e., a marble).

13. J. J. Thomson’s “Plum Pudding” Model (1898) What was the name of the U.S. battle ship that sunk in 1898? The atom is divisible Thomson postulated the existence of electrons based on experiments using cathode ray tubes. The atom is a ball of diffuse positive (+) charge with small negative (-) bits throughout its volume.

14. Thomson’s Experiment: Deflection of “Cathode Rays” by an Applied Electric Field Electromagnet connected to power supply Electron beam Electrical signal detected only when electrons collide with this positive plate Negatively charged end cap Electromagnet connected to power supply

15. Ernest Rutherford’s Model (1909) • He postulated that an atom: • has a dense positive (+) core that was called the atom’s nucleus. • has several negative (-) particles called electrons traveling around the nucleus at a large distance from the nucleus. Was New Mexico a state in the United States in 1909?

16. Rutherford’s Experiment: Deflection of “Alpha” Particles by a Thin Piece of Gold Foil

17. Some of the alpha particles were reflected almost straight back toward the emitter. Thomson’s model of the atom could not explain why the alpha particles would not simply pass through the foil all the time. Rutherford’s Experiment: Deflection of “Alpha” Particles by a Thin Piece of Gold Foil

18. Rutherford’s Experiment (continued) This alpha particle reflects back. This alpha particle missed the nucleus and did pass through the gold foil. Alpha particles emitted from a radioactive atom source This alpha particle struck the nucleus and was reflected to the side. It did not pass through the gold foil.

19. Thinking people tried to develop theories to help explain what they observed. However, each new theory always left questions unanswered. Democritus/Leucippus atomos theory (450 BC) How do different things get made and big enough to see? Aristotle’s composition theory (354 BC) How did earth, air, water, and fire get changed into trees, rocks, and living things? Dalton’s atomic theory (1808) Why do some of these indivisible particles (atoms) join together and some do not? J. J. Thomson’s “Plum Pudding” model(1898) Why do some of the alpha particles pass through a material when some of the particles do not? Ernest Rutherford’s nucleus model (1909) Why don’t the electrons eventually lose their energy for motion and crash into the atom’s positive nucleus?

20. Thinking people tried to develop theories to help explain what they observed. However, each new theory always left questions unanswered. As the models get better and better, more observations are explained and the questions to be answered get more complicated. ?