History of the OBJECTIVES. ESSENTIAL QUESTION What evidence is there for the existence of atoms and their sub-atomic particles?

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1 History of the 09/15/2016 OBJECTIVES Understand the law of definite proportions. Define a scientific law and identify how observations become a law. Explain that a scientific theory is not established until similar conclusions are yielded by a multitude of experiments conducted by various scientists Analyze and differentiate among the theories and associated scientists that led to the modern atomic theory. Explain how certain experiments led to the creation of the historical and modern atom models: for example, the plum pudding atom and the nuclear atom. Discuss the importance of certain experiments; for example, cathode ray tube and gold foil that led to the discovery of the particles that make up the atom. Discuss, using examples, how similar experiments/investigations conducted in separate locations/parts of the world may either strengthen or weaken a scientific finding. ESSENTIAL QUESTION What evidence is there for the existence of atoms and their sub-atomic particles? How does a theory differ from a law?

2 SCIENTIFIC THEORY QUICK NOTES Summarizes a hypothesis or group of hypotheses that have been supported with repeated testing If enough evidence exists, it becomes accepted as a valid explanation of a phenomenon. Theories are foundations for furthering scientific knowledge and for putting the information gathered to practical use. Scientists use theories to develop inventions or find a cure for a disease.

3 SCIENTIFIC LAW QUICK NOTES A scientific law is a statement based on repeated experimental observations that describes some aspects of the universe. A scientific law always applies under the same conditions, and implies that there is a causal relationship involving its elements.

4 Leucippus and Democritus 442 BCE Democritus: a Greek philosopher Co-originated the thought (with his teacher, Leucippus) that all matter is composed of indivisible elements.

5 Aristotle 384 BCE Studied a very wide variety of sciences. These sciences include logic, philosophy, ethics, physics, biology, psychology, politics, and rhetoric. Aristotle did not believe in the atomic theory. He thought that all materials on Earth were not made of atoms, but of the four elements, Earth, Fire, Water, and Air. Aristotle s idea, causing Democritus idea to be over-looked for about 2,000 years!

6 John Dalton 1803 Developed a theory that matter is simply composed of atoms of different weights and is combined in ratios by weight. Also proposed that these atoms are spherical, and are in motion.

7 Daltons postulates 1. Matter consists of small particles called atoms. 2. All atoms of one particular element are identical and their properties are identical. 3. Atoms are indestructible. In chemical reactions, the atoms rearrange or combine, but they are not destroyed. 4. Atoms of different elements have different properties. 5. When atoms of different elements combine they form compounds. They also combine in a ratio of whole numbers. Example: H 2 O

8 Law of Definite Proportions 5 th Postulate The Law of Definite Composition, a chemical compound always contains exactly the same proportion of elements by mass. Examples Sugar C 6 H 12 O 6 Water H 2 O 1:2:1 Ratio 2:1 Ratio

9 J.J. Thompson 1898 Discovers the electron, using properties of cathode rays. Developed the plum pudding model.

10 CATHODE RAY EXPERIMENT Over the course of three experiments J. J. Thomson discovered the existence of electrons. A Cathode Ray tube is a vacuum-sealed tube with a cathode (+) and anode (-) on one end that creates a beam of electrons travelling towards the other end of the tube.

11 Plum Pudding Model The plum pudding model is an obsolete scientific model of the atom proposed by J. J. Thomson in It was devised shortly after the discovery of the electron but before the discovery of the atomic nucleus. The atom is composed of electrons surrounded by a soup of positive charge to balance the electrons' negative charges

12 Ernest Rutherford 1909 Known as the father of nuclear physics, developed the theory for the structure of the atom. He used a GOLD FOIL EXPERIMENT, observing the scattering of alpha particles, and demonstrated for the first time the existence of the atomic nucleus.

13 Gold Foil Experiment Rutherford developed an experiment where he shot alpha rays through gold foil and he hypothesized that the alpha rays particles would pass through the gold foil with little or no deflection. However when he did the experiment he saw that although most of the alpha rays did pass right through, some of the particles were deflected greatly. This showed that the atom is made up of mainly empty space except for the nucleus in the middle which has the most mass in the atom.

14 Bohr Model Introduced by Niels Bohr in 1913 Depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus similar in structure to the solar system, but with attraction provided by electrostatic forces rather than gravity.

15 Current atomic theory Atoms are made of smaller particles called protons, neutrons and electrons (and these are made of even smaller particles quarks!) Electrons do not travel in definite paths The exact path of electron cannot be predicted. Orbitals are regions where electrons are likely to be found. They are also known as electron clouds.