1. The Development of Atomic Theory

2. I. Early Models of Atomic Structure The work of Dalton, Thomson, and Rutherford…

3. Democritus, "The Laughing Philosopher" Democritus was a pre- Socratic philosopher who said that all matter is made up of various indivisible elements which he called atoma, from which we get the English word atom. Democritus was a pre- Socratic philosopher who said that all matter is made up of various indivisible elements which he called atoma, from which we get the English word atom.

4. John Dalton’s Model of the Atom John Dalton developed his atomic theory in 1803 John Dalton developed his atomic theory in 1803 It worked well, until subatomic particles were discovered. It worked well, until subatomic particles were discovered.

5. Dalton’s atomic theory 1. All matter is made of atoms, which are indivisible and indestructible particles. 2. All atoms of an element are all identical in mass and properties. 3. Atoms of different elements have different masses and properties. 4. Compounds are formed by atoms combining in small whole number ratios.

6. J.J. Thomson’s Model of the Atom J.J. Thomson discovered the electron while studying cathode ray tubes in 1897. J.J. Thomson discovered the electron while studying cathode ray tubes in 1897. He received the Nobel Prize in 1906. He received the Nobel Prize in 1906.

7. Thomson’s cathode ray tube (a vacuum tube with 2 electrodes)

8. His discovery of the electron… Thomson found that the cathode ray was a beam of negative particles (electrons) and so atoms were NOT indivisible. Thomson found that the cathode ray was a beam of negative particles (electrons) and so atoms were NOT indivisible.

9. Thomson’s “Plum Pudding” Model of the Atom He described his atomic model as negative charged electrons scattered in a lump of positively charged material, like raisins scattered in plum pudding (a popular dessert at the time). He described his atomic model as negative charged electrons scattered in a lump of positively charged material, like raisins scattered in plum pudding (a popular dessert at the time).

10. Rutherford’s Model of the Atom In 1894, Ernest Rutherford was awarded a scholarship to be a research student at the Cavendish Laboratory under J.J. Thomson. He received the Nobel prize in 1908. In 1894, Ernest Rutherford was awarded a scholarship to be a research student at the Cavendish Laboratory under J.J. Thomson. He received the Nobel prize in 1908. Rutherford’s Gold Foil experiment led to the development of his atomic model in 1911. Rutherford’s Gold Foil experiment led to the development of his atomic model in 1911.

11. The Gold Foil Experiment Rutherford fired a beam of positively charged particles (called alpha particles) at a sheet of gold foil a few particles thick. Rutherford fired a beam of positively charged particles (called alpha particles) at a sheet of gold foil a few particles thick.

12. The Gold Foil Experiment Rutherford was expecting results in line with Thomson's model, with the stream of positive particles passing through the foil. Rutherford was expecting results in line with Thomson's model, with the stream of positive particles passing through the foil.

13. The Gold Foil Experiment Instead, he observed that some of the alpha particles were repelled, while most went through the foil unchanged. Instead, he observed that some of the alpha particles were repelled, while most went through the foil unchanged.

14. The Gold Foil Experiment

15. There was only one explanation… A dense, very positive A dense, very positive charge was condensed charge was condensed into one place, called into one place, called the nucleus. the nucleus. The rest of the atom had to be made up of mostly empty space. He described the electrons as “buzzing around the nucleus like bees buzz around a hive”. The rest of the atom had to be made up of mostly empty space. He described the electrons as “buzzing around the nucleus like bees buzz around a hive”.

16. One particularly memorable quote attributed to Rutherford is "All science is either physics or stamp collecting“. One particularly memorable quote attributed to Rutherford is "All science is either physics or stamp collecting“. Rutherford’s Model of the Atom

17. II. Understanding the basics… Let’s look at some basic ideas of physics to better understand the more recent models of the atom. We need to understand wave properties, and light energy. Let’s look at some basic ideas of physics to better understand the more recent models of the atom. We need to understand wave properties, and light energy.

18. Wave Properties –Wavelength, λ, is the distance between two like points on a wave [the unit of wavelength is the meter (m)]

19. Wave Properties –Frequency, υ, describes the number of wave cycles per second. [ The unit of frequency is cycles/second (s -1 ), or the Hertz (Hz)]

20. Wave Properties Amplitude is the maximum height of a wave, measured from the origin line of the wave. [A wave has zero amplitude at certain intervals along the wave, called nodes.]

21. The Electromagnetic Spectrum Short wavelength…………………….long wavelength High frequency………………….………..low frequency High energy……………….…………………..low energy

22. The Electromagnetic Spectrum All electromagnetic radiation (including visible light) travels at the same speed. All electromagnetic radiation (including visible light) travels at the same speed. The speed of light (c) The speed of light (c) = 3.0 x 10 8 m/s = 3.0 x 10 8 m/s

23. Questions: Which has the highest frequency, red light or green light? Which has the highest frequency, red light or green light? Which has the longest wavelength, x-rays or microwaves? Which has the longest wavelength, x-rays or microwaves? Which has the highest energy, yellow light or infrared rays? Which has the highest energy, yellow light or infrared rays?

24. The wavelength and frequency of light are inversely related. C = lu C = lu Speed of light = wavelength x frequency (Since c = 3.0 x 10 8 m/s, you will be asked to solve for wavelength or frequency.) l = c/ uu = c/ l

25. Question: Orange light has a wavelength of 620 nm. What is the wavelength in meters? What is the frequency? Orange light has a wavelength of 620 nm. What is the wavelength in meters? What is the frequency? 620 nm (10 -9 m/1 nm) = 6.2 x 10 -7 m 620 nm (10 -9 m/1 nm) = 6.2 x 10 -7 m u = c/ l 6.2 x 10 -7 m u = 3.0 x 10 m/s / 6.2 x 10 -7 m = 4.8 x 10 14 s -1