1. Probing the Atom Developing A Model We Can Use

2. 1800’s In the nineteenth century scientists were busy trying to determine the properties of atoms and discover new elements to fill in the gaps left by Mendeleev. In the nineteenth century scientists were busy trying to determine the properties of atoms and discover new elements to fill in the gaps left by Mendeleev. Many scientists were playing with electricity - zap it!! Many scientists were playing with electricity - zap it!! How does electricity interact with matter? How does electricity interact with matter?

3. Electricity What do we know about how electricity interacts with matter? What do we know about how electricity interacts with matter? How was electricity used to learn more about the atom? How was electricity used to learn more about the atom?

4. Electricity and Gas 1821 - Humphrey Davey discovers that air conducts electricity better if it is trapped and its pressure is reduced. 1821 - Humphrey Davey discovers that air conducts electricity better if it is trapped and its pressure is reduced. Other scientists tries trapping air and other gases in small gas discharge tubes connected to a power source. Other scientists tries trapping air and other gases in small gas discharge tubes connected to a power source.

5. Gas Discharge Tubes The scientists found that the gas glowed when electricity was passed through the tube. The scientists found that the gas glowed when electricity was passed through the tube. The problem was this was the 1800’s and there were no good vacuum pumps around. The problem was this was the 1800’s and there were no good vacuum pumps around. All they could do was make the tube glow a little. All they could do was make the tube glow a little.

6. Geissler Sucks! In 1855 Heinrich Geissler (glass blower and mechanic) made a much improved gas discharge tube, which had a much improved vacuum pump. In 1855 Heinrich Geissler (glass blower and mechanic) made a much improved gas discharge tube, which had a much improved vacuum pump.

7. The Geissler Tube The Geissler Tube was a glass tube with electrodes embedded at each end. The Geissler Tube was a glass tube with electrodes embedded at each end. It had a pump attached, which was capable of producing a strong vacuum. It had a pump attached, which was capable of producing a strong vacuum.

8. Discharge Tube

9. Gas Discharge Tubes Today Today gas discharge tubes are used in TV sets. Today gas discharge tubes are used in TV sets.

10. Now What? Once scientists had access to pumps and well made discharge tubes, they went to town! Once scientists had access to pumps and well made discharge tubes, they went to town! Here’s what they learned. Here’s what they learned. Gases at ordinary pressure don’t conduct electricity Gases at ordinary pressure don’t conduct electricity At low pressure they do. It makes them glow. At low pressure they do. It makes them glow. Air glows blue Air glows blue When the pressure becomes too low the glow fades When the pressure becomes too low the glow fades Then the glass wall glows green opposite the cathode Then the glass wall glows green opposite the cathode

11. Cathode Rays Cathode Rays The green glow is produced when the glass is bombarded by some kind of ray coming from the cathode. The green glow is produced when the glass is bombarded by some kind of ray coming from the cathode. Cathode rays travel through the gas toward the anode. They carry a negative charge. Cathode rays travel through the gas toward the anode. They carry a negative charge. Cathode rays Cathode rays The kind of metal the cathode was made of makes no difference in the color of the ray. The kind of metal the cathode was made of makes no difference in the color of the ray. Scientists realized that the atoms of different metals must have something in common - something negatively charged!! Scientists realized that the atoms of different metals must have something in common - something negatively charged!!

12. I am Not A Crooke!! In the 1870’s William Crooke conducted many experiments with discharge tubes. In the 1870’s William Crooke conducted many experiments with discharge tubes. What does the position of the cross tell you about the charge of the cathode rays?

13. But what are they made of?

14. The Search Was On!! IN 1894 Joseph John Thomson worked with a new version of Crookes’ tube to find out how these “corpuscles” would move in an electric field. IN 1894 Joseph John Thomson worked with a new version of Crookes’ tube to find out how these “corpuscles” would move in an electric field. Now it’s your turn to find out what he was able to infer…take a break from me …um, on me. Now it’s your turn to find out what he was able to infer…take a break from me …um, on me.

15. So What Do We Know Now? By now (late 1800’s) scientists knew… By now (late 1800’s) scientists knew… Atoms had negatively charged pqrticles that could be made to move. Atoms had negatively charged pqrticles that could be made to move. Atoms are electrically neutral Atoms are electrically neutral Ah ha!! Atoms must have positively charged particles as well!! Ah ha!! Atoms must have positively charged particles as well!! So…all atoms contain both protons and electrons So…all atoms contain both protons and electrons Electron is negative, proton is positive Electron is negative, proton is positive 1 Electron charge = 1 proton charge (but opposite) 1 Electron charge = 1 proton charge (but opposite) Proton is much more massive. Proton is much more massive.

16. Science to Technology Electron microscopes control a tight beam of electrons. Electron microscopes control a tight beam of electrons. What does that remind you of? What does that remind you of? The first electron microscope was built in 1938 in Toronto. The first electron microscope was built in 1938 in Toronto.

17. Wanna see some cool stuff?? Bugs n’ stuff Bugs n’ stuff Bugs n’ stuff Bugs n’ stuff More Bugs …in color More Bugs …in color More Bugs More Bugs Whoah!!!!!!! Whoah!!!!!!! Whoah!!!!!!!