Probing the AtomProbing the Atom

Developing A Model We Can UseDeveloping A Model We Can Use

1800’s1800’s

In the nineteenth century scientists were busy In the nineteenth century scientists were busy trying to determine the properties of atoms and trying to determine the properties of atoms and discover new elements to fill in the gaps left discover new elements to fill in the gaps left by Mendeleev.by Mendeleev.

Many scientists were playing with electricity - Many scientists were playing with electricity - zap it!!zap it!!

How does electricity interact with matter?How does electricity interact with matter?

ElectricityElectricity

What do we know about how electricity What do we know about how electricity interacts with matter?interacts with matter?

How was electricity used to learn more about How was electricity used to learn more about the atom?the atom?

Electricity and GasElectricity and Gas

1821 - Humphrey Davey discovers that air 1821 - Humphrey Davey discovers that air conducts electricity better if it is trapped conducts electricity better if it is trapped and its pressure is reduced.and its pressure is reduced.

Other scientists tries trapping air and other Other scientists tries trapping air and other gases in small gases in small gas dischargegas discharge tubes tubes connected to a power source.connected to a power source.

Gas Discharge TubesGas Discharge Tubes

The scientists found that the gas glowed when The scientists found that the gas glowed when electricity was passed through the tube.electricity was passed through the tube.

The problem was this was the 1800’s and there The problem was this was the 1800’s and there were no good vacuum pumps around.were no good vacuum pumps around.

All they could do was make the tube glow a All they could do was make the tube glow a little.little.

Geissler Sucks!Geissler Sucks!

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

The Geissler TubeThe Geissler Tube

The Geissler Tube was a glass tube with The Geissler Tube was a glass tube with electrodes embedded at each end.electrodes embedded at each end.

It had a pump attached, which was capable of It had a pump attached, which was capable of producing a strong vacuum.producing a strong vacuum.

Discharge TubeDischarge Tube

Gas Discharge Tubes TodayGas Discharge Tubes Today

Today gas discharge tubes are used in TV Today gas discharge tubes are used in TV sets.sets.

Now What?Now What? Once scientists had access to pumps and well Once scientists had access to pumps and well

made discharge tubes, they went to town!made discharge tubes, they went to town! Here’s what they learned.Here’s what they learned.

Gases at ordinary pressure don’t conduct electricityGases 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 blueAir glows blue When the pressure becomes too low the glow fadesWhen the pressure becomes too low the glow fades Then the glass wall glows green opposite the cathodeThen the glass wall glows green opposite the cathode

Cathode RaysCathode Rays

The green glow is produced when the glass is The green glow is produced when the glass is bombarded by some kind of ray coming from the bombarded by some kind of ray coming from the cathode.cathode.

Cathode rays Cathode rays travel through the gas toward the anode. travel through the gas toward the anode. They carry a negative charge.They carry a negative charge.

The kind of metal the cathode was made of makes no The kind of metal the cathode was made of makes no difference in the color of the ray.difference in the color of the ray.

Scientists realized that the atoms of different metals Scientists realized that the atoms of different metals must have something in common - something must have something in common - something negatively charged!!negatively charged!!

I am Not A Crooke!!I am Not A Crooke!!

In the 1870’s William Crooke conducted many In the 1870’s William Crooke conducted many experiments with discharge tubes.experiments with discharge tubes.

What does the position of the cross tell you about the charge of the cathode rays?

But what are they made of?But what are they made of?

The Search Was On!!The Search Was On!!

IN 1894 Joseph John Thomson worked with a IN 1894 Joseph John Thomson worked with a new version of Crookes’ tube to find out how new version of Crookes’ tube to find out how these “corpuscles” would move in an electric these “corpuscles” would move in an electric field.field.

Now it’s your turn to find out what he was Now it’s your turn to find out what he was able to infer…take a break from me …um , on able to infer…take a break from me …um , on me.me.

So What Do We Know Now?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 Atoms had negatively charged pqrticles that could be made

to move.to move. Atoms are electrically neutralAtoms are electrically neutral Ah ha!! Atoms must have positively charged particles as Ah ha!! Atoms must have positively charged particles as

well!!well!! So…all atoms contain both protons and electronsSo…all atoms contain both protons and electrons Electron is negative , proton is positiveElectron 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.

Science to TechnologyScience to Technology Electron microscopes Electron microscopes

control a tight beam of control a tight beam of electrons.electrons.

What does that remind What does that remind you of?you of?

The first electron The first electron microscope was built microscope was built in 1938 in Toronto.in 1938 in Toronto.

Wanna see some cool stuff??Wanna see some cool stuff??

Bugs n’ stuffBugs n’ stuff More Bugs More Bugs …in color…in color WhoahWhoah!!!!!!!!!!!!!!