Discovering the ElectronDiscovering the Electron

Chapter 4, Section 2Chapter 4, Section 2

Crooke’s Tube

Crooke’s tubes were developed in the 1870’s - kind of like early neon lights.

Sealed glass tube with a small amount of gas inside and metal electrodes (+, -) at either end.

Pass electricity through the tube.

Victorian Party NoveltyVictorian Party Novelty

Crookes – cathode rayCrookes – cathode ray

Working with a tube that had a coating at the end. The coating produced a flash of light when it was hit by radiation.

There were rays (radiation) traveling inside the tube from the cathode (-) to the anode (+).

Called a cathode ray.

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Movie of cathode ray tube

Cathode Rays – by late 1800’sCathode Rays – by late 1800’s

Actually a stream of charged particles.

Particles carry a negative charge.

Didn’t matter what gas (low P) was inside the tube or what metal the electrodes were made of. So the negative particles were in all forms of matter.

Divisible!!!Divisible!!!

Negative particle in all forms of matter. Called electrons.

The atom is DIVISIBLE!

An electric field An electric field or a magnetic or a magnetic field will deflect field will deflect a beam of a beam of charged particles.charged particles.

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J.J. Thomson “discovered” the electron in 1897

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Thomson’s Cathode Ray Tube

Thomson proposed that cathode rays were streams of particles much smaller than atoms.

He found the charge-to-mass ratio of the electron. (He called it a corpuscle.)

Atom is Divisible!Atom is Divisible!

Thomson’s discovery meant that the atom was divisible!

He knew there had to be an equal amount of positive charge because matter is neutral.

Thomson’s Plum-Pudding Model

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The positive The positive charge is evenly charge is evenly smeared out. smeared out. The negative The negative charge is in bits charge is in bits – like chips.– like chips.

Robert Millikan - 1909Robert Millikan - 1909

Oil drop experiment – determined the charge of Oil drop experiment – determined the charge of the electron: 1.60 X 10the electron: 1.60 X 10-19-19 coulomb. coulomb.

Thomson had determined the charge-to-mass Thomson had determined the charge-to-mass ratio as 1.76 X 10ratio as 1.76 X 1088 coulomb per gram. coulomb per gram.

So the mass of the electron is 9.09 X 10So the mass of the electron is 9.09 X 10 -28-28 grams. grams.

Animation of Oil-Drop ExperimentAnimation of Oil-Drop Experiment

Proton – Discovered by 1920Proton – Discovered by 1920

Thomson & Goldstein – 1907Thomson & Goldstein – 1907 Discovered a heavy particle with a positive Discovered a heavy particle with a positive

charge in some cathode ray tube experiments.charge in some cathode ray tube experiments. Rutherford – 1918Rutherford – 1918 Shot alpha particles at nitrogen gas and got Shot alpha particles at nitrogen gas and got

hydrogen. Figured out that the hydrogen had hydrogen. Figured out that the hydrogen had to come from the nitrogen. Suggested that the to come from the nitrogen. Suggested that the hydrogen nucleus was an elementary particle. hydrogen nucleus was an elementary particle. Named it proton.Named it proton.

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RutherfordRutherford

Famous for a lot of experiments.Famous for a lot of experiments. Discovered the proton.Discovered the proton. Figured out Figured out and and radiation. radiation. Changed our idea of the atom! NUCLEAR Changed our idea of the atom! NUCLEAR

model.model. One of the most elegant experiments in the history One of the most elegant experiments in the history

of science!of science!

Rutherford’s Experiment - 1911source

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The steel marble will move in a straight line until The steel marble will move in a straight line until it hits something. If it hits something heavy, like it hits something. If it hits something heavy, like the rim of the table, it will rebound back.the rim of the table, it will rebound back.

Rutherford’s exp’t: animation

Compared to an electron, an alpha Compared to an electron, an alpha particle is massive & fast.particle is massive & fast.

If Thomson’s model was correct, the If Thomson’s model was correct, the alpha particle wouldn’t be much alpha particle wouldn’t be much affected. No big deflections.affected. No big deflections.

electrons are tinyelectrons are tinypositive charge uniformly spreadpositive charge uniformly spread

Results of Rutherford’s ExptResults of Rutherford’s Expt

Most of the alpha particles went straight Most of the alpha particles went straight through – they didn’t bump into anything so through – they didn’t bump into anything so most of the atom was empty space.most of the atom was empty space.

Some of the alpha particles were deflected Some of the alpha particles were deflected back – they must have hit something really back – they must have hit something really heavy that Rutherford called the nucleus.heavy that Rutherford called the nucleus.

Results do NOT match Thomson’s model.Results do NOT match Thomson’s model.

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What did Rutherford What did Rutherford conclude from the conclude from the particles that went particles that went straight through?straight through?

What did Rutherford What did Rutherford conclude from the conclude from the particles that bounced particles that bounced back?back?

So how big is the nucleus compared So how big is the nucleus compared to the entire atom?to the entire atom?

If the atom was as big as a football stadium, If the atom was as big as a football stadium, the nucleus would be smaller than a flea on the the nucleus would be smaller than a flea on the 50-yard line!50-yard line!

If the atom was as big as a period at the end of If the atom was as big as a period at the end of a sentence in a standard textbook, it would a sentence in a standard textbook, it would have the mass of 70 cars!have the mass of 70 cars!

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Rutherford did Rutherford did notnot speculate on speculate on how the how the electrons were electrons were arranged around arranged around the nucleus.the nucleus.

Rutherford Rutherford proposed the proposed the nuclear atom.nuclear atom.

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Neils Bohr - 1913Neils Bohr - 1913Planetary modelPlanetary model

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•Electrons travel only in Electrons travel only in specific orbits.specific orbits.•Each orbit has a definite Each orbit has a definite energy. The orbit closest energy. The orbit closest to nucleus has the lowest to nucleus has the lowest energy.energy.•Atoms emit radiation Atoms emit radiation when an electron jumps when an electron jumps from an from an outerouter orbit orbit toto an an innerinner orbit. orbit.•Outer orbits hold more Outer orbits hold more electrons than inner orbits.electrons than inner orbits.•Outer orbits determine Outer orbits determine atom’s chemical properties.atom’s chemical properties.

Schrodinger – 1926Schrodinger – 1926

Mathematically - treated electrons Mathematically - treated electrons as waves rather than particles!as waves rather than particles!

Quantum mechanical model orQuantum mechanical model orModern model.Modern model.

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Modern ModelModern Model

Electron’s energy has only certain values – it Electron’s energy has only certain values – it is quantized. (Bohr model had quantization is quantized. (Bohr model had quantization too!)too!)

Electrons are located in “probability regions” Electrons are located in “probability regions” or atomic orbitals. These are or atomic orbitals. These are notnot circular circular orbits!orbits!

Electrons move around the nucleus at near the Electrons move around the nucleus at near the speed of light.speed of light.

Schrodinger’s ModelSchrodinger’s Model

We talk about the We talk about the probabilityprobability of locating an of locating an electron at a certain place.electron at a certain place.

Also called: Quantum Mechanical Model, Also called: Quantum Mechanical Model, Wave Mechanical Model, or Modern ModelWave Mechanical Model, or Modern Model

Orbitals – standing wave patterns with definite Orbitals – standing wave patterns with definite energy.energy.

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The development of The development of atomic theory atomic theory represents the work represents the work of many scientists of many scientists over many years.over many years.

James Chadwick - 1932James Chadwick - 1932

Discovered the neutron in cloud chamber Discovered the neutron in cloud chamber experiments.experiments. About the same mass as a proton.About the same mass as a proton. Electrically neutral.Electrically neutral.