History of the Atomic Theory

The Movers and Shakers of the

Subatomic World.

Who are these guys?

Democritus

• Greek Philosopher, lived around 400BCE• “If you kept dividing a substance by

two, could you go on forever? Or would you eventually reach the smallest piece possible?”

• The smallest, indivisible pieces of matter are called ATOMS (from “atomos” in Greek: can’t be cut)

• But he was ignored for over 2000 years!

WHY?!?!?!Aristotle and Plato had a different (and ultimately wrong!) idea: that all matter was made up of the four elements of nature. But they were more famous philosophers at the time, so everyone went along with them!

Hey bro, smell my finger!

Fwoah! Dude! That smells worse than

Democritus’ theory!

Questions...

• What did Democritus wonder?

• What did he suggest?• Why did no one believe him?

Who are these guys?

Dalton• English chemist, early 1800’s• Did some experiments that proved

Democritus was right!• Developed his own model in 1803, the

first atomic theory ever, forming the basis for all future theories and models about the atom!

• His atomic theory had 3 main points:

Dalton’s Atomic Theory

1. All matter is composed of atoms. They cannot be divided.

2. ELEMENTS contain only one type of atom, they are all the

same.

H H

O H

COMPOUNDS contain two or more types of atoms, which are different.

3. Atoms cannot be created or destroyed, but they can be rearranged to form different substances.

H H O

Questions...

• What did Dalton develop?• What were its 3 main points?

Was he right?

• Dalton’s atomic theory of matter became one of the foundations of chemistry.

• BUT it needed work…other scientists had to modify, improve and build on this theory.

Who are these guys?

Thomson

• English scientist in the late 1800’s

• His experiments showed the first hint that atoms were made up of even smaller particles

• Atoms are divisible…Dalton was wrong!

• He passed an electric current through a gas, and it gave off rays of negatively charged particles.

• These had to come from within the atom, meaning a particle smaller than the atom had to exist!

• These negative sub-atomic particles are today known as “electrons”.

• Since the gas was neutral, he figured there had to be positively charged particles too. But he couldn’t find them!

Where did they come from?

Thompson’s Model• Thompson in 1897 suggested that

atoms are made up of positively (+) and negatively (-) charged particles, spread out in the atom like

p p u l d u d m i s n g(plums in puddin’)

Thompson’s “Plum Pudding” Model

Questions...

• Which of Dalton’s ideas did Thomson disprove?

• What kind of particles did Thompson find in atoms?

• What did his idea of an atom look like?

Was Thomson Right?

• Not really…there are positively (+) and negatively (-) charged particles inside atoms.

•BUT...

Who are these guys?

Rutherford

• Kiwi-born English physicist, early 1900’s

• He stated that atoms have a small, dense, positively (+) charged

center called a nucleus.• How did he find this out?

– By shooting for gold!

He fired tiny, positively charged alpha-particles at a thin sheet of gold. Most of them passed right through. But some bounced off, as if they had hit something solid!

• Since positive charges repel other positive charges, he figured that the gold atoms had small, dense, positively-charged centers which repelled his positively-charged alpha-particle “bullets”!

• He called this center the NUCLEUS.

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+ + ++

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RUTHERFORD’S ATOMIC MODEL (1911)

TRY IT YOURSELF!

Use these materials to create Thomson’s and Rutherford’s two different atomic models, and use them to recreate Rutherford’s experiment:

Questions...

• What did Rutherford find inside the atom?

• How did he find this out?• What did Rutherford’s atomic model look like?

So...

• Particles are not spread out like plums in pudding…

• Positive particles (protons)

are in the center (nucleus).

• Where are the negative particles (electrons)?

Who are these guys?

Bohr

• Danish Physicist, early 1900’s

• Worked in Rutherford’s lab, and proposed an improvement to Rutherford’s Model:

• Electrons move in definite orbits around the nucleus, like planets around the sun.

BOHR’S MODEL (1922)

Questions...• Where were the particles in Bohr’s model of an atom?

• What is this analogous to?

• So, does this mean you could predict the location of an electron at a particular point in time?

Are we there yet?

• Bohr’s model was used for a long time…

•BUT...

Who are these guys?

Schrodinger

• Austrian physicist, mid 1900’s

• Said in 1926 that electrons aren’t particles, they’re more like waves.

• But, but, but…. Bohr said that they’re particles!

It’s time for another episode

of…

ATOMIC^

Niels Bohr VS Erwin

Schrödinger

BEGIN!

Who are these guys?

Chadwick

• English physicist, mid 1900’s• Got a Nobel prize in Physics because in

1932 he discovered the neutron, a neutral (non-charged) particle in the nucleus.

• Studied under Rutherford in Berlin in 1913

• WWI: Set up a lab in the stables of an internment camp.

• WWII: Was part of the Manhattan Project in the USA.

Who are these guys?

Heisenberg

• German physicist, mid to late 1900’s• Got a Nobel prize in Physics because in 1932

he came up with Quantum Theory. It unifies Bohr and Schrodinger’s theories:

• Electrons are particles but behave like waves,• They swirl in a sort of “cloud” around the

nucleus.• You can figure out the probability (likelihood)

that an electron is in a particular place at a particular time, but you can never be 100% certain.

Modern Atomic Model• An atom has a small,

positively charged nucleus containing positively-charged (+) protons and neutral (0) neutrons,

surrounded by a large region in which there are enough electrons (-) to make the atom neutral (0).

Copy all ofthis!

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Questions...• What theory did Heisenberg come up with?

• Do electrons have specific orbits in the modern model?

• What does the atom look like in the new model?