Atomic Structure Timeline

History of Atomic Theory

Essential Questions

• What does it mean when my science

teacher says stuff cannot be created or

destroyed?

• What is matter made up of? And who

figured it out?

Evolution of Modern Atomic Theory

Democritus (400 B.C.)

• Proposed that matter was

composed of tiny indivisible

particles

• Not based on experimental

data

• Greek: atomos

The Early Atom

• Aristotle succeeded Democritus and did not believe in atoms. Instead, he thought that all matter was continuous. It was his theory that was accepted for the next 2000 years.

Alchemy (next 2000 years)

• Mixture of science and mysticism.

• Lab procedures were developed, but alchemists did not

perform controlled experiments like true scientists.

Lavoisier 1777

• French chemist, who is considered the founder of modern chemistry.

• Lavoisier clarified the concept of an element as a simple substance that could not be broken down by any known method of chemical analysis

• He devised the Law of Conservation of Mass

Basic Laws of Matter

• Law of Conservation of Mass- mass is

neither created nor destroyed during

ordinary chemical reactions or physical

changes.

CH4 + 2O2 → 2H2O + CO2

16g + 64g → 36g + 44g

Antoine Lavoisier

stated this about 1785

Alka Seltzer in Water

• Ziploc bag

• Alka seltzer tablet

• Water

Using the reaction between the tablet and the water, prove that the Law of Conservation of matter is true.

Proust (1794)

• Proust stated the Law of Definite

Proportions.

• The law states that the ratio of elements

in a compound is always the same.

Basic Laws of Matter

• Law of Definite Proportions – no matter how much salt you have, it is always 39.34% Na and 60.66% Cl by mass.

Example: Sodium chloride always contains 39.34% Na and 60.66% Cl by mass.

2NaCl → 2Na + Cl2100g → 39.34g + 60.66g

116.88g → ? + ?

Joseph Louis Proust

stated this in 1794.

John Dalton (1808)

• Dalton add his own - the Law of multiple

proportions

• He proposed a theory to summarize and

explain these three laws

I was a school

teacher at the

age of 12!

Basic Laws of Matter

• Law of Multiple Proportions- Two or more

elements can combine to form different

compounds in whole-number ratios.

Example

John Dalton

proposed this

in 1803.

John Dalton

Dalton’s Four Postulates

1. Elements are composed of small indivisible

particles called atoms.

2. Atoms of the same element are identical.

Atoms of different elements are different.

3. Atoms of different elements combine together

in simple whole number ratios to create

compounds.

4. In a chemical reaction, atoms are rearranged,

but not changed.

John Dalton’s Elements

Was Dalton right?

Elements are

composed of small

indivisible particles

called atoms.

Subatomic particles –

electrons, protons,

neutrons, and more

Atoms of the same

element are identical.

Atoms of different

elements are different.

No, isotopes are atoms

that have the same

number of protons but

a different number of

neutrons

Was Dalton right?

Atoms of different

elements combine

together in simple

whole number ratios to

create a compound.

Yes! He was right!

In a chemical reaction,

atoms are rearranged,

but not changed.

Yes! He was right!

Essential Questions

• So, Dalton figured out the atom, how did

we figure out what the inside of an atom

looks like?

How Big (Small) Is the Nucleus

of an Atom?

• If the total atom was the size of a

football field, then the nucleus would be

the size of an apple in the middle of the

field.

How Small is an Atom?

The width of a human hair = 1,000,000

carbon atoms across

One bucket full of water contains more

atoms than there are bucketfuls of water

in the Atlantic ocean

Discovery of the Electron

• In the late 1870’s many experiments

were performed in which electric current

was passed through gases at low

pressures due to the fact that gases at

atmospheric pressure don’t conduct

electricity well.

• These experiments were carried out in

glass tubes called cathode-ray tubes or

Crookes tubes for the man that

developed them.

Crookes Tube

CRT

J. J. Thomson (1903)

• Cathode Ray Tube

Experiments

– beam of negative particles

• Discovered particles

smaller than the atom!

J.J. Thomson (1903)

• Proved that the atom was divisible and that

all atoms contain electrons.

• This contradicted Dalton’s Atomic Theory.

• This allowed a new model of the atom.

J. J. Thomson (1903)

Plum-pudding Model

– positive sphere (pudding)

with

negative electrons (plums)

dispersed throughout

http://www.youtube.com/watch?

v=GzMh4q-2HjM

Robert Millikan (1910’s)

• American physicist

• Determined the mass and charge of an

electron

• Oil drop experiment

• http://www.youtube.com/watch?v=XMfY

Hag7Liw

Oil Drop Experiment

• Millikan dropped negatively charged microscopic

oil particles into a chamber containing metallic

plates and viewed them with a microscope.

• By applying voltage to the metallic plates,

Millikan created an electric field.

He was able to suspend the

oil droplets by adjusting the

electric field to the

appropriate strength and

direction to overcome

gravity.

Oil Drop Experiment

• Knowing the mass of the droplets and

the strength of the electric field

necessary to suspend them, he was

able to calculate the charge of the

electron.

• He noticed that the charge was always

a whole-number multiple of 1.602 X10-19

Coulombs.

Ernest Rutherford (1911)

• Gold Foil Experiment

• Discovered the nucleus

– dense, positive charge in

the center of the atom

• Nuclear Model

Gold Foil Experiment

Gold Foil Experiment

• As expected, most of the alpha particles

passed straight through with little or no

deflection.

• However, 1/8000 of the positively charged

alpha particles were deflected, some back

at the source.

Gold Foil

Experiment

Gold Foil Experiment

• From this experiment, Rutherford discovered that there must be a very densely packed positively charged bundle of matter within the atom which caused the deflections.

• He called this positive bundle the nucleus.

• He tried this experiment with other metals and found the same results.

Gold Foil Experiment

• The volume of the

nucleus was very small

compared to the volume

of the atom.

• Therefore, most of the

atom was composed of

empty space.

“It was about as believable as if you had fired a 15-inch shell at a piece of tissue paper, and it

came back and hit you.”

-Ernest Rutherford

Ernest Rutherford (1911)

• Nuclear Model

• http://www.youtube.com/watch?v=XBq

Hkraf8iE

Let’s take a break

• Work with a partner to complete the

“Rutherford’s Gold Foil Experiment”

Evolution of Modern Atomic Theory

Almost there!

+

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Niels Bohr (1913)

• Met with J.J. Thomson

but didn’t impress him

• Worked with Rutherford

and liked his model of the

atom

• Incorporated idea of

quantum mechanics into

the Rutherford model

Niels Bohr (1913)

• Introduced the idea of electrons traveling

around the nucleus in orbits or energy

levels.

• Like the planets, there are multiple orbits

like concentric circles around the nucleus

• Planetary Model

Niels Bohr (1913)

• He used Bright-Line Spectrum

– tried to explain presence of

specific colors in hydrogen’s

spectrum

• Built upon Max Planck’s

concepts of quantized energy

Erwin Schrödinger (1926)

• Treats electrons as waves

• Tells us the probability of finding an electron at any given location at any given moment

• Electron cloud model– Atomic orbital: region around

the nucleus where electrons are likely to be found

Erwin Schrödinger (1926)

Electron Cloud Model (orbital)

• dots represent probability of finding an e-

not actual electrons

James Chadwick (1932)

• Studied under Rutherford

and built upon his model

• Discovered neutrons

– neutral particles in the nucleus

of an atom

• He projected alpha particles (from a

decaying polonium) towards beryllium

target.

• The particles released are uncharged

and have the same mass as protons.

• He called them neutrons.

James Chadwick (1932)

Neutron Model

• revision of Rutherford’s Nuclear Model

Models of the Atom Timeline

Let’s recap with a video (10:52)

http://www.youtube.com/watch?v=NSAgL

vKOPLQ

Dogs teaching Atoms

• http://www.youtube.com/watch?v=_M9k

hs87xQ8&list=FLTWKpM0RNaHUX5D

RoXcf7qw&index=2

Atomic Timeline Poster Project

• Divide into 7 groups and make a poster that includes:

– Year of discovery

– Name of model

– Name of scientist (s)

– Picture of the model

– Picture of equipment used in discovery

– Key information about model

Poster project

• Group 1 – Dalton (pg 104-105)

• Group 2 – Thompson (pg 108 – 110)

• Group 3 – Millikan (pg. 109)

• Group 4 – Rutherford (pg. 111-113)

• Group 5 – Niels Bohr (handout)

• Group 6 - Schrödinger (pg. 152-155)

• Group 7 – Chadwick (pg. 113-114)