Development of Atomic Theory

(and various people along the way)

The Greek Philosophers

who

thought

but did not

experiment

LeucippusTeacher of Democritus

First proposed that if matter was divided and divided over and over and over, it would reach a

smallest particle

he called an

ATOM.

MATTER IS PARTICULATE!

Why don’t you hear about Leucippus?

His writings didn’t survive.

We know about him from his student

Democritus

who wrote a lot more about the idea of

atoms.

Democritus (470 - 380 BC)

Proposed that all matter is

made up of

tiny, invisible particles

he called

ATOMS.(this was from Leucippus)

Also proposed that atoms had different

SHAPESthat gave them different

properties.(Democritus’ own ideas)

Aristotle ( 384 – 322 BC )

Aristotle believed…

If you divided matter over and over and over, you would still

be able to divide it.

Matter is continuous.

Disagreed with Democritus on the basis that there was nothing to

hold atoms together. Also, what’s in between the

atoms?

Aristotle believed everything was made up of :

•Water

•Earth

•Air

•Fire

Alchemy

• Based on Aristotle’s beliefs that all different substances had different proportions of earth, air, fire, and water, although later other elements were added.

• By changing proportions, alchemists could achieve two goals:– Change other metals to gold– Discover substance promoting eternal youth

Alchemy

Problem with alchemy:

IT

DOESN’T

WORK!!

These beliefs lasted hundreds of years –until CHEMISTRY

replaced ALCHEMY

WHY NOT CHANGE IDEAS ABOUT MATTER?

• The Roman Catholic Church supported Aristotle’s views because he also proposed, along with Ptolemy, the idea of a geocentric universe: everything revolves around the Earth. This fit Catholic theology.

• The Catholic church provided most education, so what people learned was Aristotle.

(so nothing changed for a long, long time)

Why did things change?

• Roger Bacon (13th century): first stated scientific method

• Copernicus (14th century): proposed heliocentric universe: planets orbited sun.

• Martin Luther (15th century): started the Protestant reformation (break with Catholic Church)

• Henry VIII (15th century): like Luther (but independently) broke with Catholic Church and started educational system not run by Catholic Church.

Antoine Lavoisier (1743 – 1794)

                                

Law of Conservation

of Mass:During a chemical reaction,

the total mass of all reactants = total mass of all products

1754-1836

Law of Constant Composition

A compound always contains the same

elements in the same proportion by mass.

Atomic Theory of Matter

John Dalton (1766–1844)

• Elements are made up of small indivisible particles (atoms)

• All atoms of the same element have identical properties.

• Atoms cannot be created, destroyed or transformed during a chemical reaction.

(law of conservation of mass)

• Compounds are formed when atoms of different elements combine with each other in simple numerical ratios.

• In a compound, relative numbers and kinds of atoms are constant.

(law of constant composition)

Dalton’s Law of Multiple Proportions• When two elements form a series of compounds,

the ratios of the masses of the second element combine with a fixed mass of the first element can always be expressed as the ratio of small whole numbers.

• When two elements can combine to form more than one compound and the same amount of the first element is used in each, then the ratio of the amounts of the other element will be a whole number ratio.

Dalton’s Atoms

Solid spheres

Different elements have

different masses

Benjamin Franklin (1706–1790)

Concluded from experimentation that there were 2 types of electric charge. +,-

Michael Faraday (1791-1867)

Suggested that the structure of the atom is related to

electricity

Cathode Ray Tubes (mid 1800’s)

Cathode (-) Anode (+) Electric plates (one +, one -)

partially evacuated glass tube that produces a beam between

the cathode and anode

When filled with a gas that fluoresced in the presence of electric charge, a stream

of radiation(the cathode ray) was seen

from cathode to anode.

J. J. Thomson (1856 – 1940)

Investigated cathode rays.

Thomson and Cathode Rays

• If electrically charged metal plates were placed above and below the ray, the ray bent. It ALWAYS bent toward the positive plate.

• If a different metal was used to make the cathode, he got the same results.

• If a different gas filled the tube, he got the same results.

The Atom had a Substructure

• Cathodes made of different metals gave off identical particles. Meaning: particles are in all atoms!

• Particles were attracted to positive plate and repelled by negative plate. Meaning: particles have negative charge!

• Because original atoms were neutral, same amount of positive charge remained in the atom as left with the electrons.

What are the particles?

electrons

Robert Millikan (1868 – 1953)

Calculated the mass of an electron (9.11 x 10-28 g) and its charge (-1 fundamental

charge units)

                                           

THE

OIL DROP

EXPERIMENT

Oil Drop Experiment

Henri Becquerel (1852 – 1908)

• Discovered radioactivity in 1896

                                           

Marie Curie (1867-1934)• Discovered

that radiation accompanies changes in identity of the radioactive atom

Others also investigating radioactivity:Pierre CurieOtto Hahn

Lise Meitner

Ernest Rutherford (1871 – 1938)

Discovered:

Alpha particles

and

Beta particles

The Gold Foil Experiment

• Alpha particles have a very small size and mass and a positive charge.

• Rutherford shot alpha particles at a very thin sheet of gold foil.

• Rutherford expected alpha particles to “blast” a hole in the gold foil and go straight through because he thought the positive charge was evenly spread through the foil.

Animation of

the Gold Foil Experiment

Rutherford.exe

What he Sawwas

NOTwhat heexpected

“It was quite the most incredible event that has ever happened to me in my life. It was almost as if you fired a 15-inch shell into a piece of

tissue paper and it came back and hit you.”

WHAT HE SAW AND WHAT IT MEANT: part 1

Data: Most alpha particles went straight through the foil as if it was not there.

Conclusion: there was nothing where the atoms passed through.

Meaning: most of gold atom is empty space.

WHAT HE SAW AND WHAT IT MEANT: part 2

Data: Some alpha particles bounced straight back from the foil toward the alpha particle source.

Conclusion: the alpha particle hit something much more massive and dense than itself.

Meaning: There is a very small part of the atom that contains most of the atom’s mass.

WHAT HE SAW AND WHAT IT MEANT: part 3

Data: Some alpha particles deflected: bent away without bouncing.

Conclusion: the positive alpha particle was repelled by something else that was positive.

Meaning: The part of the atom containing the mass has a positive charge.

Rutherford’s Theory

The atom contains a center which is• Very small• Positively charged• Contains almost all the atom’s mass.

He called it the

NUCLEUSHis model was the

NUCLEAR MODEL

Moseley (1887-1915)

Graduate student of Rutherford.

Used X-ray scattering to determine that positive charge in nucleus occurred in increments.

The charge was

QUANTIZED

Mosley, Rutherford, and Protons

• Moseley suggested that the increments of positive charge matched positive particles called protons.

• He was killed before he could prove it.• Rutherford finished Moseley’s experiments.• Rutherford’s data supported Moseley’s

hypothesis.

Atomic number:NUMBER OF PROTONS

Moseley’s Periodic Law

The properties of the atoms vary in a periodic fashion according to the amount of positive

charge in their nuclei.

The positive charge in the nuclei:

Atomic number:NUMBER OF PROTONS

The number of protons in a nucleus determines the identity of an element.

The number of protons in an

atom is is is is is isthe atomic number

James Chadwick (1891-1974)

Discovered the existence of

neutronsby bombarding

elements Be to K with alpha particles.

                                           

Max Planck and the beginning of the WEIRD

STUFFIn some cases, the energy of vibrating objects is

QUANTIZED:There are some amounts of energy that are

“allowed”…

And some that are “not allowed.”

What’s the deal with quantization (quantized things)?

Compare to a ramp vs. steps:

Different places along the ramp have different heights above the ground. Any height is possible. Situations like this are called

CONTINUOUS.

Steps have specific heights above the ground but cannot have “in-between” heights.

Situations like this are called QUANTIZED.

Things believed about light• Light is a wave, which is continuous.• Light can be measured with wave measurements:

wavelength (), frequency (), speed (c), and amplitude.

• The speed of light is always the same.c=

• The energy of light depends on how bright it is (amplitude).

• Color of light is determined by wavelength or frequency.

Big Al and the photoelectric effect

The photoelectric effect: when light shines on a metal, the metal gives off electrons.

Brighter lights have no effect on the number of electrons given off.

The color of light (which depends on the frequency) has a HUGE effect on how many electrons are given off.

Big Al and the photoelectric effect

Conclusion:Light occurs in quantized bundles of

energy called PHOTONS.Photon energy is determined by the light

frequency.Electrons can absorb a whole photon or

none of it (not part of a photon).If the electron absorbs a photon with the

right energy, it jumps out of the atom.

This was a very strange idea about light.

Light (like a rainbow) was believed to be

CONTINUOUSbut Einstein said it was

PARTICULATE.

WAVE-PARTICLEDUALITY

Light is a wave (continuous).

Light is a particle (quantized).

Niels Bohr (1885- 1962)

Developed atomic model where

electrons move in quantized energy

levels.

Bohr’s ideas came from absorption and emission spectra.

When an element is heated up, it gives off light of specific colors:

emission spectrum

When light goes through an element, it absorbs light of specific colors:

absorption spectrum

Bohr’s model• Electrons can be described mathematically (but not

physically) as orbiting the nucleus in circular orbits like planets around the sun.

• The orbits are called energy levels.• The orbits have quantized energy.• Electrons absorb energy (as photons – light of specific

energy) to “jump up” to a higher energy level.• Electrons emit energy (as photons – light of specific

energy) to “drop down” to a lower energy level.

Bohr’s model was called the

Planetary ModelIt had one problem.

It only worked for hydrogen.

No other elements.

DeBroglie and matter waves• Einstein said light (which had always be

perceived as a wave) behaved like a particle.

• DeBroglie said matter (which had always been perceived as particles) behaved like waves.

• Why don’t we notice matter waves? They’re so small they are only noticeable with things that are very small and moving very fast, like electrons.

Heisenberg, who was uncertainThe Heisenberg Uncertainty Principle:

The more you know about where an electron is, the less you know about where it’s going.

You’ll never be able to describe an orbit for an

electron.

Schrodinger solves the case• Electrons behave like waves (deBroglie).

• Formula for the wave describes a region where there is a high probability (Heisenberg) of finding an electron with a specific, quantized energy (Bohr).

• Region of high probability: ORBITAL

• Each orbital is described by QUANTUM NUMBERS which identify size, shape, arrangement in space, and number of electrons in that orbital.

Schrodinger’s Model

Wave model

Wave model

Quantum Mechanical model

(This is what we use today.)