Discovering the Atom…Discovering the Atom…

A Process of DiscoveryA Process of DiscoveryFrom Philosophy to Modern Exploration From Philosophy to Modern Exploration

JournalJournal

• In the first part of this unit we will be exploring the atom. What comes to your mind when I say the word atom?

Lesson ObjectivesLesson Objectives

• To explore the process that accompanied the composition of the Atomic Theory.

• To evaluate the methods used by various contributors.

• To develop an understanding of the anatomy of the atom

Deep Thoughts by DemocritusDeep Thoughts by Democritus

• All materials in the world are composed of Atoms and Empty Space.

• 400 B.C.• No evidence, just a hypothesis!• This explanation of matter was not

accepted.

Atoms: Building Blocks of MatterAtoms: Building Blocks of Matter

AristotleAristotle

• Saw matter as being continuous…• 350 B.C.• Composed of “Hyle”• This hypothesis was believed!!!

Whew…they bought it!!!Whew…they bought it!!!

• Aristotle’s hypothesis was adopted and believed for nearly 2200 years!

Philosophy vs. SciencePhilosophy vs. Science

Philosophy• No experimentation• No proof• Diverging beliefs• Acceptance based on

popularity.

Science• Experiments• Evidence• Converging beliefs• Acceptance based on

the validity and reproducibility of your experiments.

A Breakthrough…A Breakthrough…

• In the 1700’s Scientists like Sir Issac Newton began to Question Aristotle’s reasoning!

• Still no evidence!

NICE…NICE…

In the 1700’s the balance was In the 1700’s the balance was invented.invented.

Here comes that evidence we wanted…Here comes that evidence we wanted…

Dalton Explodes on the scene!!!

• Dalton…the keen observer.

• Was the first to generate a legitimate hypothesis about the structure of the atom.

• HE HAD EVIDENCE TO BACK IT UP!!!− Lavosiere & Proust

From Lavosiere…• Conservation of matter!

• When observing a NORMAL chemical reaction in a closed environment, the final mass after the reaction MUST equal the initial mass before the reaction!

From Proust…• Law of Definite Proportions

• Elements exposed to one another will combine in small, whole number ratios.

Dalton’s Hypothesis…Five Major Points

Point #1:

• All Matter is composed of tiny particles called atoms.

Point #2:

• Atoms are the indestructible, they cannot be subdivided, created or destroyed.

Point #3:

• All atoms of the same element are identical.

Point #4:

• Atoms of each element are unique, they are different from all other elements.

Point #5:

• Atoms combine in simple, whole number ratios to form chemical compounds.

Dalton’s Law of Multiple Proportions…

• The law of multiple proportions states that with the number of one element held constant, there are more than one compound that can form.

• e.g. CO, CO2

The Dawn of the The Dawn of the

Subatomic EraSubatomic Era

J. J. ThomsonJ. J. Thomson18971897

Thomson’s Thomson’s Research…Research…• Cathode Ray tubes…Cathode Ray tubes…

– Discovered the ray had a negative Discovered the ray had a negative charge.charge.

– Determined the ray was composed Determined the ray was composed of “electrons.”of “electrons.”

– Determined the mass to charge Determined the mass to charge ratio for the electron.ratio for the electron.

Dalton’s First Law FallsDalton’s First Law Falls Dalton’s hypothesis that claims that the Dalton’s hypothesis that claims that the

atom is the smallest and simplest form of atom is the smallest and simplest form of matter was proved wrong through the matter was proved wrong through the experimentation of Thomson and experimentation of Thomson and Milliken.Milliken.

Thomson and Milliken found and named Thomson and Milliken found and named the first subatomic particle…the electron.the first subatomic particle…the electron.

Robert Milliken (1903)Robert Milliken (1903)

• American scientist determined American scientist determined the actual magnitude of the the actual magnitude of the charge of an electron.charge of an electron.

• The charge was -1.The charge was -1.• Used his charge and Thomson’s Used his charge and Thomson’s

ratio to determine the mass to ratio to determine the mass to be 1/1837 mass of hydrogen.be 1/1837 mass of hydrogen.

Oil Drop Experiment…Oil Drop Experiment…

Oil Drop Experiment…Oil Drop Experiment…

Rutherford’s Crew…Rutherford’s Crew…Two years of intense Two years of intense

discovery!discovery!1912 - 19131912 - 1913

Geiger and Marsden…Gold FoilGeiger and Marsden…Gold FoilExperimental SetupExperimental Setup

The Fruits of Their LaborThe Fruits of Their Labor

• Atom is mostly empty space.Atom is mostly empty space.• Only 1 of 8000 positively charge Only 1 of 8000 positively charge

particles returned to the particles returned to the source…source…– What do you think this proved?What do you think this proved?

The Fruits of Their LaborThe Fruits of Their Labor• Atom is mostly empty space.Atom is mostly empty space.• Only 1 of 8000 positively charge Only 1 of 8000 positively charge

particles returned to the source…particles returned to the source…– What do you think this proved?What do you think this proved?

•This proved that the atom was mostly This proved that the atom was mostly empty spaceempty space

•That the center was small and had a That the center was small and had a positive charge.positive charge.

Rutherford’s Publication…Rutherford’s Publication…

• Rutherford theorized that the Rutherford theorized that the center held all of the positive center held all of the positive charge and that it was very charge and that it was very small.small.

• Rutherford also named the Rutherford also named the positive center the nucleus.positive center the nucleus.

MOOOOOOOseleyMOOOOOOOseley•Using X-Ray tubes determined that Using X-Ray tubes determined that

each element had a characteristic each element had a characteristic wavelength.wavelength.

•The magnitude of the wavelength The magnitude of the wavelength was directly related to the number of was directly related to the number of positive particles in its nucleus.positive particles in its nucleus.

•Called the positive particles protons.Called the positive particles protons.•Devised the idea of Atomic Number!Devised the idea of Atomic Number!

The Missing Link…the The Missing Link…the Neutron!Neutron!• 1920 – Rutherford first to 1920 – Rutherford first to

hypothesize the existence of a hypothesize the existence of a third massive particle in the third massive particle in the nucleus.nucleus.

• 1930 – Walter Bothe first 1930 – Walter Bothe first experimentation.experimentation.

• 1932 – Chadwick repeats Bothe’s 1932 – Chadwick repeats Bothe’s experiments and finds definite experiments and finds definite sign of massive, neutral particles sign of massive, neutral particles with high energy in the nucleus.with high energy in the nucleus.