Discovering the Atom

One day (back when very little “scientific” information existed) you are

doing an experiment.

Your experiment involves grinding a substance with a mortar and pestle for a very long time until it becomes a very fine powder. What conclusions could you make from this

experiment?

Democritus (460 B.C.)

A similar observation is what led Democritus to propose the ‘Atomos’ – tiny individual particles that made up

matter and were indivisible.

However…

Aristotle (384 B.C.) disagreed with Democritus and so his ideas, while fairly accurate, were ignored for many years.

Over the Next 2000 Years…

Scientists discovered many elements. Additionally Alchemy thrived, particularly in

the Middle East.

Ja’far Al-Sadiq (702 A.D.)

Refuted Aristotle’s theory of matter…but no one listened to him either.

"The universe was born out of a tiny particle,

which had two opposite poles. That particle

produced an atom. In this way matter came into

being. Then the matter diversified. This

diversification was caused by the density or

rarity of the atoms."

Experiments that Got Attention…

You observe a number of chemical reactions and find that in reactions between two elements, the final mass is equal to the

combined mass of the reagent elements.

What could you conclude about the nature of matter from these

results?

John Dalton (1766)

As a result of this type of work, Dalton proposed an atomic theory.

Defining Atom

Dalton defined it as the extremely small particles that make up elements and cannot be divided….

How do we define it

today ?

Another Experiment…

At this point, atomic theory is accepted. You have a glass tube filled with a gas and connected at each end to a battery.

Battery

One night you are working late and you notice a spark inside the tube in the dark. To investigate this spark, you try

switching the gas in the tube and switching the type of battery used, but the results are always the same- you

notice sparks that are moving toward the anode.

What does this work tell you about the nature of matter?

Electrons!

Two main conclusions emerged- a cathode ray is a stream of charged particles and the

particles (regardless of element) had a negative charge.

The Next Important Experiment

You decide to shoot small particles at a piece of gold foil. What you find is that the particles enter the foil in a

straight path, but as they pass through the foil their path is sometimes diverted.

What does this experiment suggest about the nature of the atom?

Nucleus!

The work done by Ernest Rutherford’s group disproved the plum pudding model of the

atom and showed that atoms have a small but dense center.

Within the Nucleus…

• Rutherford (1920s) did work to show that protons (positively charged subatomic particles) exist in the nucleus.

• Chadwick (1932) did work to show that the nucleus also contains neutrons (neutral subatomic particles).

Review Atomic Structure

A proton, neutron, and electron walk into a restaurant and have dinner. At the end of the meal the waiter comes up and …

Subatomic Particles

Particle

proton

neutron

electron

Charge

+ ve charge

-ve charge

No charge

1

1

nil

Mass

Example - Carbon

What color are the protons, the neutrons, and the electrons?

Current Atomic Model

Atomic Number

Different elements can be described in terms of their atomic number. Atomic

number = number of protons = number of electrons.

Mass Number

Mass number is a descriptor that lets us know when we are dealing with isotopes. Mass number = # protons + # neutrons

Nuclear Reactions and Isotopes

Not all atoms of the same element are identical and in nature they exist as a

mixture of isotopes.

How Can we Change the Nucleus?

If nuclear reactions involve a change in the nucleus, what different ways can we think of

that they can occur?

Nuclear Reactions

Nuclear reactions can produce many types of radiation. All nuclear reactions move atoms toward stability.

We learned about:

Beta – (an electron)

Alpha – (2 protons, 2 neutrons)

Gamma – (no mass, no charge)

Carbon dating

All organisms pick up carbon in their life time- this is called carbon sequestration.

Can you spot what is incorrect in the diagram below?

N

n C

Carbon Dating

During our lives we have a set amounts of 12C and 14C in our bodies. 12C is the most

common for of carbon.

The 14C isotope decays by releasing beta radiation to become 14N (nitrogen).

Carbon Dating

This looks like:

146C 14

7N + 0-1B

(B= beta particle, electron)

After we die, this radioactive decay continues slowly over time such that half of the amount of

14C that is in your body when you die will be there

Carbon Dating

After we die, this radioactive decay continues slowly over time such that half of the amount of 14C

that is in your body when you die will be there 5730 years later (half life). Therefore, Carbon is good for dating things up to 50,000 years old.

How is this socially relevant?

So… How is it socially relevant?

Nanotechnology

Another cool quality of carbon is that it can form complicated molecules all on its lonesome! These

molecules are quite stable and have implications in industry and medicine.

Implications for development of medical nanotechnologies

What are the implications for development of medical nanotechnologies? (think population

characteristics)