Chapter 4

The Atom – Part 1

In Search of Giants Part 1: Atoms and the Periodic Table (2:02)

What is the atom?

• Sulfur

What is the atom?

• Lead

• An atom is the smallest particle of an element that retains that element’s properties.

• From the Greek “a-tomos” which means “not divisible”.

• The concept of the atom was first proposed in Greece over 5000 years ago.

Democritus

• First proposed the concept that matter was not continuous but was instead made up of tiny particles he termed the “atoms”.

Aristotle

• Believed in the four elements: Earth, Air, Fire and Water.

• Proposed a fifth element “ether”.

• Because of Aristotle’s vast influence on later generations his ideas would persist until the Renaissance. (Philosophy vs. Science)

Sir Francis Bacon (1561-1626)

• Father of “Inductive Reasoning”.

• In deductive reasoning, a conclusion is reached from general statements, but in inductive reasoning the conclusion is reached from specific examples.

• “Knowledge is Power”

Robert Boyle (1627 – 1691)

•Robert Boyle is generally regarded as the first modern chemist

Antoine Lavoisier (1743 – 1794)

• Father of Modern Chemistry.

• Law of Conservation of Mass.

• Abolished the Phlogiston Theory.

• Recognized and named oxygen and hydrogen.

• Introduced the metric system.

• Wrote the first extensive list of elements.

Antoine Lavoisier (1743 – 1794)

• Declared a traitor during “The Reign of Terror”.

• “It took them only an instant to cut off his head, but France may not produce another like it in a century” (LaGrange).

John Dalton (1766 – 1844)

• Proposed the first scientifically supported atomic theory.

Dalton’s Model of the Atom

• Dalton's model was that the atoms were tiny, indivisible, indestructible particles and that each one had a certain mass, size, and chemical behavior that was determined by what kind of element they were.

• What made us change Dalton’s Atomic Model?

Cathode Ray Tube

• Physicists in the 19th century found out that if they constructed a glass tube with wires inserted in both ends, and pumped out as much of the air as they could, an electric charge passed across the tube from the wires would create a fluorescent glow.

Cathode Ray Tubes

Scientific Debate• William Crookes discovered that a tube

coated in a fluorescing material at the positive end, would produce a focused ‘dot’ when cathode rays hit it.

• With more experimentation, researchers found that the cathode rays moved with the properties of waves. However, other researchers argued that the focused nature of the cathode ray beam meant that they had to be made up of particles.

Scientific Debate• Physicists knew that the ray carried a negative charge but

were not sure whether the charge could be separated from the ray. They debated whether the rays were waves or particles, as they seemed to exhibit some of the properties of both. In response, J. J. Thomson constructed some unique experiments to find a definitive and comprehensive answer about the nature of cathode rays.

• Wave – Particle Duality is a concept that will be discussed at a later time.

THOMSON’S FIRST CATHODE RAY EXPERIMENT

• Thomson built a cathode ray tube with an electrometer, a device for catching and measuring electrical charge.

electrometer

THOMSON’S FIRST CATHODE RAY EXPERIMENT

• Thomson wanted to see if, by bending the rays with a magnet, he could separate the charge from the rays.

• He found that when the rays were bent toward the electrometer that it registered a large amount of negative charge and that when they were bent away the electrometer did not register much electric charge.

• This showed that the negative charge and the cathode rays must somehow be “stuck together”.

electrometer

THOMSON’S FIRST CATHODE RAY EXPERIMENT

• Thomson’s first experiment therefore allowed him to “speculate” that the cathode rays were made of negatively charged particles.

THOMSON’S SECOND CATHODE RAY EXPERIMENT

• Thomson developed the second stage of the experiment, to prove that the rays carried a negative charge.

• To prove this hypothesis, he attempted to deflect the cathode rays with an electric field. Earlier experiments had failed to back this up, but Thomson thought that the vacuum in the tube was not good enough, and found ways to improve the quality of the vacuum.

THOMSON’S SECOND CATHODE RAY EXPERIMENT

• Thomson's second experiment proving cathode rays had electric charge.

• Cathode rays have charge: they are attracted toward the positive plate D and repelled by the negative plate E.

cathode

anode

THOMSON’S SECOND CATHODE RAY EXPERIMENT

• Thomson concluded from his first two experiments: "I can see no escape from the conclusion that [cathode rays] are charges of negative electricity carried by particles of matter."

• By deflecting the cathode rays with the electric charge, Thomson’s second experiment proved, that the cathode rays were made up of negatively charged particles.

THOMSON’S SECOND CATHODE RAY EXPERIMENT

• This result was a major discovery in itself, but Thomson resolved to understand more about the nature of these particles.

• Thomson asked: "What are these particles? Are they atoms, or molecules, or matter in a still finer state of subdivision?“

• He then moved on to a third experiment.

THOMSON’S THIRD CATHODE RAY EXPERIMENT

• Thomson's third experiment sought to determine the basic properties of the particles.

• He could measure how much the rays were bent by a magnetic field, and how much energy they carried.

• From this data he could calculate the ratio of the mass of a particle to its electric charge (m/e).

THOMSON’S THIRD CATHODE RAY EXPERIMENT

• Thomson found out that the mass to charge ratio was so small that the particles had to be nearly 2000 times smaller than a hydrogen atom.

Do you understand the significance of this?

THOMSON’S THIRD CATHODE RAY EXPERIMENT

• Thomson speculated that the cathode rays were made of negatively charged particles that must come from within the atoms. Since they were much smaller than the atom itself.

• Therefore the atom was not the smallest particle of matter and could be broken apart.

• He had discovered the first subatomic particle.

The Electron

THOMSON’S CATHODE RAY EXPERIMENTS

• Thomson’s work with cathode rays shows how a series of experiments can gradually uncover truths.

• Many great scientific discoveries involve performing a series of interconnected experiments, gradually accumulating data and proving a hypothesis.

THOMSON’S CATHODE RAY EXPERIMENTS

• Thomson received the Nobel prize for physics in 1906 for this work.

J. J. Thomson (1856 – 1940)

• Credited for the discovery of the electron.

• Invented the mass spectrometer which led to his discovery of isotopes.

Thomson’s Model of the Atom

Thomson’s model of the atom stated that the atom is a tiny, spherical and neutral body, which is made up of negatively charged particles (electrons) in a positive mass which neutralized the charge of the negative electrons.

This model is commonly referred to as the plum-pudding model of the atom.

In Search of Giants Part 2: The Discovery of the Electron (2:50)

James Chadwick (1891 – 1974)

• Discovered the neutron.

• Paved the way for nuclear power and the nuclear bomb.

Protons• Somewhere between Thomson and

Chadwick, physicists realized that there are positively charged particles in the atom which we call 'protons'.

• The way this happened was a gradual process, and that is why it is hard to say exactly who discovered the proton, although Ernest Rutherford, is often given credit.

Lord Ernest Rutherford (1871 – 1937)

• Discovered the nucleus of the atom.

• Pioneered the orbital theory (planetary model) of the atom.

Rutherford’s Gold Foil Experiment

Rutherford’s Gold Foil Experiment

Rutherford’s Gold Foil Experiment

Rutherford’s Atomic ModelThe “Planetary Model” of the Atom

• The nucleus is very small, dense, and positively charged.

• Electrons surround the nucleus.• Most of the atom is empty space

In Search of Giants Part 3: The Discovery of the Nucleus (3:20)