Chapter 4

The Atom – Part 1

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”. • “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.

J. J. Thomson (1856 – 1940)

• Credited for the discovery of the electron and of isotopes, and the invention of the mass spectrometer.

J. J. Thomson was one of the great scientists of the 19th century; his inspired and innovative cathode ray experiment greatly

contributed to our understanding of the modern world.

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. This cathode ray tube also became known as an ‘electron gun’.

Scientific Debate• Later improved cathode ray experiments found that certain

types of glass produced a fluorescent glow at the positive end of the tube. William Crookes discovered that a tube coated in a fluorescing material at the positive end, would produce a focused ‘dot’ when rays from the electron gun hit it.

Scientific Debate• With more experimentation, researchers found that the

‘cathode rays’ emitted from the cathode could not move around solid objects and so traveled in straight lines, a property of waves. However, other researchers, notably Crookes, argued that the focused nature of the beam meant that they had to be 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 ending in a pair of metal cylinders with a slit in them. These cylinders were in turn connected to an electrometer, a device for catching and measuring electrical charge.

• 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 entered the slit in the cylinders, the electrometer measured a large amount of negative charge. The electrometer did not register much electric charge if the rays were bent so they would not enter the slit.

• Thomson’s first experiment indicated that the negative charge and the cathode rays must somehow be stuck together: you cannot separate the negative charge from the cathode rays.

THOMSON’S FIRST CATHODE RAY EXPERIMENT

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 them 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 experiment indicated as he expected, that the cathode rays were deflected by the electric charge, proving beyond doubt that the cathode rays were made up of negatively charged particles.

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

THOMSON’S SECOND CATHODE RAY EXPERIMENT

Thomson concluded from these two experiments, "I can see no escape from the conclusion that [cathode rays] are charges of negative electricity carried by particles of matter." But, he continued, "What are these particles? are they atoms, or molecules, or matter in a still finer state of subdivision?"

THOMSON’S THIRD CATHODE RAY EXPERIMENT

• Thomson's third experiment sought to determine the basic properties of the particles. Although he couldn't measure directly the mass or the electric charge of such a particle, 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 found out that the charge to mass ratio was so large that the particles either carried a huge charge, or were a thousand times smaller than a hydrogen ion. He decided upon the latter and came up with the idea that the cathode rays were made of negatively charged particles that came from within the atoms.

• He had discovered the electron.

THOMSON’S THIRD CATHODE RAY EXPERIMENT

• Schematic drawing of one of the tubes used in Thomson's third experiment. Rays originate at the cathode on the left and pass through a slit in the anode into a bell jar containing gas at low pressure. The deflected paths of the rays are photographed against a ruled glass plate.

THOMSON’S CATHODE RAY EXPERIMENT

• Thomson’s work with cathode rays show 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 received the Nobel prize for physics in 1906 for this work.

• Thomson was not actually the first to notice the electron, but he was the first to determine anything about its properties and so he remains credited with its discovery.

Thomson’s Model of the Atom

The atom as tiny, spherical and neutral body, which made up of positive and negative charged particles. These positively charged particles which later termed as proton, distributed in equally in atom and negatively charged particles (electron) embedded in these particles just like plum-pudding.

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

The 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