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The History of Atomic Theory. By Daniel Cooke. Presentation Outline. Introduction Ancient Atomic theory The Modern Atomic Theory Rutherford's Experiment Bohr’s Model Quantum Theory of the Atom. Introduction.

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The History of Atomic Theory

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The History of Atomic Theory

By Daniel Cooke

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Presentation Outline

  • Introduction

  • Ancient Atomic theory

  • The Modern Atomic Theory

  • Rutherford's Experiment

  • Bohr’s Model

  • Quantum Theory of the Atom

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Atomic theory first originated with Greek philosophers around 2500 years ago. This basic theory remained unchanged until the 19th century when it first became possible to test the theory with more sophisticated experiments.

As science has rapidly advanced over the past few centuries the atomic theory has been refined in accordance with the accepted scientific principles and theories of the time.

Advances in technology and theory that have allowed experiments to be designed that enable us to probe matter to microscopic scales.

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Ancient Atomic Theory

  The atomic theory of matter was first proposed by Leucippus, a Greek philosopher who lived at around 400BC. At this time the Greeks were trying to understand the way matter is made.

According to Anassagora, it is possible to subdivide matter in smaller and smaller parts, and he proposed that this process can be continued with no limit. In Anassagora's view, you can always divide a bit of substance into two parts, and each of these parts is also divisible into two parts, and so on--no matter how small each part gets there is no problem dividing it again into even smaller parts.

But according to Leucippus, eventually you arrive at small particles which can not be further subdivided. Leucippus called these indivisible particles atoms (from the Greek word atomos, meaning “indivisible”).


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Ancient Atomic Theory continued..

Leucippus's atomic theory was further developed by his disciple, Democritus who concluded that infinite divisibility of a substance belongs only in the imaginary world of mathematics.

Democritus suggested the atomic theory, explaining that all things are "composed of minute, invisible, indestructible particles of pure matter which move about eternally in infinite empty.". If a sample of a pure element was divided into smaller and smaller parts, eventually a point would be reached at which no further cutting would be possible—this was the atom of that element.


According to the ancient Greeks, atoms were all made of the same basic material, but atoms of different elements had different sizes and shapes. The sizes, shapes, and arrangements of a material’s atoms determined the material’s properties. It was believed that there were four elements that all thing were mare from; Earth, Air, Fire and Water.

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The Modern Atomic Theory

For centuries scientists did not have the methods or technology to test their theories about the basic structure of matter, so people accepted the ancient Greek view.

In the 19th century John Dalton made inferences that exhibited how atoms bond together in definite proportions. Dalton was able to say that atoms of different elements combine in whole number ratios.

This theory, to go along with four other theories, made up what Dalton called the "Modern Atomic Theory.“ Included in these were two theories that stated atoms could not be divided or destroyed, a theory that stated different elements contain different chemical properties, and atoms of the same element contain the same chemical properties

John Dalton

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The Modern Atomic Theorycontinued…

Dalton made two assertions about atoms: (1) atoms of each element are all identical to one another but different from the atoms of all other elements, and (2) atoms of different elements can combine to form more complex substances.

Although the two theories that speculated atoms couldn't be divided were false, Dalton contributed greatly to the advances of atomic theory, and would greatly influence J.J. Thompson in his own discoveries.

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Expanding the Modern Atomic Theory

J.J. Thompson is the person who is credited for discovering the electron. Thompson created a tube that had a positively charged anode on one side and a negatively charged cathode on the other side. Thompson then applied a magnet to the middle of the tube and discovered that negatively charged particles were emanating towards the positive magnetic field. From this, Thompson concluded that negatively charged particles, called electrons, were present in atoms.

J.J. Thompson

Thompson then created the Plum Pudding model, which suggested that electrons and protons were randomly placed throughout the atom.

The “Plum Pudding”

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Rutherford's Experiment

In 1911 British scientist Ernest Rutherford set out to test Thomson’s proposal by firing a beam of charged particles at atoms.

Alpha particles are heavy particles with twice the positive charge of a proton. Alpha particles are now known to be the nuclei of helium atoms, which contain two protons and two neutrons.

Ernest Rutherford's experiment was to emit alpha particles towards a thin gold sheet. Rutherford would then determine where the deflections of the alpha particles would go, and therefore be able to theorize what kind of placement protons and electrons had.

Ernest Rutherford

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Rutherford's Experiment continued…

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Rutherford's Experiment continued…

Rutherford observed that most of the alpha particles went strait through the foil. However a large proportion were deflected through small angles an some (though very few) deflected straight back.

Rutherford then theorized that there was something called a nucleus, which contained a high density of positively charged particles. Rutherford was able to say there was a nucleus because alpha particles that deflected right back must have hit something more massive and with a strong positive charge. This led Rutherford to propose a very different model for the atom.

Instead of supposing that the positive charge and mass were spread throughout the volume of the atom, he theorized that it was concentrated in the center of the atom. Rutherford called this concentrated region of electric charge the nucleus of the atom.

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Bohr’s Model

Danish physicist Niels Bohr used new knowledge about the radiation emitted from atoms to develop a model of the atom significantly different from Rutherford’s model.

Scientists of the 19th century discovered that when an electrical discharge passes through a small quantity of a gas in a glass tube, the atoms in the gas emit light. This radiation occurs only at certain discrete wavelengths, and different elements and compounds emit different wavelengths.

Niels Bohr

Bohr developed a theory by which he could predict the same wavelengths scientists had measured radiating from atoms with a single electron. He concluded that because atoms emit light only at discrete wavelengths, electrons could only orbit at certain designated radii, and light could be emitted only when an electron jumped from one of these designated orbits to another.

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Quantum Theory of the Atom

To make his theory work, Bohr had to propose special rules that violated the rules of classical physics. He concluded that, on the atomic scale, certain preferred states of motion were especially stable. In these states of motion an orbiting electron (contrary to the laws of electromagnetism) would not radiate energy.

The quantum mechanical view of atomic structure is that the nucleus is at the center of the atom and provides the electrical attraction that binds the electrons to the atom. Contrary to Bohr’s theory, however, the electrons do not circulate in definite planet-like orbits. Due to the wavelike character of electrons and provides the framework for viewing the electrons as fuzzy clouds of negative charge.

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Electrons still have assigned states of motion, but these states of motion do not correspond to fixed orbits. Instead, they tell us something about the geometry of the electron cloud—its size and shape and whether it is spherical or bunched in lobes like a figure eight. Physicists called these states of motion orbitals.

The way electrons fill up orbitals determines the number of electrons that end up in the atom’s valence shell. This in turn determines an atom’s chemical and physical properties, such as how it reacts with other atoms and how well it conducts electricity.

It is through today’s understanding of the nature of matter that advancement of technology is able to continue at an increasing rate.

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