The Development of Modern Atomic Theory

“If I have seen farther than other men, it is because I have stood upon the shoulders of giants”
–Sir Isaac Newton.
John Dalton

John Dalton

John Dalton, the Conservation of Mass Guy

John Dalton–a Quaker schoolteacher from Manchester, England–laid down some of the fundamentental ideas about modern atomic theory in 1808.

1. All matter is made of atoms
2. Atoms of a given element have identical properties; atoms of different elements differ in size, mass and other properties.
3. Atoms cannot be subdivided, created or destroyed.
4. In chemical reactions, atoms are simply combined, separated or rearranged. (Law of Conservation of Mass)
5. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. (Law of Definite Proportions).

The first three items were disproven rather quickly, but the last two explain the law of conservation of mass and the law of definite proportions.

This YouTube video is a good demonstration of Dalton’s ideas:

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Reflection Question:
It’s a common misconception that rusting iron doesn’t change weight. Does rusting iron get heavier or lighter? How would you explain to this to someone who thinks the weight stays the same?

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The Law of Definite Proportions:

Every chemical compound always contains the same elements in exactly the same proportions by mass.

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That means that 1 gram, 1 kilogram or even a Megagram of sodium chloride (NaCl) will always be 39.34% Sodium and 60.66% Chlorine by mass.

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The Law of Multiple Proportions:

When two elements combine to form two or more compounds, such as nitrogen oxide (NO) and nitrogen dioxide (NO2). The mass of elements in each will be in small whole number ratios of each other. (There is twice as much oxygen in NO2 than in NO, duh.)

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Ernest Rutherford

Lord Rutherford of Nelson

Ernest Rutherford, the Nuclear Man

Two years after Dalton’s landmark theory, in 1810, A New Zealander, Ernest Rutherford, discovered a phenomenon that had big implications for how we understand the atom’s nucleus.

Rutherford’s Gold Foil experiment led him to conclude that the mass of an atom is concentrated in a densely-packed bundle of matter that has a positive charge.

The “alpha particles” that Rutherford shot at the gold foil are actually helium nuclei, produced by larger atoms that are under decay. Prior to gold foil “Rutherford Skattering,” Rutherford had already earned a Nobel Prize in 1908 for his work on radioactivity and half-life.

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Reflection Question:

It’s a common misconception that the space between electrons and the nucleus is composed of air. How would you convince someone that this is not true?

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JJ Thompson

Joseph John Thompson

 Joseph John “J.J.” Thompson, Electron Man I: Cathode Rays

The video below allows you to see the phenomenon that led J.J. Thompson, in 1897 to theorize that all cathode rays are composed of identical negatively charged particles, which were named electrons:

(Check out the classic periodic table behind the chemistry teacher. What’s different about it?)

Cathode rays are produced in low-pressure gasses in the cathode ray tube. Thompson experimented with many elemental gases, and because cathode rays always displayed the same properties regardless of the element used to produce them, he concluded that all elements have electrons and that electrons are one of the atom’s main parts.

Charge to Mass Ratio
Thompson also realized that electrons have a very high charge compared to their mass. He measured this quantity, and called it the electron’s charge to mass ratio.

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Robert Millikan

Robert A. Millikan

Robert A. Millikan, Electron Man II: The Oil Drop

12 years later in 1909, American Physicist, Robert Millikan measured the charge of an electron in his famous Oil Drop Experiment. Using Thompson’s charge to mass ratio, Millikan also deduced the electron’s mass, about 1/1800 of a proton’s mass. He was awarded a Nobel Prize, in part because of this research.

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Reflection Question:

If you knew an electron’s charge and its charge-to-mass ratio, do you think you could solve for its mass? Use our method for dimensional analysis to solve for the mass of an electron in kg:

Charge to mass ratio = Q/m = -1.6×10−11 C/kg

Charge = −1.6×10−19 C

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James Chadwick
James Chadwick

James Chadwick, Neutron Man

James Chadwick discovered a new particle in 1932. Because it had no charge, it was called the neutron.


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Review of The Main Sub-Atomic Particles
Proton:
*Positively charged particle.
*Mass slightly less than 1 amu
*Number of protons gives an atom its identity

Neutron: Neutrally charged particle.
Mass slightly more than 1 amu.
Slightly more massive than proton.
Plays a role in the stability of the nucleus.

Nucleus:
Dense center of atom.
Composed of proton and neutron.
Bound together by “Strong Nuclear Force”
1/10000th the volume of entire atom

Electron:
Negatively charged particle.
1/1800 the mass of proton.
Outside the nucleus.
Electrically attracted by positive protons
If # electrons = # protons, atom is electrically neutral
If # electrons do not equal # protons, atom is an ion, meaning “charged atom.”

4 thoughts on “The Development of Modern Atomic Theory

  1. Mr. Crockett

    No sweat, Josh. I’m just going to update them each year. I like your TopChemist avatar. One eye in the middle of the head is a good look.

    Reply
  2. Evan Sell

    It’s really cool how the electrons can produce a ray of light like that. It sort of reminds me of a light saber.

    Reply

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