IDG Notes

(Important Dead Guys)

The History of Atomic Theory

• Atomic theory states that all matter is composed of discrete units called atoms
• The Greek word atomos means “indivisible”
• The ancients were the first to speak an atomic theory, but they only had ideas, no experiments were done. (460 – 370 BC)
• Democritus
• Aristotle

Matter is made of atoms.

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In between these atoms is empty space.

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Therefore, matter is discrete, not continuous.

Democritus

Aristotle believed that all matter consisted of four elements:

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earth, wind, water, and fire.

I hope no one asks me to prove this with data…

Aristotle

John Dalton

• Early 1800’s, Englishman John Dalton forms the following postulates:
• Each element is composed of a single type of indivisible atom, so atoms of the same element are identical.
• Atoms of different elements are different.
• Different atoms can combine in whole-number ratios to form compounds.
• Chemical reactions don’t change atoms, they just change how these atoms are arranged.

Dalton’s Model

Thomson

• 1897 - J. J. Thomson performs an experiment that proves that part of an atom must have an electric charge (negative).
• The cathode ray tube experiment used a positively-charged plate to deflect a cathode ray (a beam of electrons).
• Since the positive plate would have to attract negative particles, Thomson knew that negative particles existed. He called them “corpuscles”.
• To balance things out, he knew there must be positive particles as well.

The Cathode Ray Tube Experiment

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+ Positive field

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- Negative field

A More 3-D Depiction of Thomson’s Experiment

Thomson Model

• So, Thomson proposes that atoms are not the smallest particle possible. Positive and negative subatomic particles do exist.
• His model is called the “plum pudding model”.

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Proper understanding

of electrons

Thomson’s

understanding

Plum Pudding…

yummy…

Rutherford

• 1909 – A student of Thomson’s named Ernest Rutherford proposed a new model, which he based on someone else’s experiment – the famous “gold foil experiment”
• Rutherford claimed that atoms were mostly empty space, and that the positive charge of an atom was located at the center in a dense nucleus.

Gold Foil Experiment

A Simpler View

• Thomson’s plum pudding model would have predicted this (all particles pass through undeflected).

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• The gold foil experiment showed that some positive particles were deflected (obviously by a positive nucleus).

Rutherford Model

• A dense, positive nucleus with negative electrons orbiting in the space around it.
• Often called the “planetary model”

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Niels Bohr

• Niels Bohr (1920’s) worked for Thomson and Rutherford, and proposed that electrons travel in orbits that are discretely separated.
• He came up with the idea of energy levels.
• Electrons had to be in one energy level or the next one, not anywhere between.
• When electrons moved down in energy, they released a photon of light.
• The color of this light was related to the energy of the photon, or it’s wavelength.
• He based his findings on experiments with light.

Bohr’s Model

• Notice:

2-Dimensional

The Modern Understanding

• Some fellows by the names of Schrodinger, Planck, and Heisenberg did a bunch of quantum physics type experiments that expanded on Bohr’s “orbits”.
• The modern understanding changes these 2-D orbits to 3-D “orbitals,” and we know the probabilities that electrons are in the orbitals at a certain time.
• Hard to measure because electrons are so small
• Heisenberg uncertainty principle

Orbitals

Now, we know that protons, neutrons, and electrons are actually composed of even smaller particles. But that’s another course…

Thomson

Rutherford

Bohr

Modern Atom

Dalton

Unit 3: �Atomic Structure and Periodic Table