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The History of Atomic Theory A piece by piece progression through the structure of the atom.

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Presentation on theme: "The History of Atomic Theory A piece by piece progression through the structure of the atom."— Presentation transcript:

1 The History of Atomic Theory A piece by piece progression through the structure of the atom

2 What is Atomic Theory ► Atomic theory states that all matter is composed of discrete units called atoms  The Greek word atomos means “indivisible”  Before this, people believed that matter could be endlessly subdivided ► An idea of Aristotle ► Began as a philosophical idea  Shared by Hindu philosopher Kanada (600 BC) and Greek philosopher Democritus (500 BC)  No experiments, just ideas

3 Any piece of matter can be broken down into smaller pieces. Therefore, matter is continuous, not discrete. I hope no one asks me to prove this with data…

4 Matter is made of atoms In between these atoms is empty space Therefore, matter is discrete, not continuous.

5 The first understanding… ► Early philosophers believed that atoms were all made of the same material but had different shapes and sizes  For example, liquids had smooth atoms while solids were made of rough ones ► Islamic philosophers of the Islamic Golden age (c AD) first propsed that smaller particles than atoms might exist  Still, no experiments

6 The birth of the modern theory ► 1789 – Antoine Lavoisier formulates the law of conservation of mass, stating that in a chemical reaction matter is not destroyed ► Joseph Louis Proust proved the law of definite proportions, whereby small portions of compounds have the same relative composition as large portions  1 gram of CO 2 has twice as many atoms of Oxygen as atoms of Carbon. So does 50 grams, and so does 3400 grams.

7 John Dalton John Dalton ► Early 1800’s, Englishman John Dalton builds off these two laws and proposes that:  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

8 Dalton’s model

9 An atom divided ► 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

10 The Cathode Ray Tube Experiment + Positive field - Negative field

11 A more 3-D depiction of Thomson’s Experiment

12 Thomson Model 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” Proper understanding of electrons Thomson’s understanding Plum Pudding… yummy…

13 Let’s get NUCLEAR! ► 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.

14 Gold Foil Experiment

15 A simpler view ► Thomson’s plum pudding model would have predicted this (all particles pass through undeflected) ► The gold foil experiment showed that some positive particles were deflected (obviously by a positive nucleus)

16 Rutherford Model Rutherford Model ► A dense, positive nucleus with negative electrons orbiting in the space around it. ► Often called the “planetary model” To visualize, the atom is spread out like this: If the whole atom were pirate stadium, the nucleus would be your class ring sitting on the 50-yard line

17 This is Bohring… ► Niels Bohr worked for Thomson and Rutherford, and proposed that electrons travel in orbits that are discretely separated  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 ► Based his findings on experiments with light

18 Bohr’s model Bohr’s model ► Notice: 2-Dimensional

19 The Modern Understanding 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

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

21 ► Do Mean Teachers Mosly read science books ► Dalton, Mendeleev, Thomson, Mosley, Rutherford, Schrodinger, Bohr


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