Alchemy: The Quest for Gold and Knowledge (2:48)
1. In ancient times, people tried to convert elements and compounds into gold.
2.
In Ancient Greece in the 15th century BC, Empedocles thought all things consisted of earth, fire,
air, and water.
3. Leucippus (and Democritus) thought people and things were composed of atoms, which means
indivisible.
4. He thought atoms were all made up of the same material but have different sizes.
John Dalton: Elements, Compounds, and Atomic Theory (2:47)
1.
In 1808, John Dalton proposed his atomic theory, which begins by stating that elements are
composed of atoms which preserve their individuality in all chemical changes.
2.
All atoms of the same element are the same in all respects. Differing elements have atoms
differing in weight.
3. Chemical combinations of atoms occur when they join together in simple, numerical ratios. This
means that there cannot be 2 atoms of one element combining with 0.5 atoms of another
element, etc.
The History of Atomic Theory (3:27)
1. In 1898, J.J. Thompson discovered that the atom contained a smaller, negatively charged
particle called an electron.
2. His model of the atom is called the Plum Pudding Model.
3. Ernest Rutherford shot helium nuclei at a sheet of gold foil. Most particles passed right
through, but few hit something and were deflected.
4. Rutherford then determined that the positive particles were centered in a small core of the
atom called the nucleus.
5. In 1913, Niels Bohr suggested that electrons were in orbits, much like the solar system.
6. The current model shows electrons arranged in shells that surround the atom in different
energy levels.
7. Atoms are positioned on the periodic table according to the arrangement of their electrons.
Rutherford’s Model of the Atom (3:08)
1. Rutherford calculated that the nucleus was 10,000 times smaller than the atom itself.
2. Because most of the alpha particles passed through the gold foil, Rutherford concluded that the
atom was mostly empty space.
Bohr’s Model
1. Bohr attempted to look for clues about the atom’s structure by examining the nature of light.
2. Substances glow with distinct colors; these colors associated with different substances are called
spectra. Bohr realized that this tells us something about the inner structure of the atom.
3. Bohr described the atom as a multi-story building.
4. When an electron jumps from a higher floor to a lower floor, it gives off light.