Ancient Greek Models of Atoms

Transcription

1 Atomic Theory

2 Ancient Greek Models of Atoms The philosopher Democritus believed that all matter consisted of extremely small particles that could not be divided. He called these particles atoms from the Greek work atomos, which means uncut or indivisible.

3 Aristotle believed that all substances were built up from only four elements earth, air, fire, and water. These elements were a combination of four qualities cold, hot, dry, and wet. Fire was a combination of hot and dry, while water was a combination of cold and wet.

4 Dalton s Atomic Theory John Dalton was a teacher in England who spent time doing scientific experiments. Dalton gathered evidence for the existence of atoms by measuring the masses of elements that combine when compounds form.

5 He noticed that all compounds have something in common. No matter how large or how small the sample, the ratio of the masses of the elements in a compound is always the same. In other words, compounds have a fixed composition.

6 Dalton proposed the theory that all matter is made up of individual particles called atoms, which cannot be divided.

7 The main points of Dalton s theory are: 1) All elements are composed of atoms. 2) All atoms of the same element have the same mass, and atoms of different elements have different masses. 3) Compounds contain atoms of more than one element. 4) In a particular compound, atoms of different elements always combine in the same way.

8 Thompson s Model of the Atom JJ Thompson, used an electric current to learn about atoms. He used the fact that like electric charges repel and opposite electric charges attract.

9 Thompson used a set up that included a sealed tube filled with gas. He put two metal discs on each side of the tube with a source of electric current.

10 When the current is turned on, one metal disk becomes negatively charged and the other disk becomes positively charged. A glowing beam appears in the space between the disks. The beam was attracted by the positively charged plate and repelled by the negatively charged plate.

11 Thompson concluded that the particles in the glowing beam had a negative charge because of this. Thompson s experiments provided the first evidence of the existence of subatomic particles.

12 Thompson s atomic model Thompson thought that negative charges were evenly scattered throughout an atom filled with a positively charged mass of matter. (Scientists have dubbed this model the plum pudding model )

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14 Rutherford s Atomic Theory In 1899, Ernest Rutherford discovered that uranium emits fast moving particles that have a positive charge. He called these particles alpha particles. In 1909, Rutherford and one of his students, Ernest Marsden, did a gold foil experiment.

15 Rutherford hypothesized that most particles would travel in a straight path from their source to a screen that lit up when struck. Those few that did not pass straight through would be deflected only slightly.

16 What actually happened was that more particles were deflected than was predicted. About one out of every 20,000 was deflected by more than 90 degrees. Some of the alpha particles behaved as though they had struck an object and bounced straight back.

17 Rutherford concluded that the positive charge of an atom is not evenly spread throughout the atom. It is concentrated in a very small, central area that Rutherford called the nucleus. The nucleus is a dense, positively charged mass located in the center of the atom.

18 Rutherford was the first scientist to suggestthatprotonsandneutronswere found in the nucleus and that electrons orbited around the nucleus.

19 Bohr s Model Niels Bohr was a danish chemist who also studied the atom. Bohr s model was similar to Rutherford s but he suggested that electrons were found in energy levels.

20 According to Bohr s model, electrons can be only certain distances from the nucleus. Each distance corresponds to a certain quantity of energy that an electron can have.

21 An electron that is as close to the nucleus as it can be is in its lowest energy level. The farther an electron is from the nucleus, the higher the energy level that the electron occupies.

22 The difference in energy between two energy levels is known as a quantum of energy.

23 Bohr s model states that an electron can be in only one energy level or another, not between energy levels. Bohr also thought that an electron did not give off energy while in a given energy level.

24 Schroedinger In the 1920 s, Edwin Shroedinger discovered that electrons moved around in orbitals in an electron cloud. This was the beginning of the modern theory of the atom.