1. Models of the Atom Particle model of matter: All matter is composed of tiny particles called atoms. The combination of the types of particles in a substance determines what kind of matter it is.

2. Models of the Atom Through Time: 500 BC: Democritus Democritus inferred that all matter is composed of small, indestructible particles he named atomos. He taught that there was an infinite number of them, and that atoms differed in their size, shape, and weight. He also thought that different substances were made up of different kinds of atoms.

3. 400 BC: Aristotle Aristotle didn’t believe in atoms. He thought all matter was composed of a combination of four elements- air, fire, water and earth. His model was, of course, accepted for the next 2000 years.

4. 1803: John Dalton After experimenting with gases, especially hydrogen, methane and oxygen, Dalton rediscovered the particle theory of matter.

5. Dalton was able to infer 4 things about matter and atoms: 1. Atoms of one element have masses that are different from atoms of other elements 2. Atoms of the same element are identical 3. An atom of one element cannot be changed into an atom of another element 4. Atoms of different elements can be combined and /or rearranged to form new substances.

6. Like Democritus, Dalton thought an atom was a tiny, irreducible sphere that could not be destroyed.

7. 1890: JJ Thomson Excited by the discovery of X-rays a couple of years earlier, Thomson played around with radioactive waves and discovered that atoms have electrical charges. Not only that, he discovered that those charges come from small particles which make up part of the atom. He called the particles electrons.

8. Thomson thought that atoms consisted of positively-charged main bodies, and attached to these were the negatively-charged electrons (plum pudding model). He was wrong about the details, but his discovery that atoms can be broken down into even smaller parts earned him science immortality. + --- + + +

9. 1909: Earnest Rutherford: A student of Thompson, Rutherford designed a very famous experiment that proved that the atom is mostly empty space: he bombarded a very thin piece of gold leaf with alpha particles. Although some alpha particles were reflected, most passed through, proving that a dense nucleus was at the center of the atom, but not much else.

10. Rutherford’s model of the Atom

11. 1912: Niels Bohr Bohr improved Rutherford’s model by noticing that energy levels in atoms went up and down by specific, “pre-set” amounts. He suggested that electrons move around the nucleus of an atom like planets around the sun, and that they move from orbit to orbit as they gain and lose energy.

12. This model of the atom is still the most famous of all models, and though it is not the most accurate, it predicts chemical reactions very well. (It is important to remember, though, that atoms probably do not really look like this.)

13. 1920-present: Particle-Wave models Since Bohr, most scientists have tried to describe atoms using mathematical models instead of physical ones. Generally, in these modern models, they keep the nucleus where Rutherford and Bohr left it, and try to describe the location and movement of electrons, which can best be described as a “cloud” which surrounds the nucleus. Even more recent theories suggest that matter, at its most basic level, isn’t even made of particles at all, but consists of really tiny fields of vibrating energy.

14. Wave Model of Atoms:

15. According to modern physicists, the location and/or motion of electrons cannot be known for sure. The best we can do is to talk about the probability of some event occurring or some particle existing at a given location.

16. Structure of a Bohr Atom: Electron- negative charge (-) Neutron-no charge Proton- positive charge (+)