1. Greeks Before 400 BC Observers (not scientists) did not use scientific experiments Matter could be endlessly divided Matter is made of fire, water, air, and earth

2. Democritus Around 400 B.C. An observer who thought that atoms were: the smallest particles of matter that could not be further divided That different types of atoms existed for every type of matter. Atomos: “uncuttable”

3. Dalton Dalton used the ideas and models of other scientists to come up with the Atomic Theory (~1803) All matter is made up of atoms. Atoms are indestructible and cannot be divided into smaller particles. (Not currently true) All atoms of one element are exactly alike, but they are different from atoms of other elements. (Not currently true)

4. Thomson Observed negative particles from the atom using a cathode ray tube. Atoms were not just neutral spheres, but are composed of electrically charged particles. Atoms were composed of a positive sphere (the pudding) and had negatively charged particles called electrons (plums) embedded into the pudding. The positive sphere and negative electrons together make the atom neutral. The Cathode Ray Tube Experiment shows negative charges (electrons) moving towards a positive charge

5. Rutherford 1909 Ernest Rutherford experiments with alpha particles from Radioactive polonium and Gold foil. He shows that much of an atoms volume is actually empty space Alpha particles are positively charged particles of matter. Most of the alpha particles passed right through the gold foil but some were deflected to the side and back. Because most of the particles passed right through the foil Rutherford and his colleagues concluded that the atom is nearly all empty space. Rutherford’s Model: An atom is made of a small, positively -charged, dense nucleus and negative “empty space” called the electron cloud.

6. Bohr Bohr observed how electron interacted with energy. His experiments looked at the emission spectrum of atoms, particularly hydrogen. Bohr observed that electrons absorbed energy, and then released the energy in the form of light. Bohr’s Model: Electrons move around the atom within energy levels and only a certain amount of electrons can exist on one energy level at a time. Bohr’s model lead towards the current Quantum Mechanical Model of the Atom

7. Current Models Focus on the Nucleus: The Discovery of the Neutron was the last of the classic sub atomic Particles. Neutrons are neutral in charge. Therefore experiments looking for charged particles would not have detected them. This is one type of current model showing the 3 particles: Protons, Neutrons, and Electrons. It also shows their Locations. Protons have a positive Charge Neutrons have a Neutral Charge Electrons have a Negative Charge Nucleus Energy Levels

8. Current Models: The Quantum Model Focus on the Electrons The Current Model of the atom accounts for the movement of the electrons. Rather than moving in circular orbits, electrons make clouds called orbitals that exist with sublevels of the energy levels. Clouds are modeled by their shape Clouds are known by a code: 2s, 3p, 6d,2p, 4s… A picture cannot suffice for a moving object. Up to 2 electrons can occupy an orbital (cloud). Electrons also have their own movement within the cloud they produce. This is called spin. However they must spin the opposite direction of one another within the same cloud. Spin is noted by the up and down arrows. 1s 2 2s 2 2p 2

9. Overview As you can see models have been important towards our understanding of the atom. Each model has had thought, and hopefully, scientific evidence to support it. As we develop our understanding of the atom we will be able to visualize something that we cannot see. Remember, a model is not just a picture. It can be mathematical, symbolic, anecdotal (story like). All of these will be used to understand the atom