1. Atomic Models: Developing the Structure of the Atom

2. The Greek Model  Around 440 BC, Leucippus originated the atom concept. He and his pupil, Democritus, refined and extended it in future years.  Five major points to their atomic idea. –All matter is composed of atoms, bits of matter too small to be seen. These atoms CANNOT be broken apart –There is a void (empty space) between atoms –Atoms are completely solid –Atoms of different materials are different in size, shape, and weight  Around 440 BC, Leucippus originated the atom concept. He and his pupil, Democritus, refined and extended it in future years.  Five major points to their atomic idea. –All matter is composed of atoms, bits of matter too small to be seen. These atoms CANNOT be broken apart –There is a void (empty space) between atoms –Atoms are completely solid –Atoms of different materials are different in size, shape, and weight

3. The Dalton Model (1803)  John Dalton of England introduced the atomic idea to “Modern” chemistry  The basic ideas in Dalton's atomic theory: –Chemical elements are made of atoms –The atoms of an element are identical –Atoms of different elements have different masses –Atoms only combine in small, whole number ratios such as 1:1, 1:2, 2:3 and so on.  John Dalton of England introduced the atomic idea to “Modern” chemistry  The basic ideas in Dalton's atomic theory: –Chemical elements are made of atoms –The atoms of an element are identical –Atoms of different elements have different masses –Atoms only combine in small, whole number ratios such as 1:1, 1:2, 2:3 and so on.

4. Dalton’s Atom, 1807 The atom is a solid indivisible sphere that can undergo chemical reactions but is unchanged by the reaction.

5. The Thomson Model,1903  In 1897, J.J. Thomson discovered the first subatomic particle, the electron, using a cathode ray tube.  First model showing an atom that is divisible (has smaller parts).  In 1897, J.J. Thomson discovered the first subatomic particle, the electron, using a cathode ray tube.  First model showing an atom that is divisible (has smaller parts). Plum Pudding Model

6. The nucleus was discovered. The Gold Foil Experiment

7. The Rutherford Model 1. The atom contains a tiny dense center: “nucleus” –the nucleus is about 1/10 trillionth the volume of the atom 2. The nucleus contains almost the entire mass of the atom. 3. The nucleus is positively charged. 4. The electrons move around in the empty space (the void?) surrounding the nucleus 1. The atom contains a tiny dense center: “nucleus” –the nucleus is about 1/10 trillionth the volume of the atom 2. The nucleus contains almost the entire mass of the atom. 3. The nucleus is positively charged. 4. The electrons move around in the empty space (the void?) surrounding the nucleus

8. Rutherford’s Atom 1908 3p+ e-

9. The Bohr Model (1913)  Niels Bohr applies mathematics to Rutherford's model. This eventually led to the idea of energy levels for the electrons.  Led scientists to the understanding that valence electrons, those in the highest energy level, determine chemical reactivity.  Niels Bohr applies mathematics to Rutherford's model. This eventually led to the idea of energy levels for the electrons.  Led scientists to the understanding that valence electrons, those in the highest energy level, determine chemical reactivity.

10. Chadwick (1935)  Hypothesized that the mass of the nucleus could not be explained by the masses of protons alone.  Be foil experiment discovered neutrons, particles that had mass but no charge.  Hypothesized that the mass of the nucleus could not be explained by the masses of protons alone.  Be foil experiment discovered neutrons, particles that had mass but no charge.

11. Subatomic Particles 3 protons 4 neutrons 3 electrons Lithium

12. Chemical versus Nuclear Reactions ChemicalNuclear Stable atoms (Not radioactive) Electrons exchanged between atoms Atoms are conserved (unchanged but rearranged) Reactions are affected by temperature and pressure Radioactive nuclei = unstable, which decays Nucleus ejects/absorbs neutrons, protons, electrons. Atoms change into new elements/isotopes! Atomic # or mass # changes Reactions NOT affected by temperature or pressure.