2. DEMOCRITUS ~ 460 BC 2000 years ago Democritus and Leucippus suggested all matter was made up of tiny invisible, indestructible particles. These particles were called atomos. There was NO experimental data to support this idea, just careful thinking and reasoning.

3. Antoine Lavosier ~ 1770s Gave the 1 st experimental evidence for the law of conservation of matter Law of conservation of matter: matter can neither be created nor destroyed.

4. Joseph Proust ~ 1779 A French chemist who worked on the problem of determining the composition of compounds. Law of definite proportions- the proportion by mass of the element in a given compound is always the same

5. Dalton ~ 1800s Law of Multiple Proportions: In all cases the masses of 1 element combine that combine with a fixed mass of another element will form simple, whole number ratios.

6. Dalton’s Atomic Theory (1800s) 1)All elements are composed of atoms which can’t be created or destroyed 2)Atoms of the same element are identical 3)Atoms of different elements are different…. different masses 4)Compounds are formed by joining 2 or more elements. In any compound the atoms of the elements in the compound are joined in definite whole number ratio

7. Dalton’s Atomic Model (1800s)

8. J.J. Thompson ~ 1870s 1 st discovered electrons using a Cathode Ray tube Electrons are 1/1,840 times lighter than + protons. Protons are single positively charged particles with a mass of 1.67x 10^ (-24)

9. Ernest Rutherford~ 1909 Showed atoms are not solid spheres Atoms consist of small, massive positively charged nuclei, surrounded by empty space

10. Ernest Rutherford~ 1909 Electrons move around the empty space Negative charge of the electrons offsets the positive charge of the nucleus, accounting for the electrically neutral atom.

11. James Chadwick ~ 1932 Discovered the neutron A neutron has approximately the same mass as a proton but “0” (no) charge.

12. Niels Bohr ~ 1913  Stated there are definite orbits in which electrons can travel 1.Each orbit is circular and a fixed distance from the nucleus 2.An electron in a given orbit has a certain definite amount of energy 3.The greater the radius of the shell, or distance of the electron from the nucleus the greater the energy of the electrons in that shell 4.Orbits or shells are also known as energy levels

13. Niels Bohr ~ 1913  Under normal conditions electrons in an atom occupy the lowest energy levels are available to them. Ground State: When all the lowest energy levels are occupied in an atom Excited State: When an atom absorbs energy from an outside source, the energy may cause an electron or electrons to move to a higher energy level.

14. Niels Bohr ~ 1913 Electrons do NOT remain at these higher levels for very long. The electrons return (or fall back) to the original state. When they do the electrons emit (give off) a quantum of energy (the same amount of energy needed to raise the electron to the higher state) in the form of light