1. Atomic Theory Bellwork

2. Early Atomic Theory Democritus’ Atomic Theory – All matter consists of invisible particles called atoms. – Atoms are indestructible. – Atoms are solid but invisible. – Atoms are homogenous. – Atoms differ in size, shape, mass, position, and arrangement. Solids are made of small, pointy atoms. Liquids are made of large, round atoms. Oils are made of very fine, small atoms that can easily slip past each other.

3. Early Atomic Theory

4. John Dalton (1766-1844) Dalton’s Atomic Theory (1803) – 5 Basic Principles All matter consists of tiny particles called atoms. Atoms are indestructible and unchangeable. Elements are characterized by the weight of their atoms. In chemical reactions, atoms combine in small, whole-number ratios. When elements react, their atoms may combine in more than one whole-number ratio.

5. Early Atomic Theory

6. J.J. Thomson (1897) – Discovered negatively and positively charged particles within an atom – “Plum Pudding Model” The negative electrons represented the raisins in the pudding and the dough contained the positive charge. – Thomson's model of the atom did explain some of the electrical properties of the atom due to the electrons, but failed to recognize the positive charges in the atom as particles.

7. Thomson’s Plum Pudding Model

8. Early Atomic Theory Robert Millikan (1868 – 1909) – Determined the charge of the electron. – Today we call it -1. – He then used this to determine the mass of an electron was 9.1 x 10 -28 or 1/1840 th mass of a hydrogen atom

9. Early Atomic Theory Rutherford (1899) – Gold Foil experiment. – Fired alpha particles (protons) at a sheet of gold foil. Most passed straight through but a few bounced back. They concluded that the atom was mostly empty space with a tiny dense region called the nucleus.

10. Rutherford Gold Foil Experiment

11. Early Atomic Theory Eight years after the gold foil experiment, he concluded that the nucleus contained positively charge particles or +1

12. Rutherford’s Model

13. Early Atomic Theory Niels Bohr “The Bohr Model” (1913) The Bohr model consists of four principles: Electrons assume only certain orbits around the nucleus. These orbits are stable and called "stationary" orbits. Each orbit has an energy associated with it. For example the orbit closest to the nucleus has an energy E1, the next closest E2 and so on. Light is emitted when an electron jumps from a higher orbit to a lower orbit and is absorbed when it jumps from a lower to higher orbit. The energy and frequency of light emitted or absorbed is given by the difference between the two orbit energies

14. The Bohr Model aka “The Planetary Model”

15. Early Atomic Theory James Chadwick (1932) – Showed that the nucleus also contained a neutral particle called a neutron with about the same mass as a proton.

16. History of Atomic Theory Summary Democritus = “small round spheres” Dalton = atoms Thomson = “Plum Pudding Model;” electrons Rutherford = positively charged nucleus Bohr = “Planetary Model”; electrons in orbits Chadwick = Neutrons

17. Modern Atomic Theory The Quantum Model The modern model of the atom describes the positions of electrons in an atom in terms of probabilities.modern model of the atom An electron can potentially be found at any distance from the nucleus, but, depending on its energy level, exists more frequently in certain regions around the nucleus than others

18. Modern Atomic Theory