1. Atomic Theory v1.1 Created June 2007 Modified February 2008

2. Early Ideas Democritus, (shown on the right), was one of several philosophers that proposed that matter was made of small indivisible particles, which he called atomos.

3. John Dalton (shown on the right), formalized this several thousand years later.

4. Dalton’s Atomic Theory 1.All matter is made of atoms, which are particles too small to see. 2.Each element has its own kind of atom. Atoms of the same element have the same size and mass. Atoms of different elements have different sizes and masses.

5. 3.Compounds are created when atoms of different elements join to form compound atoms. In a compound, all the compound atoms are alike. (We now call Dalton’s compound atoms molecules.) 4.Atoms cannot be created or destroyed.

6. Inside the Atom Atoms are the smallest particle of an element, but are not the smallest particles. Atoms are made of subatomic particles: –protons –neutrons –electrons

7. The next problem was where these subatomic particles existed. J.J. Thomson proposed the “plum pudding” model.

8. The “plum pudding” model has the atom as a positively charged sphere, with the electrons evenly distributed on the surface. show the shockwave “rutherford.swf”

9. Rutherford’s Model Ernest Rutherford’s gold foil experiment led him to propose a nuclear atom, where there was a heavy nucleus in the center, with the electrons occupying the outer areas of the atom.

10. Protons, Neutrons and Electrons ParticleLocationMassCharge Proton (p)Nucleus1 u+1 Neutron (n)Nucleus1 uno charge Electron (e) Outer Areas –1 1836 1 u Note: 1 u = 1 atomic mass unit (approximately the mass of a proton or neutron)

11. Numbers of Particles in Atoms Protons Electrons Atomic Mass Mass Number Neutrons = Atomic Number (in a neutral atom) = Average mass of an element = Protons + Neutrons = Mass Number – Atomic Number *If you’re not told anything else, you could make a reasonable guess at a mass number by taking the atomic mass and rounding it to the nearest whole number. *

12. For lithium, determine the following: atomic number___ # of protons___ atomic mass___# of electrons___ mass number___# of neutrons___ 6.9 3 7 3 3 4