1. The Atom Pg. 29

2. Early Theories There was no experimentation Democritus- atoms are solid, homogeneous, indestructible, and indivisible Aristotle- believed everything was made of either fire, water, earth, or air. John Dalton- all matter composed of atoms; all atoms of one element are identical; atoms can’t be created or destroyed (actually used experiments)

3. Evolution of the Atomic Theory The electron- discovered by JJ Thomson using a cathode ray tube; Robert Millikan discovered the negative charge JJ Thomson came up with the plum pudding model. An atom was a mass ( pudding) with electrons scattered throughout (plums) Ernest Rutherford discovered the nucleus using the gold foil experiment; said atom is mostly empty space with dense positive nucleus in the middle

5. Evolution of the atomic theory Rutherford did more experiments and concluded the proton was in the nucleus James Chadwick- discovered a neutral particle in the nucleus called the neutron Atom- smallest particle of an element that still retains that elements properties

6. BEHOLD! An atom is born Atom is spherical with a lot of empty space where electrons (with little mass) exist. In the middle, there is a small dense nucleus (contains most the mass)that contains positively charged protons and neutral neutrons

7. Atomic Number The periodic table can be used to find the number of protons, electrons, and neutrons. The atomic number of an element (labeled Z) tells you how many protons there are. On the periodic table, the elements are listed as neutral elements so the number of electrons=the number of protons.

8. Z

9. Calculating # of neutrons The mass number (labeled A) is the number of neutrons + protons. Mass number=atomic mass rounded to the nearest whole number The atomic mass is located under the element symbol. You can find the number of neutron by subtracting the Atomic number (Z) from the Mass number (A) A-Z = # of neutrons

10. Shorthand notation There is a way to represent atomic mass and number Steps: –1-start with the element symbol –2- place the mass number on the top left side of the symbol –3-place the atomic number on the bottom left side of the symbol

11. Al 27 13