1. T HE A TOM

2. T HE A TOM, ACCORDING TO J OHN D ALTON All elements are composed of atoms All atoms of the same element have the same mass Atoms of different elements have different masses Compounds contain atoms of more than one element

3. J.J. T HOMSON ’ S CONTRIBUTIONS TO THE A TOM Atoms are made of smaller particles An atom is neutral An atom has postive and negative charges

4. R UTHERFORD ’ S A TOMIC T HEORY All of an atom’s positive charge is concentrated in its nucleus Nucleus – a dense, positively charged mass located in the center of the atom

5. S UBATOMIC P ARTICLES Protons – a positively charged subatomic particles Found in the nucleus Electrons – a negatively charged particle Found in the space outside the nucleus Neutrons – Neutral subatomic particle Found in the nucleus

6. S UMMARY OF S UBATOMIC P ARTICLES ParticleSymbolRelative Charge Relative Mass Actual Mass (g) Protonp+p+ 1+11.674x10 -24 Electrone-e- 1-1/18369.11x10 -28 Neutronn011.675x10 -24

7. A TOMIC N UMBER Atomic number – equal to the number of protons in an atom Atoms of different elements have different numbers of protons Mass number – the sum of the protons and neutrons of that atom Example: Aluminum has 13 protons and 14 neutrons, what is its mass number? 27 Carbon has a mass number of 12. It has 6 protons. How many neutrons? How do you know? 6. Mass number – Protons = Neutrons

8. P RACTICE What is the mass number of the following elements: 1. Calcium (20 protons, 20 neutrons) 2. Silver (47 protons, 61 neutrons) 3. Mercury (80 protons, 121 neutrons) 4. Iodine (53 protons, 74 neutrons) 5. Cesium (55 protons, 77 neutrons) 6. Magnesium (12 protons, 12 neutrons) 7. Iron (26 protons, 30 neutrons) 8. Sulfur (16 protons, 16 neutrons) 9. Xenon (54 protons, 77 neutrons) 10. Krypton (36 protons, 48 neutrons)

9. P RACTICE How many neutrons are in the following elements? 1. Tin (33 protons, 75 mass number) 2. Platinum (78 protons, 195 mass number) 3. Tungsten (74 protons, 184 mass number) 4. Polonium (84 protons, 209 mass number) 5. Molybdenum (42 protons, 96 mass number) 6. Yttrium (39 protons, 89 mass number) 7. Nitrogen (7 protons, 14 mass number) 8. Rubidium (37 protons, 85 mass number) 9. Argon (18 protons, 40 mass number) 10. Cobalt (27 protons, 59 mass number)