1. Atomic Structure and the Periodic Table Continued

2. Atomic Mass Mass = the amount of matter in an object What does an atom weigh? What does an atom weigh? Unit of Measurement = Atomic Mass Units or u 1u = 1.67 X10 -24 g or 0.00000000000000000000000167 g This is a REALLY SMALL NUMBER!!!!!

3. Mass Number Mass Number = the sum of the number of protons and neutrons in the nucleus of an atom Protons and Neutrons each have a mass around 1 u Protons and Neutrons each have a mass around 1 u Example: An atom has 6 protons and 6 neutrons: What is the mass number? Example: An atom has 6 protons and 6 neutrons: What is the mass number? 6 protons + 6 neutrons = 12 u Carbon 6 protons + 6 neutrons = 12 u Carbon

4. How Do You Know How Many Neutrons Are In An Atom Subtract the Atomic # (which equals the # of Protons) from the Mass # Important Point: Atomic mass and thr mass # are not the same thing Atomic Mass

5. HUH!!!!!! To get the mass number from the atomic weight, either round the atomic weight up or down (.0 -.49 round down;.5 -.99 round up): Example: Oxygen Atomic Mass = 15.9994 Atomic Mass = 15.9994 Mass Number = 16 Mass Number = 16

6. Back to the original question: For Oxygen the mass number is 16 and the number of protons is 8 16 mass # – 8 Protons = 8 Neutrons

7. Do all atoms of the same element have the same number of neutrons? No, they can be different Atoms of the same element that have different number of neutrons are called: Isotopes!!!

8. Isotopes How are isotopes written: The top number is the mass # and the bottom number is the atomic number To get the # of neutrons just subtract the two numbers

9. Why are isotopes important? Isotopes are used in radiometric dating Used in ecological studies Biochemical Studies Cancer therapies Forensics

10. What does having different # of neutrons mean for the element? This means the different isotopes of the same element have different masses!!!! Average Atomic Mass: The average mass of a mixture of its isotopes The average mass of a mixture of its isotopes This is the number we find on the periodic table of elements This is the number we find on the periodic table of elements