1. BELLWORK 8-27-2014 1.What is a good definition for an isotope? In other words, what are isotopes? If you write I don’t know, you will not get a stamp, look at your worksheet 2.Write the isotope symbol for Potassium–40 (hint: you will need to look at a periodic table)

2. **Go over isotope worksheet **Any questions on other worksheets?

3. HOMEWORK ANSWERS Nitrogen Magnesium Hydrogen

4. AVERAGE ATOMIC MASS To calculate, you need 2 things: 1.Relative abundance of each isotope – the percentage of atoms that exist as each isotope 2.Average mass of each isotope (usually in amu – atomic mass units) -Mass of one proton or neutron = 1 amu

5. AVERAGE ATOMIC MASS Calculate: ∑(Relative abundance x average mass of isotope) = average atomic mass ∑ = sum Do the calculation above for each isotope and add them all together to get average atomic mass.

6. PRACTICE At Robot HS, all girl robots weigh 150 pounds. All boy robots weigh 195 pounds. In robot chemistry class 40% are girl robots and 60% are boy robots. What is the average weight of a robot student in chemistry class?

7. HONORS: WORK ON PRACTICE PROBLEMS Practice problems due at the end of the class Get a homework problem sheet

8. REVIEW FOR QUIZ Get out your slates!

9. What is the equation for density? D = m/v

10. What is the density of a substance that weights 15.6g and has a volume of 47.9mL? D = m/v D = 15.6g = 0.33 g/mL 47.9mL

11. What is the volume of a substance that has a density of 3.45g/mL and has a mass of 52g? D = m/v 3.45g/mL = 52g volume V = 15.07mL

12. Draw the correct Bohr model for Silicon (Si)

13. How many neutrons does Silver (Ag) have? 61

14. How many electrons does Iodine (I) have? 53

15. Atomic number is also equal to ___________. Number of protons

16. How many neutrons are in the isotope of Lithium below? 4

17. What is the mass number of the Bromine isotope below? 81

18. Draw an isotope symbol for Argon-41 41 18 Ar

19. Do all isotopes have the same atomic number (number of proton)? Why or why not? Yes, because if you change atomic number you change the element too.

20. Do all isotopes of an element have the same mass number? Why or why not? No, because when you change the number of neutrons, it changes the mass

21. What is a good definition for an isotope? Elements with different number of neutrons but the same number of protons (atomic number)

22. How would you make a positive ion? Take away electrons (less negative charge)

23. How would you make a negative ion? Add electrons, (more negative charge)

24. Which scientist came up with the model of the atom we are currently using? Bohr

25. Which scientist used cathode ray tubes to discover the electron? Thomson

26. Which scientist discovered the proton by shooting positively charged alpha particles at gold foil? Rutherford

27. How did Chadwick discover the neutron? Observing weight of atoms (Helium had 4x more mass than Hydrogen and thus neutrons must be present)