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# What is Atomic Mass?

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• 1. Launch: 9/16
• 4. How does atomic number vary throughout the periodic table?
• a. it is fixed and does not change as you move across the periodic table
• b. it decreases from left to right, and from top to bottom
• c. it increases from left to right, and from top to bottom
• d. it increases from right to left, and from top to bottom
• 1. All atoms of an element have
• a. the same number of protons.
• b. the same number of neutrons.
• c. the same number of electrons.
• d. neither mass nor volume.
• 2. What is atomic mass? Mr. Heffner Chemistry 9/15/09
• 3. What is an element?
• All atoms in a specific element have the same atomic number
• Atomic number = # of protons
• The periodic table is organized by increasing atomic number
• 4. What is atomic mass?
• Atomic mass is…
• the number of protons and neutrons in an atom
• the number of particles in the nucleus
• Mass = weight
• So, atomic mass is the weight of an atom
• Units = amu (atomic mass units)
• 5. What is an element? Atomic number Symbol Name Atomic mass
• 4.00
• 2
• He
• Helium
• 6. What is atomic mass?
• Atomic mass vs. atomic number
Mass Number
• Protons + neutrons
• Just protons
• Weight of atom
• What defines atom
• On bottom
• On top
• Decimal
• Whole-number
• 7. What is atomic mass?
• How to calculate the number of neutrons
• Atomic mass – atomic number = # of neutrons
• How many neutrons does He (Helium) have?
• 4 – 2 = 2 neutrons
• How many neutrons does Li (Lithium) have?
• 7 – 3 = 4 neutrons
• 8. Summary
• Atomic mass is the number of protons and neutrons in an atom
• Atomic mass – atomic number = # of neutrons
• 9. Foldable
• Fold in half 3X, then unwrap and fold in windows
• Cut with scissors
• 3 hole-punch
• Terms: only 2!
• Atomic mass
• Neutron (charge, mass, location)
• Quiz yourself when finished
• 10. Practice Questions
• Answer the following in complete sentences:
• What is atomic mass?
• How is atomic mass different from atomic number?
• Draw and label the representation of C (Carbon) including atomic mass.
• How many neutrons do the atoms of the following elements have?
• N (Nitrogen)
• Mg (Magnesium)
• Au (Gold)
• 11. What is atomic mass?
• Trend for atomic number
• INCREASES from Left to Right
• INCREASES from Top to Bottom
• An exception to the rule!
• Co (Cobalt) and Ni (Nickel)
• The periodic table is organized by atomic number, so Co must come before Ni
• 12. What is an isotope?
• What is going on here?
• Why are there exceptions?
• Why are atomic masses not whole numbers?
• An isotope is…
• an atom that has the same number of protons as other atoms of the same element, but has a different number of neutrons
• the same atomic number, but a different atomic mass
• 13. What is an isotope?
• Most elements have a few stable isotopes
• Carbon-12, atomic mass = 12.00
• Carbon-13, atomic mass = 13.00
• Carbon-14, atomic mass = 14.00
• Why is atomic mass a decimal?
• Because it is an average of the masses of the naturally occurring isotopes
• 14. What is an isotope?
• How to calculate the number of neutrons in isotope
• Atomic mass – atomic number = # of neutrons
• How many neutrons does Carbon-13 have?
• 13 – 6 = 7 neutrons
• How many neutrons does Carbon-14 have?
• 14 – 6 = 8 neutrons
• 15. Summary
• Atomic mass INCREASES from
• Left  Right
• Top  Bottom
• Isotopes have a different number of neutrons
• Atomic mass = average mass = decimal
• Atomic mass – atomic number = # neutrons
• 16. Foldable
• Fold in half 3X, then unwrap and fold in windows
• Cut with scissors
• 3 hole-punch
• Terms: only 2!
• Isotope
• Trend for atomic number and atomic mass
• Quiz yourself when finished
• 17. Practice Questions
• Answer the following in complete sentences:
• What is an isotope?
• Why is Co placed before Ni on the periodic table?
• Why is atomic mass a decimal and not a whole #?
• What is the trend for atomic mass?
• How many protons and neutrons are in:
• K
• P
• Hydrogen-2
• Oxygen-18
• 18. Exit Slip
• 1. How many neutrons does Carbon-12 have?
• a. 2
• b. 6
• c. 12
• d. 18
• 2. In general, how does atomic mass vary throughout the periodic table?
• a. it is fixed and does not change as you move across the periodic table
• b. it decreases from left to right, and from top to bottom
• c. it increases from left to right, and from top to bottom
• d. it increases from right to left, and from top to bottom
• 19. Exit Slip
• 3. Which of the following is true for isotopes?
• a. they change the identity of an atom to a different element
• b. they have varying atomic numbers
• c. they have varying atomic masses
• d. they have varying numbers of electrons
• 4. Why are atomic masses listed as decimals on the periodic table instead of whole numbers?
• a. they are a weighted average of the naturally occurring isotopes
• b. their units are grams/moles
• c. they were measured with a balance that read to only two decimal places
• d. none of the above
• 20. Exit Slip
• 5. On the periodic table, why is Co (Cobalt) placed before Ni (Nickel) even though Ni has a higher atomic mass?
• a. Nickel has one more proton
• b. Cobalt has fewer electrons
• c. Nickel has a few more electrons
• d. Cobalt has a lower density
• 21. Homework
• Finish Practice Questions
• Finish Foldable
• Textbook:
• Pg. 86 Practice: #1, 2, 4
• Pg. 89 Section Review: #2, 4, 6, 8