What is Atomic Mass?

1. Launch: 9/16
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.
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

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?
2
He
Helium
Atomic
number
Symbol
Name
Atomic
mass 4.00

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:
2.What is atomic mass?
3.How is atomic mass different from atomic number?
4.Draw and label the representation of C (Carbon)
including atomic mass.
5.How many neutrons do the atoms of the following
elements have?
sN (Nitrogen)
sMg (Magnesium)
sAu (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:
2.What is an isotope?
3.Why is Co placed before Ni on the periodic table?
4.Why is atomic mass a decimal and not a whole #?
5.What is the trend for atomic mass?
6.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