1. 4.2

2. = # electrons in a neutral atom
ATOMIC NUMBER
Elements are different because they contain different numbers of protons.
Atomic number
Chemical symbol
Chemical name
Average atomic mass
Atomic number
= # protons
= # electrons in a neutral atom

3. 82 82 O 8 8 Zn 30 30

4. 1 neutron = 1 amu Atomic Mass Units amu: atomic mass units
× grams
amu: atomic mass units
Unit used to measure the mass of VERY SMALL particles (atoms)
Standard is the carbon-12 atom
1 carbon-12 atom = 12 amu
1 amu = 1/12 carbon-12 atom mass
1 proton = 1 amu
1 neutron = 1 amu

5. Mass Number – Mass of a specific atom
Mass number = # protons + # neutrons

6. The total number of protons
REVIEW:
Atomic Number:
The total number of protons
Atomic number = # protons = # electrons
Mass Number:
The total number of protons and neutrons
Mass number = # protons + # neutrons

7. Isotopes
Each element may have different numbers of neutrons => ISOTOPES
The mass number is different for different isotopes.

8. Isotopes – Naming (Two ways)
Element name – mass number
Ex. carbon-12 or neon-22
Using the element symbol:

9. Isotopes - Naming
How would you write Uranium-235 using the symbol? (atomic number = 92)

10. Atomic Mass The weighted average mass of the isotopes of that element.
*different than mass number

11. Calculating Average Atomic Mass
All isotopes of an element do not exist in equal abundance in the world
Percent abundance in world:
90.48% % %

12. Average atomic mass (amu)
Example #1
Calculate the average atomic mass of Neon
Atomic Mass = (% abundance A)(mass A) +
(% abundance B)(mass B) + ….
Isotope
Mass (amu) (M)
Percent abundance (P)
M x P÷100
Neon-20
20.0
90.48%
Neon-21
21.0
0.27%
Neon-22
22.0
9.25%
Average atomic mass (amu)
18.1
0.057
2.03
20.2

13. Atomic Mass Neon-20 Which isotope is most abundant?
If Neon has 3 isotopes:
Neon-20, Neon-21, Neon-22,
Weighted average atomic mass is 20.2 amu.

14. Example #1
Calculate the average atomic mass of Neon
Atomic Mass = (% abundance A)(mass A) +
(% abundance B)(mass B) + ….
Another way to write it out: Atomic Mass = (90.48/100)(20.0) + (0.27/100)(21.0) + (9.25/100)(22.0) = 20.2 amu

15. Average atomic mass (amu)
Example #1
Calculate the average atomic mass of Lithium
Atomic Mass = (% abundance A)(mass A) + (% abundance B)(mass B) + ….
Isotope
Mass (amu) (M)
Percent abundance (P)
M x P÷100
Lithium-6
6.01
7.5%
Lithium-7
7.01
92.5%
Average atomic mass (amu)
0.45
6.49
6.94 OR
Atomic Mass = (7.5/100)(6.01) + (92.5/100)(7.01) = 6.94 amu

16. Average atomic mass (amu)
Example #3
Calculate the average atomic mass of Cesium
Atomic Mass = (% abundance A)(mass A) + (% abundance B)(mass B) + ….
Isotope
Mass (amu) (M)
Percent abundance (P)
M x P÷100
Cesium-132
132.0
75.0%
Cesium-133
133.0
20.0%
Cesium-134
134.0
5.0%
Average atomic mass (amu)
99.0
26.4
6.7
132.1
OR Atomic Mass = (75.0/100)(132.0)
+ (20.0/100)(133.0) + (5.0/100)(134.0) = amu

17. Questions 1) What particles make up an atom?
Protons, Neutrons and Electrons
2) What are the charges on these particles?
Protons (+), Neutrons (none), Electrons (-)
3) What particles make up the nucleus?
Protons and Neutrons

18. More Questions: Atomic Number tells us: Atomic Mass tells us:
The identity of the element; the number of protons and electrons
Atomic Mass tells us:
The average mass in amu of all naturally occuring isotopes
Mass

19. The Development of Atomic Models
The timeline shoes the development of atomic models from 1803 to 1911.
These illustrations show how the atomic model has changed as scientists learned more about the atom’s structure.

20. The Development of Atomic Models
5.1
The Development of Atomic Models
The timeline shows the development of atomic models from 1913 to 1932.
These illustrations show how the atomic model has changed as scientists learned more about the atom’s structure.