4.2
ATOMIC NUMBER
Elements are different because they contain different
numbers of protons.
Atomic number
= # protons
= # electrons
in a neutral
atom
Atomic number
Chemical symbol
Chemical name
Average atomic mass
82
O
8
Zn
30
82
8
30
Atomic Mass Units
amu: atomic mass units
1 atomic mass unit =
1.66053892 × 10-24 grams
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
Mass Number
Mass Number – Mass of a specific atom
Mass number = # protons + # neutrons
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
Isotopes
• Each element may have different
numbers of neutrons => ISOTOPES
• The mass number is different for
different isotopes.
Isotopes – Naming (Two ways)
• Element name – mass number
• Ex. carbon-12 or neon-22
• Using the element symbol:
Isotopes - Naming
• How would you write Uranium-235 using
the symbol? (atomic number = 92)
Atomic Mass
The weighted average mass of the isotopes of
that element.
*different than mass number
Calculating Average Atomic Mass
All isotopes of an element do not exist in equal
abundance in the world
Percent abundance in world:
90.48%
0.27%
9.25%
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)
Neon-20
20.0
90.48%
Neon-21
21.0
0.27%
0.057
Neon-22
22.0
9.25%
2.03
Average atomic mass (amu)
M x P÷100
18.1
20.2
Atomic Mass
Which isotope is most abundant?
Neon-20
If Neon has 3 isotopes:
Neon-20, Neon-21, Neon-22,
Weighted average atomic mass is 20.2 amu.
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
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)
Lithium-6
6.01
7.5%
Lithium-7
7.01
92.5%
Average atomic mass (amu)
M x P÷100
0.45
6.49
6.94
OR
Atomic Mass = (7.5/100)(6.01) + (92.5/100)(7.01)
= 6.94 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%
99.0
Cesium-133
133.0
20.0%
26.4
Cesium-134
134.0
5.0%
6.7
Average atomic mass (amu)
OR
132.1
Atomic Mass = (75.0/100)(132.0)
+ (20.0/100)(133.0) + (5.0/100)(134.0)
= 132.1 amu
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
More Questions:
Atomic Number 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
The Development of Atomic Models
The timeline shoes the development of atomic
models from 1803 to 1911.
5.1
Defining the Atom
> The Development of Atomic Models
The timeline shows the development of atomic
models from 1913 to 1932.
Slide
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© Copyright Pearson Prentice Hall
End Show