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```Chemistry 103
Lecture 5
Outline
I. Review from last lecture
II. Periodic Table
– Atomic Mass
III. Electronic Structure
Structure of the Atom
An atom consists
• Of a nucleus that
contains protons and
neutrons
• Of electrons in a large
empty space around
the nucleus
Publishing as Benjamin Cummings
Atoms:
Composed of 3 Subatomic Particles
Symbol
Relative
Charge
Mass
Proton
p or p+
+1
1.673x10-24g
Neutron
n or no
0
1.675x10-24g
Electron
e-
-1
9.110x10-28g
Atomic Symbols
Mass number =
number of protons +
number of neutrons
Charge =
A
Z
Atomic number =
number of protons
Defines the
element
X
Q
Number of protons number of electrons
Atomic Symbols
Mass number =
number of protons +
number of neutrons
Charge =
A
Z
Atomic number =
number of protons
X
Q
number of protons –
number of electrons
Atomic Symbols
Mass number =
number of protons +
number of neutrons
Charge =
A
Z
Atomic number =
number of protons
X
Q
number of protons –
number of electrons
Atomic Symbol Notation vs.
The Periodic Table Notation
20
40
20
2+
Ca
Ca
40.078
Learning Check
An atom of zinc has a mass number of 65.
1. How many protons are in this zinc atom?
A) 30
B) 35
C) 65
2. How many neutrons are in the zinc atom?
A) 30
B) 35
C) 65
3. What is the mass number of a zinc atom that has
37 neutrons?
A) 37
B) 65
C) 67
Learning Check
Write the nuclear symbols for atoms with
the following subatomic particles:
47p+, 60 n, 47 e-
12p+, 12n, 10e-
___________
___________
10
Determine the number of
subatomic particles for each:
• Co-59
• Pb
•
39K+
•
19F-
Z = 82 A = 207
Exam type multiple choice
question
• The radioactive isotope strontium-90, a
component of fallout from atmospheric nuclear
testing, is particularly hazardous because its 2+
ion may substitute for that of calcium in bone.
90Sr2+ has
• a. 38 protons, 52 neutrons, and 36 electrons
• b. 38 protons, 52 neutrons, and 38 electrons
• c. 38 protons, 52 neutrons, and 40 electrons
• d. 52 protons, 38 neutrons, and 50 electrons
• e. 52 protons, 38 neutrons, and 54 electrons
How Much Does an Atom Weigh?
• The mass of atoms is measured in units of amu
– atomic mass unit
• Defined as 1/12th the mass of 12C
Therefore, C-12 is exactly 12 amu on this scale
• Masses reported on the periodic table are
weighted averages
• Takes into account the fact that element samples
consist of mixtures of different isotopes
Atomic Mass
The atomic mass of an element,
• Is listed below the symbol of each element on
the periodic table
• Gives the mass of an “average” atom of each
element compared to 12C
Na
22.99
• Is not the same as the mass number
14
Mass Number v.
Atomic Weights/Mass
•Mass Number = the number of protons plus the number of neutrons
•ALWAYS A WHOLE NUMBER
•Atomic Weight/Mass = the mass of an average atom
•Found on the Periodic Table
20
•An average of the masses of all the
isotopes of an element
Ca
40.078
Examples of Isotopes and Atomic
Masses
Most elements have
two or more isotopes
that contribute to the
atomic mass of that
element.
16
Calculating Atomic Mass
The calculation for atomic mass requires the:
• Percent (%) abundance of each isotope
• Atomic mass of each isotope of that element
• Sum of the weighted averages
mass isotope1(%) + mass isotope2(%) + …
100
100
17
Calculating an Average
• Exam Scores (10 students):
• 95, 85, 85, 82, 79, 79, 79, 79, 65, 51
• How do you determine an average?
Calculating an Average
• Exam Scores (10 students):
• 95, 85, 85, 82, 79, 79, 79, 79, 65, 51
95+85+85+82+79+79+79+79+65+51 = 77.9
10
Calculating an Average
• Exam Scores:
• 95, 85, 85, 82, 79, 79, 79, 79, 65, 51
• 95+85+85+82+79+79+79+79+65+51 = 77.9
10
Alternate way of writing:
• 95+(85 x 2)+82+(79 x 4)+65+51 = 77.9
10
Calculating an Average
•
•
•
•
•
Exam Scores:
95, 85, 85, 82, 79, 79, 79, 79, 65, 51
10% = 95, 1/10 x 100% = 10%
20% = 85, 2/10 x 100% = 20% etc….
Total percentages = 100%
Weighting factor:
(95 x 0.1)+(85 x 0.2)+(82 x 0.1)+(79 x 0.4)+
(65 x 0.1)+(51 x 0.1) = 77.9
Isotopes of Magnesium
In naturally occurring magnesium,
there are three isotopes.
24Mg
12
25Mg
12
26Mg
12
22
Calculating Atomic Mass of Mg
Isotopes Mass of Isotope
Abundance
Weighted portion
24Mg
= 23.985 amu x
78.70/100
=18.88 amu
25Mg
= 24.986 amu x
10.13/100
= 2.531 amu
26Mg
= 25.983 amu x
11.17/100
= 2.902 amu
Atomic mass (average mass) Mg
= 24.31 amu
Mg
24.31
23
Calculating Atomic Mass of Mg
Isotopes Mass of Isotope
Abundance
Weighted portion
24Mg
= 23.985 amu x
.7870
= 18.88 amu
25Mg
= 24.986 amu x
.1013
= 2.531 amu
26Mg
= 25.983 amu x
.1117
= 2.902 amu
Atomic mass (average mass) Mg
= 24.31 amu
Mg
24.31
24
Group Work:
Average Atomic Weights
• Naturally occurring carbon is composed of 98.93% 12C
and 1.07% 13C. If the mass of one 12C atom is 12 amu
and the mass of one 13C atom is 13.00335 amu, what is
the average mass of naturally occurring carbon?
Atomic Mass Calculation
• Naturally occurring sulfur consists of 4
isotopes – calculate the atomic mass of S
Sulfur Isotopes
Atomic Mass
% Abundance
31.9721
95.0%
32.9715
0.76%
33.9679
4.22%
35.9671
0.014%
```
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