AP Chemistry
Unit 3 - Elements
Lesson 4 – Isotopes
Book Section: 2.3
Isotopes


Isotopes are atoms of the same element with
different masses.
Isotopes have different numbers of neutrons.
11
6
C
12
6
C
13
6
C
14
6
C
Atomic Mass

Atomic and
moleculear
masses can be
measured with
great accuracy
with a mass
spectrometer.
Average Mass


Because in the real world we use large
amounts of atoms and molecules, we use
average masses in calculations.
Average mass is calculated from the isotopes of
an element weighted by their relative
abundances.
Sample Problem

Naturally occurring chlorine is 75.78% 35Cl,
which has an atomic mass of 34.969 amu, and
24.22% 37Cl, which has an atomic mass of
36.966 amu. Calculate the average atomic
mass (that is, atomic weight) of chlorine.
Sample Problem


Naturally occurring chlorine is 75.78% 35Cl,
which has an atomic mass of 34.969 amu, and
24.22% 37Cl, which has an atomic mass of
36.966 amu. Calculate the average atomic
mass (that is, atomic weight) of chlorine.
35.45 amu
AP Problem


A)
B)
C)
D)
E)
1989 MC #24
The mass of element Q found in 1.00 mole of
each of four different compounds is 38.0 grams,
57.0 grams, 76.0 grams, and 114 grams,
respectively. A possible atomic weight of Q is
12.7
19.0
27.5
38.0
57.0
AP Problem


A)
B)
C)
D)
E)
1989 MC #24
The mass of element Q found in 1.00 mole of
each of four different compounds is 38.0 grams,
57.0 grams, 76.0 grams, and 114 grams,
respectively. A possible atomic weight of Q is
12.7
19.0 – 64% correct, easy
27.5
38.0
57.0
AP Problem


1984 MC #19
Which of the following represents a pair of isotopes?
Atomic Number
A)
B)
C)
D)
E)
Mass Number
I.
II.
I.
II.
I.
6
7
6
14
6
14
14
7
14
14
II.
14
28
I.
II.
I.
7
7
8
13
14
10
II.
16
20
AP Problem


1984 MC #19
Which of the following represents a pair of isotopes?
Atomic Number
A)
B)
C)
D)
E)
Mass Number
I.
II.
I.
II.
I.
6
7
6
14
6
14
14
7
14
14
II.
14
28
I.
II.
I.
7
7
8
13 -- 87% correct
14 – very easy
10
II.
16
20
HW: 2.22, 2.24, 2.26, 2.28, 2.30,
2.32, 2.34

This Week:








Tuesday – Periodic Table Structure (2.4, 7.1)
Wednesday – Quantitative Analysis of a Soluble Sulfate
Day 2
Thursday – Wave-Particle Duality (6.1, 6.2)
Friday – Gravimetric Analysis of a Chloride Salt Lab
10/7 – Quantitative Analysis of a Soluble Sulfate
Due
10/18 – Gravimetric Analysis of a Chloride Salt Due
10/20 – Elements Exam
10/21 – Problem Set 2 Due