Warm Up
O
8 p+
8n
8 e-
2O
8 p+
8n
10 e-
2+
Zn
30 p+
35 n
28 e-
Same Element Different AtomIsotopes
All atoms of a particular element are not exactly alike.
Some elements have atoms with different masses (isotopes)
Learning Objectives
You will know what isotopes are
You will be able to figure out the number of
neutrons for each isotope
You will be able to calculate the atomic mass of an
element that has different isotopes.
Isotope Example
35Cl
37Cl
17
17
chlorine - 35
chlorine - 37
Isotopes of Carbon
Naturally occurring carbon consists of three isotopes, 12C, 13C, and
14C. State the number of protons, neutrons, and electrons in each of
these carbon atoms.
12C
6
13C
14C
6
6
#P _______
_______
_______
#N _______
_______
_______
#E _______
_______
_______
Solution
12C
6
13C
14C
6
6
#P __6___
_ 6___
___6___
#N __6___
_ _7___
___8___
#E __6___
_ 6___
___6___
Zinc problem
An atom of zinc has a mass number of 65.
A. Number of protons in the zinc atom
1) 30
2) 35
3) 65
B. Number of neutrons in the zinc atom
1) 30
2) 35
3) 65
C. What is the mass number of a zinc isotope
with 37 neutrons?
1) 37
2) 65
3) 67
Solution to Zinc Problem
An atom of zinc has a mass number of 65.
A. Number of protons in the zinc atom
1) 30
B. Number of neutrons in the zinc atom
2) 35
C. What is the mass number of a zinc isotope
with 37 neutrons?
3) 67
Atomic Mass
 Listed on the periodic table
Na
22.99
 Gives the mass of “average” atom of each element
compared to 12C
 Average atom based on all the isotopes and their
abundance %
 Atomic mass is not a whole number … mass number is a
whole number
Calculating Atomic Mass
 Percent(%) abundance of isotopes
 Mass of each isotope of that element
 Weighted average =
mass isotope1(%) + mass isotope2(%) + …
100
100
Atomic Mass of Magnesium
Isotopes
Mass of Isotope
Abundance
24Mg
=
24.0 amu
78.70%
25Mg
=
25.0 amu
10.13%
26Mg
=
26.0 amu
11.17%
(24)(.787) + (25)(.1013) + 26(.1117) =
18.888 + 2.5325 + 2.9042 = 24.3 amu