Name: __________________
Period: ____
Date: _____________
Chemistry I: Chapter 4 - Atomic Structure practice
Part A: Complete the following table using the following relationships. Use the periodic table in the text.
Atomic No. = number of protons = number of electrons (for a neutral atom)
Mass No. = number of protons and neutrons
ELEMENT
NAME
ATOMIC
NUMBER
MASS
NUMBER
Hydrogen
1
Lithium
7
Sodium
23
NUMBER
OF
PROTONS
NUMBER
OF
NEUTRONS
NUMBER OF
ELECTRONS*
ELEMENT
SYMBOL
4
He
24
Mg
5
11
6
12
7
14
8
16
9
19
20
10
9
4
Part B: Calculating Average Atomic Mass using Isotope Abundance Data
Calculating average atomic mass is similar to calculating a weighted GPA or weighted grade in class.
Example: Your semester grade is calculated based on the following criteria:
First Quarter Grade = 45%
Second Quarter Grade = 45% Semester Exam = 10%
A student has a grade of 80 for the first quarter, 60 for the second quarter and 90 semester exam grade.
Calculate the student’s semester grade:
ANSWER: The grade is calculated as follows – Each grade is multiplied by the assigned percent
(converted to decimal). Then the products are summed to yield the AVERAGE GRADE:
First Quarter:
Second Quarter :
Semester Grade:
80 * .45 = 36
60 * .45 = 27
90 * .10 = 9
---------
SEMESTER GRADE =======
72
Atomic Mass example:
Carbon: 99% is Carbon-12
Calculate the average atomic mass of Carbon
0.9% is Carbon-13
0.1% is Carbon-14
Answer: 0.99 * 12 = 11.88
0.009 *13 = 0.117
0.001 * 14 = 0.014
atomic mass= 12.01
Solve the following problems on another piece of paper. Show your work to received full credit.
1) The term “average atomic mass” is a _________________________average, and so is calculated differently
from a “normal” average.
2) Rubidium is a soft, silvery-white metal that has two common isotopes, 85Rb and 87Rb. If the abundance of 85Rb
is 72.2% and the abundance of 87Rb is 27.8%, what is the average atomic mass of rubidium?
3) Uranium is used in nuclear reactors and is a rare element on earth. Uranium has three common isotopes. If
the abundance of 234U is 0.01%, the abundance of 235U is 0.71%, and the abundance of 238U is 99.28%, what is the
average atomic mass of uranium?
4) Titanium has five common isotopes: 46Ti (8.0%), 47Ti (7.8%), 48Ti (73.4%),
49Ti (5.5%), 50 Ti (5.3%). What is the average atomic mass of titanium?
5) Copper used in electric wires comes in two flavors (isotopes): 63Cu and 65Cu. 63Cu has an atomic mass of
62.9298 amu and an abundance of 69.09%. The other isotope, 65Cu, has an abundance of 30.91%. The average
atomic mass between these two isotopes is 63.546 amu. Calculate the actual atomic mass of 65Cu?
6). The element copper has naturally occurring isotopes with mass numbers of 63 and 65. The relative
abundance and atomic masses are 69.2% and 30.8% respectively. Calculate the average atomic mass of copper.
7) Calculate the average atomic mass of sulfur if 95.00% of all sulfur isotopes are Sulfur-32, 0.76% are Sulfur-33
and 4.22% are Sulfur-34.
8) The four isotopes of lead are shown below, each with its percent by mass abundance and the composition of
its nucleus. Using the following data, first calculate the approximate atomic mass of each isotope. Then calculate
the average atomic mass of lead.
82p
122n
1.37%
82p
124n
26.26%
82p
125n
20.82%
82p
126n
51.55%
9) There are three isotopes of silicon. They have mass numbers of 28, 29 and 30. The average atomic mass of
silicon is 28.086amu. What does this say about the relative abundances of the three isotopes?
10) Calculate the average atomic mass of bromine. One isotope of bromine has an atomic mass of 78.92amu and
a relative abundance of 50.69%. The other major isotope of bromine has an atomic mass of 80.92amu and a
relative abundance of 49.31%.