Masses of
atoms
17.2
Warm up
 Label the different parts of an
atom.
 Complete the table with the
appropriate term or number.
Particle
Location
Charge
Mass
Proton
Neutron
Electron
Objectives
 Compare between the atomic number and the mass number of
an atom
 Calculate the number of electrons, protons, and neutrons in an
atom given its mass and atomic numbers and vise verse
 Explain the existence of the majority of the mass of an atom in
its nucleus
 Define isotopes
 Explain why isotopes have different mass number and similar
chemical properties
 Define the average atomic mass
 Calculate the average atomic mass of an element based on the
natural abundances rate of its isotopes
Introduction
 Can you guess what is the mass of your laptop in Kg?
 Can you predict the mass of the laptop after you place a
paperclip on top of it?
 How can you compare the mass of the laptop with and
without the paperclip to the mass of the atom and its
particles?
 Which of the following: laptop of paper clip refers to
the mass of the electrons in an atom?
Atomic mass
Complete the following table
Particle
Protons
Neutrons
Electrons
Mass (g)
Mass in (Kg)
Atomic mass
 The unit of measurement used for atomic particles
is the atomic mass unit (amu)
 Mass of p+ = mass of No = 1 amu
 Mass of e- <<<<<<<<<<< mass of p+ or No
 can you conclude where is the majority of the mass
of an atom is located?
Atomic mass
 Atomic mass is the mass of the atom (mass of protons
and neutrons) in amu
Protons identify the elements
 Each type of atom of an element has a different number
of protons.
Compare number of P+ and e-
Application
 Find, using the periodic table, which element has an
atom with the following number of protons:
1- six protons
2- 8 protons
3- 12 protons
4- 20 protons
What is the atomic number of hydrogen, helium, argon,
lithium, magnesium, chlorine, sulfur, phosphorous (use the
periodic table)
Mass number
 The mass number is the sum of the number of the protons and
the neutrons in the nucleus of an atom.
 Number of neutrons= mass number – atomic number ( # of P+)
Atomic and mass number
Application
 Find the Atomic number (Z) and the mass number (A)
for the following.
 Protons are colored red and neutrons are blue
Application
Element
Symbol
Atomic
number
# of
protons
# of
neutrons
Mass
number
Boron
11
Carbon
12
Oxygen
16
Sodium
23
Copper
63
Average
atomic
mass
 What is the difference and the similarity between the following
pictures?
Isotopes
 Atoms of the same element
with different number of
neutrons can have different
properties.
 Ex: C-12 and C-14
 Isotopes are atoms of the same
element that have the same
atomic number (number of
protons) but different mass
number ( number of
neutrons)
Identifying isotopes
 An isotope can be
identified by writing the
name or symbol of the
atom followed by its
mass number. For
example:
Carbon-14 or C-14 &
Chlorine-35 or Cl-35
Application
 Naturally occurring elements of Boron have mass
number of 10 or 11. Calculate the number of neutrons
of B-10, and B-11.
 Uranium-238 has 92 protons. How many neutrons does it
have?
Application
 Find the number of protons, electrons and neutrons for
each isotope of Lithium.
 Write the name or the symbol of the three isotopes of
Lithium.
Application
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
13C
14C
6
6
6
#P _______
_______
_______
#N _______
_______
_______
#E _______
_______
_______
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
Write the atomic symbols for atoms with
the following:
A. 8 p+, 8 n, 8 e-
___________
B. 17p+, 20n, 17e-
__________
C. 47p+, 60 n, 47 e-
___________
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) 14
2) 16
3) 34
B. Its mass number is
1) 14
2) 16
C. The element is
1) Si
3) 34
2) Ca
3) Se
D. Another isotope of this element is
1) 34X
16
2) 34X
14
3) 36X
14
Atomic mass
Listed on the periodic table
Na
22.99
Gives the mass of “average” atom of each
element
Average atomic mass is based on all the
isotopes and their abundance %
Atomic mass is not a whole number
Application
Using the periodic table, specify the atomic
mass of each element:
A. calcium
__________
B. aluminum
__________
C. lead
__________
D. barium
__________
E. iron
__________
Calculating average
Calculate the weighted
average of the student quiz
scores if the following
results were obtained: 5
students scored 80%; 19
students scored 90%
Calculating average atomic mass
 The atomic mass of an element is the weighted average
mass of all of its naturally occurring isotopes
 Weighted average =
mass isotope1(%) + mass isotope2(%) + …
100
100
Application
Isotopes Mass of Isotope Abundance
24Mg
=24.0 amu
78.70%
25Mg
= 25.0 amu
10.13%
26Mg
= 26.0 amu
11.17%
Atomic mass (average mass) Mg = 24.3 amu
Mg
24.3
Application
Gallium is a metallic element found in
small lasers used in compact disc
players. In a sample of gallium, there
is 60.2% of gallium-69 (68.9 amu)
atoms and 39.8% of gallium-71 (70.9
amu) atoms. What is the average
atomic mass of gallium?
Wrap up
 What is meant by atomic number?
 What is the mass number?
 How can we calculate the number of protons, neutrons
and electrons of an element?
 What are isotopes?
 How can we identify an isotope?
 How can we calculate the average atomic mass of an
element?
Home work
Section review questions
Page 515 # 1 to 7
Take a snap shot of the complete
periodic table and save it on your
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