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Section 4.2—Atomic Structure
Objectives:
•determine the number of protons, neutrons, &
electrons in atoms
•distinguish between the properties of protons,
neutrons, & electrons
•define isotopes
•differentiate between mass number and
average atomic mass
•calculate weighted average atomic masses
What are atoms?
An atom is the smallest piece of an
element that has all the chemical
properties of that element.
What is in an atom?
An atom is made of three sub-atomic particles
Particle
Proton
Location
Nucleus
Mass
Charge
1
amu(1.6710-27
+1
kg)
Neutron
Nucleus
Electron
Outside the
nucleus
1 amu
(1.6710-27 kg)
0.00055 amu
(9.1010-31
kg)
0
-1
What gives an atom its identity?
What makes an atom “carbon” as opposed
to “oxygen”?
Every atom has a different number of
protons.
The number of protons determines the
identity of the atom
On a periodic table, the atomic number
shows the number of protons.
The Nucleus & Mass
 Since the nucleus has protons & neutrons, and
the mass of each one is 1 amu…
 The mass of the nucleus (called the mass
number) is the number of protons + neutrons
 Since electrons have relatively no mass (0.054%
of one proton or neutron), we don’t need to
worry about them when determining mass of an
atom.
 The mass number (protons + electrons)
represents the mass of an atom.
Charges in an Atom
Protons have a positive charge (+)
Electrons have a negative charge (-)
Neutrons have no charge
Overall charge of an atom = 0 because the
number of protons equals the number of
electrons in any atom.
Ions
An ion is any charged particle; specifically,
if an atom LOSES or GAINS electrons, it
will become an ion.
If an atom loses electrons, there will be
more protons than electrons. It will
become a positively-charged ion.
If an atom gains electrons, there will be
more electrons than protons. It will
become a negatively-charged ion.
Element symbols
Element Symbol
1 or 2 letters, found on
the periodic table
Mass number
# protons + # neutrons
Atomic number
# of protons
Charge
A
X
Z
C
# protons - # electrons
(assumed to be “0” if
blank)
Example: Element symbols
Element Symbol
O = Oxygen
Charge
-2
16
Mass number
16
Atomic number
8
-2
O
8
Another element symbol
It is possible for atoms to have the same
number of protons but different numbers of
neutrons. Such atoms are called isotopes.
To make it easy to identify the various
isotopes of an element, scientists add a
number after the element’s name.
This number is the mass number - the
sum of the number of protons and
neutrons in the nucleus of an atom of the
isotope.
Example
16 O
8
17 O
8
18 O
8
Let’s Practice
Example:
Fill in the
missing
values
Symbol
Name
Atomic
#
Mass
#
Magnesium-25
8
Charge Proton Neutron Electron
+2
3Li
15
53
127
-1
16
18
Isotopes
What are isotopes?
Isotopes - n. Atoms of the same element
with a different number of neutrons
Some isotopes are radioactive—but not
all…many are quite stable!
Isotopes Example
Mass # = 2 amu
Mass # = 1 amu
Hydrogen-1
Hydrogen-2
 If they have different number of neutrons, and
neutrons have a mass of 1 amu…
 Then isotopes of the same element will have
different masses!
 But because their protons are the same, they
are the same element!
Identifying Isotopes
Isotopes can be differentiated by their
different mass numbers in the element
symbol or by the mass number following
their name.
12
C
Carbon-12
13
C
Carbon-13
Isotopes
Elements are found in nature as a mixture
of all of their isotopes.
The percent abundance of the isotopes
(percent of each isotope in a sample) is
constant no matter where the sample is
found.
Isotopes have the same chemical behavior
because chemical behavior is determined
by the number of ELECTRONS an element
has.
Atomic Mass
The atomic mass of an element is the
weighted average mass of the isotopes of
that element.
Atomic mass, therefore, takes into
account the percent abundance of each
isotope as well as the mass of an atom of
that isotope.
The isotope in greater abundance will
have a greater effect on determining the
atomic mass.
Analyzing Atomic Mass
Analyzing an element’s atomic mass can
SOMETIMES give you a clue as to what
the most abundant isotope for that
element is.
The atomic mass of fluorine is 18.998
amu. From this you can conclude that
most fluorine occurs in the form of fluorine19.
Mass Number versus Atomic Mass
Mass Number
Average Atomic Mass
# of protons + # of neutrons
Average of actual masses
Always a whole number
Not a whole number
For one specific isotope
only
Weighted average of all
isotopes
Is not found on the periodic
table
Is found on the periodic table
Calculating Atomic Mass
Chlorine-35
Chlorine-37
1 Multiply the % abundance of each isotope by its mass.
(75%)(35) = 26.25 amu
(25%)(37) = 9.25 amu
2 Add the mass contributions
of all the isotopes together.
35.5 amu is the calculated atomic mass of chlorine.
Practice Problems
Isotope
Percent
Abundance
lithium-6
7.6%
lithium -7
92.4%
Calculate the atomic mass of lithium.