CHEMISTRY 30S – MODULE 3
CHEMICAL REACTIONS
LESSON 1  Isotopes, Ions, and the Periodic Table
The processes in our bodies that allow us to breathe, digest food and convert food to
energy are all a result of chemical reactions. The burning of natural gas to keep our
houses warm in the winter and the burning of gasoline in our cars are chemical reactions.
Chemical reactions occur everywhere in the world around us. They play a large role in
our lives. In this module we will examine several types of chemical reactions and the
quantities chemists use to describe chemical reactions.
Hopefully you have learned about the basic structure of the atom and have used the mass
number and the atomic number to determine the number of neutrons in an atom, using the
information on the periodic table.
When you have completed this lesson, you will be able to:

Define average atomic mass with respect to isotopes and relative abundance.

Research the importance and applications of isotopes

Use the periodic table to determine the subatomic particles that make up atoms

Explain why atoms bond to make compounds, and define ions
All atoms (except hydrogen) are made of 3 basic particles: protons, neutrons and
electrons. Protons are Positively charged, electrons are negatively charged, and
NEUTRons are NEUTRal 
The mass of atoms come from the nucleus, which contains all the protons and
neutrons, while electrons circle around the outside of the atom. The electrons have no
mass compared to the protons and neutrons, so we consider the total mass of an atom
to be contained in the sum of its protons and neutrons.
Each element has its own number of protons, which is its atomic number (we use Z
to represent atomic number).
For example, the number of protons in Lithium is 3, and its atomic number is 3. In
Hydrogen, (atomic number 1) there is one proton.
Thus we can tell the number of protons in an element’s atoms, by looking at the
periodic table, and noting the atomic number of the element. And since atoms all
have the same number of electrons as they have of protons, that atomic number is
also equal to the number of protons.
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The number of neutrons in each atom varies, even between atoms of the same
element. For example, potassium can exist as three different kinds of atoms. All three
kinds of potassium atoms contain 19 protons, but one kind has 20 neutrons, one kind
has 21 neutrons and yet another has 22 neutrons. Atoms that have the same number of
protons but differ in their number of neutrons are called isotopes. Most elements exist
as more than one isotope.
If different isotopes have different numbers of neutrons, they will also have different
masses.
The atomic mass unit (often designated as u, μ, or amu) is defined as 1/12 the mass of a Carbon
atom. Why Carbon? Because it is a very common element. The amu is also the mass of a proton
and of a neutron.

Chemists have designed a symbol for each isotope that includes the element’s
symbol, its atomic number (Z) and its mass number (A).
Different variations of atoms of the same element occur in nature. These variations
are called isotopes. The average mass of the isotopes for each element is a
characteristic of that element.
Isotopes are atoms of the same element (which means they have the same number of protons)
with different numbers of neutrons. They have identical atomic numbers (number of protons) but
different mass numbers (number of protons plus number of neutrons).
If we consider the potassium isotopes previously mentioned, the isotope containing
19 protons and 20 neutrons will have a mass number of 39 (19 + 20). We call this
isotope potassium-39. The isotope that has 19 protons and 21 neutrons will have a
mass number of 40 (19 + 21) and is called potassium-40.
Z = atomic number = number of protons
A = mass number = number of protons plus number of neutrons
Number of neutrons is found by A – Z (mass number – atomic number)
Isotopes are usually represented in a few different ways:
Example:
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The symbol for potassium-39 would be:
The symbol for potassium-40 would be:
The relative abundance of an isotope is the fraction of each isotope found in an average
sample of the element.
Look at the periodic table, and notice that the atomic mass shown for each element on a
periodic table is rarely a whole number; there are decimals following the numbers. If
each proton and neutron have an atomic mass of 1 amu, then how does this happen? This
is because it is actually an average mass of all isotopes of that element.
Complete the following chart, assuming the most common forms of the atoms, rather
than isotopes:
COMPOSITION OF SELECTED NEUTRAL ATOMS
Symbol
Ba
Cr
Atomic #
Mass #
12
# of protons
10
# of neutrons
4
# of electrons
Ag
36
19
Calculating Average Atomic Mass
Example 1. Mass spectrometers have shown that magnesium exists as three isotopes:
magnesium-24, magnesium-25 and magnesium-26. In an average sample of magnesium,
78.99% is magnesium-24, 10.00% is magnesium-25 and 11.01% is magnesium-26. If the
atomic mass of magnesium-24 is 23.985 amu, magnesium-25 is 24.986 amu and
magnesium-26 is 25.982 amu, calculate the the average atomic mass of magnesium.
Solution
You will notice that the actual atomic mass of magnesium-24 is not 24 amu, but 23.985
amu and similarly for the other isotopes. This is because a single amu is not exactly equal
to the mass of a proton or a neutron.
The average atomic mass is the weighted average of the relative abundances of each
isotope. We can think of the weighted average in terms of how your marks are determined
in this, or any course. Tests may be worth 30%, quizzes may be worth 15%, labs and
assignments may be 25% and the final exam 30%.
For magnesium, that would be:
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If you look at the periodic table, you will see that the average atomic mass of magnesium
is about 24.3 amu. You will also notice that the average mass of the magnesium is closer
to 24 amu than 25 amu or 26 amu. This is because the magnesium-24 has a greater
abundance than the other two isotopes.
Radioisotopes
The nuclei of some isotopes can be unstable. As a result, the nuclei may release energy
and/or particles. These type of isotopes are called radioisotopes or radioactive isotopes.
As the energy/particles are released from the nucleus, the atom may be converted into
another isotope of the same element or an isotope of a completely different element. This
release of energy is called radioactive decay or just decay. There are several types of
decay related to the types of particles released. We will not discuss these types of decay
because it is beyond the scope of this course. If you are interested, there are many internet
websites and textbooks you can use to investigate this further.
A term often associated with radioisotopes is half-life. Half-life refers to the amount of
time it takes for half the radioisotope to be converted into another isotope. For example,
238
U has a half-life of 4.46 x 109 years. This means for a 10-gram sample of 238U it takes
4.46 x 109 years for half of it, or 5 grams, to decay into another isotope. Another way to
think of half life is after 4.46 x 109 years only 5 grams of the original 10-gram sample of
238
U will remain. Not all isotopes have such a long half-life; chlorine-38 has a half-life of
87.3 minutes, while element 106 has a half-life of 0.8 seconds.
Exercise:
1. Complete the following table to calculate the average atomic mass of each
element.
Element Symbol
Carbon
Silicon
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Relative
Average
Mass
Mass (µ) Abundance Atomic Mass
Number
(%)
(µ)
C-12
12
12(exactly)
98.98
C-13
13
13.003
1.11
Si-28
28
27.977
92.21
Si-29
29
28.976
4.70
Si-30
30
29.974
3.09
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2. Define the term isotope. Explain how an element’s atomic mass is related to the
abundances of its different isotopes.
3. Using the graph below, calculate the average atomic mass of copper.
The Periodic Table as a Classification System
Jacob Berzelius suggested the chemical symbols that everybody uses today.
Berzelius used letters to represent the atoms of each element. For example: C for carbon,
H for hydrogen, I for iodine, O for oxygen, P for phosphoprus, S for sulphur, N for
nitrogen and F for fluorine.
With over 100 elements and only 26 letters in the alphabet it became necessary to
include a second letter with the first. The second letter of the symbol is usually the
second letter in the name of the element or a main consonant in the name. The first letter
of the symbol is always written as an upper case or capital letter. The second letter is
always written as a lower case letter (i.e., Ca not CA). The full name of the element is
written with lower case (noncapital) letters only. For example:
Al
Ba
Br
Ca
Mg
Zn
Cl
As
---------
aluminum,
barium,
bromine,
calcium,
magnesium,
zinc,
chlorine and
arsenic.
The name and symbol for all new elements is now established by IUPAC (The
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International Union of Pure and Applied Chemistry). IUPAC generally respects the
recommendation(s) of the scientist(s) who discovered the element. IUPAC now requires
two letters for any new element symbol.
The individual particles making up compounds (and certain elements) are referred to
as molecules. If you have a glass of water, a container of oxygen gas, or a cube of sugar,
you have a substance which consists of molecules. A molecule can be defined as a
collection of two or more atoms held together strongly enough to form an individual
particle. The attractive forces holding the atoms together in a molecule are known as
chemical bonds. There is no absolute limit to the size of a molecule. Some molecules,
like hydrogen chloride (HCl) and water (H2O) consist, respectively, of only two and three
atoms. Other molecules are so large that they are sometimes referred to as “giant
molecules”. Proteins are examples of such giant molecules.
Although most elements are composed of individual atoms, a few elements naturally
consist of pairs of atoms. These elements are made up of diatomic molecules – two atoms
per molecule. It is the diatomic molecule that has the properties of the element. For
example,the element oxygen consists of diatomic molecules. The molecule oxygen is
represented by O2 When we inhale oxygen, along with the other gases in air, we inhale
O2 molecules and not O atoms. ln fact, individual O atoms do not exist permanently in
the atmosphere. All the properties of oxygen are due to the O2 molecules.
Compounds are formed when atoms combine. The individual particles making up
the compounds are molecules. Since molecules are composed of atoms, it follows that
once the masses of the atoms are known it is possible to determine the mass of a
molecule. This mass is known as the molecular mass. This will be covered in module 2.
In chemical reactions, a rearrangement of only the electrons takes place -- the nucleus is
not affected. Understanding chemistry depends upon understanding how the electrons of
atoms are arranged around their nuclei and upon how the electrons of atoms interact with
the electrons of other atoms.
When atoms approach each other closely, their electrons become simultaneously attracted
by the positive nuclei of other atoms. The simultaneous attraction for the same electrons
by the nuclei of two or more atoms causes electron rearrangements among atoms. The
electron rearrangements may be considered to be of two distinct types:
1. the loss and gain of electrons
2. the sharing of electrons
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The electron rearrangement is normally called “bonding”, and it results in chemical
changes, and the formation of different compounds. This will be discussed in more detail
later.
Simple Ions
In chemical reactions, atoms may lose or gain electrons to acquire the more stable
electron structure of the nearest noble gas. The atoms lose or gain electrons and acquire
an electron structure with the maximum possible number of electrons (i.e., 2, 8 or 18
electrons) in their outermost energy level.
The loss or gain of electrons results in a more stable electron energy level structure. The
loss or gain of electrons also unbalances the number of positive (proton) and negative
(electron) charges. Atoms are neutral (zero net charge) because of an equal number of
positive (proton) and negative (electron) charges. Ions are charged (have a net charge)
because of a different number of protons than electrons. The difference in the number of
protons and electrons results from a loss or gain of electrons; the number of protons will
never change in a chemical reaction.
Make sure you have a periodic table in front of you for this section. All of the simple
ions in the first three periods (rows of the periodic table) will have the same number of
electrons as the nearest noble gas. A noble gas is one of the elements in the far right
column (family) of the periodic table. Elements close to the noble gases will try to
obtain the electron structure (have the same number of electrons, whether they need to
gain or lose) of the nearest noble gas. Each occupied energy level will contain the
maximum number of electrons (i.e., 2, 8 and 18)
The name of a nonmetallic ion ends in “ide” while the name for a metallic ion uses just
the name of the metal (i.e., chloride ion and magnesium ion). Metallic vs. nonmetallic
will be discussed further in lesson 3. For now, notice the “zig-zag line” in the periodic
table, towards the right hand side of the table. The elements to the left of the line are
metals, the elements to the right are nonmetals. Memorize this fact!
Hydrogen atoms may either gain or lose an electron to form a one positive ion or a one
negative ion.
Simple ions of the transition elements cannot be explained by the Bohr Model of the
atom. The quantum mechanical model of the atom explains the transition metal (the
metals in the “dip” of the table, from Sc to Zn and those under them) ions in terms of
energy sublevels. This is covered in later chemistry courses and is beyond the scope of
this course.
LESSON 1 ASSIGNMENT 2
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The extranuclear region of the atom, which makes up most of the volume of
the atom, is occupied by _________________
2.
Nearly all of the mass of any atom is made up of_________________ and
___________________
3.
An atom has 53 protons in its nucleus. In a neutral atom it will also
have________ electrons and it will (gain I lose) ___________
(number)___________ electron(s) to acquire the electron population of the
nearest noble gas, _______________
4.
The atomic number of a K atom is (greater / less)
than the atomic number of a Na atom.
5.
The name of the ion formed by a bromine atom is
_________________________
6. The name of the ion formed by a calcium atom is _____________________
7. The number of_________________ in the nucleus of chlorine atoms may
vary.
8. Atoms with the same number of protons but with a different number of
neutrons in the nucleus are called ______________________
9. The average mass of atoms for a particular element is called the
______________________________
10. Be sure you are able to define the following terms:
a)
b)
c)
d)
e)
f)
g)
h)
i)
j)
k)
l)
m)
n)
element
compound
electron
proton
neutron
nucleus
atom
valence electron
simple ion
group
period
atomic number
atomic mass
isotope
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