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Transcript
Ch. 1 Fun with the
Periodic Table
Activity 9 Part B
What Determines and Limits an Atom’s
Mass?
Investigate
 1. The nucleus has a positive charge due to
all of the protons there. The electrons
surrounding the nucleus have negative
charges.
 a) What kind of electric force (attractive or
repulsive) exists between the nucleus of an
atom and its electrons?
 b) What kind of electric force exists
between pairs of protons in the nucleus?
Investigate
 2. The nucleus is a very crowded place. The protons
in the nucleus are very close to one another. If these
protons are repelling each other by an electrostatic
force (and they are!), there must be another force, an
attractive force, which keeps them there. The
attractive force is the nuclear force, also called the
strong force. This force is much stronger than the
electric force. It acts between pairs of protons, pairs
of neutrons, and protons and neutrons. The electron
is not affected by the nuclear force.
 a) Copy and complete the table on the next slide. The
first row has been completed for you.
Fill in the following table.
Particles
electron-proton
electron-neutron
proton-proton
proton-neutron
neutron-neutron
Coulomb
electrostatic
force
Strong
nuclear
force
attractive
none
Investigate
 If the nucleus were too large, the protons on one side
of the nucleus would be too far away to attract the
protons on the other side of the nucleus. The protons
can still repel one another since the coulomb
electrostatic force is long-range. The repulsive
electrostatic force wins over the strong nuclear force
and the nucleus won’t form.
 A large nucleus will break apart when the
electrostatic repulsion between the protons is too
great. The repulsion pushes the fragments of the
nucleus apart, releasing a great amount of energy.
This process of splitting an atom into smaller atoms is
called fission. It occurs in uranium when an
additional neutron is added and causes instability.
Investigate
 One example of the fission process is when
uranium breaks into krypton and barium.
Three neutrons and energy are released.
 This can be represented as follows:
235
1
94
139 Ba + 31 n + energy
U
+
n
→
Kr
+
92
0
36
56
0
Investigate
 3.Answer the following questions.
 a) What is the total mass number on each side? Is
the mass number (the top number) conserved on
both sides of the reaction?
 b) What is the total atomic number on each side? Is
the atomic number (the bottom number) conserved
on both sides of the reaction?
 c) Why does the neutron have a mass number of 1?
 d) Why is the atomic number of a neutron equal to 0?
Investigate
 Small nuclei can also combine to form a larger
nucleus and release energy. This process is called
fusion. An example of fusion is the combining of
hydrogen to make helium in three steps on the Sun.
In the first step of the process, two hydrogens
produce an isotope of hydrogen (deuterium) with one
neutron. In the second step, a hydrogen combines
with a deuterium to produce a helium isotope with
one neutron. In the third step, two helium isotopes
produce a helium with two protons and two neutrons
and two hydrogens. Along the way, other particles
and energy are also released. The corresponding
nuclear equations are shown below.
Step 1:
+ 11 H → 21 H + 01e neutrino
 (positron: particle with a mass equal to that
of an electron but with a positive charge)
 (neutrino: a very energetic subatomic particle
with zero charge and near zero mass).

1
1H
Step 2:

1
1H
+ 21 H → 32He + gamma radiation
Step 3:
 4. Answer the following questions

3
3 He → 4 He + 1 H + 1 H
He
+
2
2
2
1
1


a) Is the atomic mass number conserved on
both sides of the reaction?
b) Is the atomic number conserved on both
sides of the reaction?
Investigate
 5. The neutrons and protons in the nucleus have a
substructure and consist of particles called quarks.
Quarks come up in many types. The ones that
combine to make protons and neutrons are “up”
quarks with a charge of 2/3 and “down” quarks with a
charge of 1/3. These combine in sets of three quarks
to make protons and neutrons.


a) Show how a proton (charge = 1) can be made up of
a combination of up and down quarks.
b) Show how a neutron (charge = 0) can be made up
of a combination of up and down quarks.
Explain- Chem Talk
 Read pp. 82-88 and answer the Checking up
questions.
 1. Explain the difference between atomic mass and





atomic number.
2. What two forces are at work in the nucleus of an
atom? Explain the characteristics of each.
3. What is an isotope?
4. Why are some isotopes unstable?
5. Construct a table or diagram to compare and
contrast the nuclear processes of fission and fusion.
6. Formulas are used to show the mass of different
isotopes. For example, Cl-35 and Cl-37 would be
expressed as 3517 Cl and 3717 Cl. Compare and contrast
Cl-35 and Cl-37.
Evaluate1- Chem Essential Questions
 You know that in nuclear fission, uranium-235 is
bombarded with neutrons and this reaction will cause
the nuclei to split to smaller elements, releasing a
large amount of energy. How does the Sun create
energy using fusion?
 The periodic table is ordered by atomic number,
which corresponds to the number of protons in the
element. How do you know that it should not be
ordered by the atomic mass, which corresponds to
the total number of protons and neutrons in the
element?
Evaluate 2: Chem to Go
 1. If lithium loses an electron to become Li+, what is the average
atomic mass of the lithium ion? Explain how you arrived at your
answer.
 2. Hydrogen has three isotopes with mass numbers of 1, 2, and
3. Write the complete chemical symbol for each isotope.
 3. Give the complete chemical symbol for the element that
contains 16 protons, 16 electrons, and 17 neutrons.
 4. Use the periodic table to complete the table below:
Chemical Symbol
39
19
Atomic Number
K
9
Atomic Mass
15
Number of Protons
Number of Electrons
Number of Neutrons
53
10
16
127
Evaluate 2: Chem to Go
 5. Neutrons can be used to bombard the nucleus of an





atom like uranium. Why would it be more difficult to inject
the nucleus of uranium with a proton?
6. Complete the following reaction:
235 U + 1 n → 38 Sr + ______ + 21 n.
92
0
94
0
7. Radon is a threat to the health of people in their
homes. It emits radioactive particles at a significant rate.
Complete the following radioactive decay equation:
222 Rn → 218 Po + ________ .
86
84
8. Explain why a helium atom is able to exist. What keeps
the two electrons, two protons, and two neutrons
together?