1. Ch. 1 Fun with the Periodic Table
Activity 9 Part B
What Determines and Limits an Atom’s Mass?

2. 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?

3. 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.

4. Fill in the following table.
Particles
Coulomb
electrostatic
force
Strong
nuclear
electron-proton
attractive
none
electron-neutron
proton-proton
proton-neutron
neutron-neutron

5. 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.

6. 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:
23592U + 10n → 9436Kr Ba + 310n + energy

7. 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?

8. 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.

9. Step 1: 11H + 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).

10. Step 2:
11H + 21 H → 32He + gamma radiation

11. Step 3: 4. Answer the following questions
32He + 32He → 42He + 11H + 11H
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?

12. 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.

13. Explain- Chem Talk
Read pp 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.

14. 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?

15. 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 K
Atomic Number 9
Atomic Mass 15
Number of Protons 53
Number of Electrons 10 16
Number of Neutrons
127

16. 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:
23592U + 10n → 3894Sr + ______ n.
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:
22286Rn → 21884Po + ________ .
8. Explain why a helium atom is able to exist. What keeps the two electrons, two protons, and two neutrons together?