Isotopes and Strong Force: Go to the PhET: Build an Atom simulation, and start
in “atom” mode.
1. Look up definitions for element, isotope and ion.
Element – a basic building block of matter, has a specific number of protons
Isotope – a form of an element that has a specific number of neutrons. Many elements
have more than one stable isotope. Labeled by mass number, eg carbon-14)
Ion – a form of an element that has a charge (unequal protons and electrons)
2. Start by placing 3 protons, 2 neutrons and 3 electrons in the simulation. Is this
stable? Nope
What is the atomic number (Z)? Z is always # protons, in this case 3
What is the mass number (A)?A is always # protons + neutrons, in this case 5
If you add a neutron, what changes (element, isotope, ion? Stability? A? Z?) The
element stays lithium, the charge stays neutral (not ionic), but the isotope
changes to Lithium – 6 (also written 6Li). Z changes but A increases and the
nucleus becomes stable.
3. How do you change this atom to be a stable neutral Be atom? Explain what you
had to add and why.
Add 1 proton to change the element. The number of protons always defines the atom.
Add at least 2 neutrons to find a stable isotope. The more protons in a nucleus, the
more those protons repel. Neutrons act to provide a little bit of space between the
positive charges. Since the repulsion depends on d2, even a small increase in distance
decreases the repulsions significantly.
Add 1 electron because you are asked for a neutral atom, not an ion. Adding 1 proton
means we had to add 1 electron to preserve the neutral charge.
4. Switch to symbol mode. Sketch where each number goes and explain how it is
calculated.
Top left A (protons + neutron), bottom left Z (protons, tells what element), top right
charge (protons – neutrons)
Note that element/isotope/ion symbols are NOT what is
shown on the periodic table. On the periodic table, the
numbers include the average mass of all stable isotopes in
nature, and Z goes at the top not the bottom.
5. Write the correct symbol for the stable nuclei, or write
unstable:
a. 4 protons, 4 neutrons unstable
b. 8 protons, 6 neutrons unstable
c. 8 protons, 10 neutrons $""𝑂"# (this is just nucleus,
no electrons)
d. 9 protons, 9 neutrons unstable
e. 9 protons, 10 neutrons $&&𝐹 &# (this is just nucleus, no electrons)
What is the role of neutrons in the nucleus? Neutrons stabilize the nucleus
by spread out the protons but still allowing the strong force to operate (very
small distances)
6. Now switch to game mode. Try to earn at least 10 points (you can use any/all of
the 4 games). Notes:
Answers will vary.
Now go to PhET: Isotopes and Atomic Mass interactive
7. What is the most common isotope of carbon? Which is the next-most-common?
What does the mixture of these isotopes look like in nature?
Most common carbon-12 (99%), second most common carbon-13 (1%).
Numberofneutrons
8. For each element in the simulation, identify the most prevalent isotope in nature.
Make a graph with # of neutrons on the y axis and Z on the x axis. Your x axis
should go up above 50.
Hydrogen-1 (1 p, 0 n)
Helium-4 (2p, 2n)
Nucleusofmostcommonisotope
Lithium-7 (3p, 4n)
60
Berylium -9 (4p,5n)
50
Boron-11 (5p, 6n)
40
Carbon-12 (6p 6n)
30
Nitrogen-14(7p, 7n)
20
Oxygen-16 (8p, 8m)
10
Fluorine-19 (9p, 9n)
0
Neon-20 (10p, 10n)
0
10
20
30
40
50
Sodium-23 (11 p, 12n)
Numberofprotons(Z)
Magnesium-24(12p,12n)
Aluminum-27(13p, 14n)
Silicon-28(14p, 14n)
Phosphorus-31(15p, 16n)
Sulfur-32(16p,16n)
Chlorine-25(17p, 18n)
Argon-40(18p, 22n)
9. Use ptable.com to figure out the most common isotope for 3 elements with Z>
30. Label these points on your graph. A force called the strong nuclear force holds
together nuclei. At low Z values, there are (more/less/about equal) protons
compared to neutrons. At high Z values, there are (more/less/about equal)
protons compared to neutrons. Reflect on why this might be:
At higher Z values, there is more charge in the nucleus. Then
there is more proton-proton repulsion. If we think of the
protons in clusters, then neon contains 2 groups of 5
protons. Zirconium contains 2 groups of 20 protons, so each
charge is 4 times as large and the repulsion is 16 times as
great. We need more distance between the 40 protons than
between the 10 protons to keep the repulsion manageable.
Thus Zr has 50 neutron spacers compared to neon’s 10.