Question: How does alpha decay change the nucleus of a radioactive atom?

Student Exploration: Nuclear Decay

Prior Knowledge Questions (Do these BEFORE using the Gizmo.)

The chart below gives the locations, charges, and approximate masses of three subatomic particles. The approximate mass of each particle is given in universal mass units (u).

Particle	Location	Charge	Approximate mass
Proton	Nucleus	1+	1 u
Neutron	Nucleus	0	1 u
Electron	Orbitals	1–	0 u

1. The mass number of an atom is equal to the sum of protons and neutrons in the nucleus. 
   A helium atom has 2 protons and 2 neutrons. What is the mass number of this atom?

1. The atomic number of an element is equal to the number of protons in each atom of the element. All helium atoms have 2 protons. What is the atomic number of helium?

Gizmo Warm-up

While most atoms are stable, some are radioactive, which means that they have a tendency to undergo spontaneous nuclear decay. The decay of radioactive atoms generally results in the emission of particles and/or energy.

Several types of nuclear decay can be explored with the Nuclear Decay Gizmo™. On the Gizmo, check that Alpha decay and Uranium are selected.

1. Click Play () and then click Pause () when the alpha particle is clearly visible. What is an alpha particle made of? Click Play and observe. Besides the alpha particle, what else is emitted from the nucleus during alpha decay?. Gamma rays are energetic electromagnetic waves; they are often emitted in nuclear decay.

Activity A: Alpha decay

Get the Gizmo ready: Click Reset (). Check that Alpha decay and Uranium are selected.

Question: How does alpha decay change the nucleus of a radioactive atom?

1. Predict: As you observed in the warm-up activity, an alpha particle consists of two protons and two neutrons. How will the emission of an alpha particle affect the following?

1. The atomic number of the atom:
2. The mass number of the atom:
3. Calculate: Turn on Write equation. What you see is an equation that shows the original uranium atom on the left. The boxes on the right represent the daughter product—the atom produced by radioactive decay—and the emitted alpha particle.

1. In the top left box, write the mass number of the daughter product and press “Enter”on your keyboard. What is this number?
2. In the bottom left box, write the atomic number of the daughter product and press “Enter.” What is this number?
3. In the next set of boxes, enter the mass number and atomic number of the alpha particle, which has the same composition as the nucleus of a helium (He) atom. You may need to write the numbers and element symbols below the image.   Use the format 238/92 U for example.  

234/90 TH + 4/2 HE

1. According to your equation, what isotope remains after the alpha decay of 
   uranium-238? (Note: You can look up element symbols on the periodic table.)

1. Check: Turn on Show equation and click Play. The equation will appear at the end of the animation. Was your prediction correct?
2. Practice: Click Reset, turn off Show equation, and select Polonium. Write an equation for the alpha decay of polonium, and then use the Gizmo to check your answer. You may need to write the numbers and element symbols below the image.   Use the format 212/84 Po for example.  

      What isotope remains after the alpha decay of polonium-212?

1. Practice: Americium-241 is a radioactive isotope used in smoke detectors. Write an equation for the alpha decay of Americium-241 below. You may need to write the numbers and element symbols below the image.   Use the format 241/95 Am for example.  

1. Analyze: In each equation, how is the mass number on the left side of the arrow related to the sum of mass numbers on the right side of the arrow? Is this true for atomic numbers?
2. Think and discuss: Helium is the second most abundant element in the universe, but it is rare in Earth’s atmosphere. Most of the helium used to fill balloons and blimps must be extracted from Earth’s crust. How do you think this helium formed?

Activity B: Beta decay

Get the Gizmo ready: Click Reset, and turn off Show equation. Select Beta decay from the Type of decaymenu. Check that Carbon is selected.

Question: How does beta decay change the nucleus of a radioactive atom?

1. Observe: Click Play and watch the animation.

1. What happens to the decaying neutron during beta decay?
2. What is emitted from the nucleus during beta decay?
3. What is the mass number and charge of the emitted particle?

Mass number: Charge:

1. Predict: During beta decay, a neutron is transformed into a proton and an electron (the beta particle), which is emitted. Gamma rays are often emitted during beta decay as well. How will beta decay affect the atomic number and mass number of the atom?
2. Calculate: Turn on Write equation. Fill in the first set of boxes with the mass number and atomic number of the daughter product and the next set of boxes with the mass number and atomic number of the beta particle. (Note: The atomic number of an electron is -1.) You may need to write the numbers and element symbols below the image.   Use the format 14/6 C for example.  

Check your answer by turning on Show equation and clicking Play. Modify your equation if necessary. What isotope is produced by the beta decay of carbon-14?

1. Practice: Turn off Show equation. Fill in the equations for the beta decay of iodine-131 and sodium-24 in the spaces below. Use the Gizmo to check your answers. You may need to write the numbers and element symbols below the image.   Use the format 131/53 I or 24/11 Na for example.