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Radioactive Decay Types

To get started with our review of radioactive decay let’s look at the different types. We won’t focus on understanding exactly how they work per se, but on the critical information you need to know about each decay type to answer the most common questions you are likely to see on this material.

What is Radioactive Decay?

At its core, radioactive decay is the process where an unstable atomic nucleus loses energy by emitting radiation. During this process, a ‘parent’ nucleus transforms into a ‘daughter’ nucleus. How this parent decays and what daughter forms ultimately depends on the type of decay.

Alpha Decay

Alpha decay involves the emission of an alpha particle, which is essentially a helium nucleus. This results in the mass decreasing by 4 units and the element moving two places back on the periodic table. We can remember this by imaging the alpha symbol, as a laughing clownfish since it already looks a bit like a fish. The “He, He, He” of the fishes laugh represents the loss of a helium nucleus and the fish is pointed to the left to help you remember that you go two back in the periodic table.

\[ \text{Uranium-238} \rightarrow \text{Thorium-234} + \text{Alpha Particle} \]

Beta Decay

In beta decay, a nucleus emits a beta particle, either an electron (β-) or a positron (β+). The superscript indicates the type of particle emitted. This is key because the sign is the reverse of how you move on the periodic table. We can remember this using the mnemonic “Beta Backwards,” where β+ results in movement backwards on the periodic table and β- moves it forward.

Beta-plus (β+) Example:
\[ \text{Nitrogen-13} \rightarrow \text{Carbon-13} + \text{Positron} \]

Beta-minus (β-) Example:
\[ \text{Carbon-14} \rightarrow \text{Nitrogen-14} + \text{Electron} \]

Unlike alpha-decay nothing is lost from the nucleus and instead protons and neutrons are converted into one another. In β- decay, a neutron is transformed into a proton, with no mass change. Conversely, in β+ decay, a proton turns into a neutron, also without altering the mass. So long as you remember how you are supposed to move in the periodic table you don’t have to memorize this fact because gaining a proton moves you forward one in the periodic table (β-) and vice versa.

Electron Capture

Electron capture is essentially the reverse of beta-minus decay. The nucleus captures an orbiting electron, which combines with a proton to form a neutron, leading to a one-place backward movement on the periodic table.

\[ \text{Potassium-40} + \text{Electron} \rightarrow \text{Argon-40} \]

Gamma Decay

Gamma decay is characterized by the emission of high-energy photons, known as gamma rays, from the nucleus. This type of decay does not result in a change in the element itself. We can remember this because the gamma symbol looks like a shooting star indicating the emission of a photon and an infinity symbol indicating that we end and start in the same place.

\[ \text{Technetium-99m} \rightarrow \text{Technetium-99} + \text{Gamma Ray} \]

Summary Table of Radioactive Decay Types

Decay TypeMass ChangePeriodic Table Movement
Alpha Decay-4Two places back
Beta-minus (β-)No changeOne place forward
Beta-plus (β+)No changeOne place back
Electron CaptureNo changeOne place back
Gamma DecayNo changeNo movement

Deciphering Decay Questions

There are a couple of different types of decay questions, when you encounter them use these strategies:

Simple Decay

1. Check for Mass Change: If there’s a change, it’s alpha decay.
2. Check for Elemental Change: No change indicates gamma decay; otherwise, follow the specific decay type movements.

Sample Question

Question: \( ^{14}\text{C} \) undergoes beta-minus decay. Which element does it become?
A. \( ^{14}\text{N} \)
B. \( ^{28}\text{Si} \)
C. \( ^{14}\text{B} \)
D. \( ^{30}\text{S} \)


  1. Check for Mass Change: Beta-minus decay does not alter the mass, so the mass number remains 14. We can eliminate answer choice B and D because of this.
  2. Check for Elemental Change: Use the mnemonic “Beta Backwards” – in beta-minus decay this means we will move one element forward (opposite to the superscript) on the periodic table.
  3. Answer: Carbon (\( ^{14}\text{C} \)) moving one place forward on the periodic table becomes Nitrogen (\( ^{14}\text{N} \)). Therefore, the correct answer is A. \( ^{14}\text{N} \).

Series Decay

1. Calculate Total Mass Change: Remember you are going to lose 4 mass for each alpha decay.
2. Determine Net Periodic Table Movement: Sum up the movements from each decay type to understand the overall transformation.

Sample Question

Question: An element undergoes two sequential decays: first an alpha decay, then a beta-minus decay. If the starting element is \( ^{238}\text{U} \) (Uranium-238), what is the resulting element?
A. \( ^{236}\text{U} \)
B. \( ^{236}\text{Th} \)
C. \( ^{234}\text{Pa} \)
D. \( ^{234}\text{Ra} \)


1. Calculate Total Mass Change:

  • The first alpha decay decreases the mass by 4 units (from 238 to 234).
  • The beta-minus decay does not change the mass.
  • Total mass after both decays: 234. Therefore we can eliminate both answer choices A and B.

2. Determine Net Periodic Table Movement:

  • Alpha decay: Moves two places back on the periodic table (Uranium to Thorium).
  • Beta-minus decay: Moves one place forward (remember the “Beta Backwards” mnemonic; beta-minus moves opposite to the superscript, i.e., forward).
  • Net movement: One place back from Uranium (U → Pa).

3. Answer: The resulting element after both decays is Protactinium-234 \(( ^{234}\text{Pa} )\), making the correct answer C. \( ^{234}\text{Pa} \).

Original Nucleus Questions

1. Approach Normally: First approach the question normally as though it is asking for simple decay or series decay
2. Flip the movement: Once you know how your mass and position on the periodic table changes flip those movements to find the original nucleus

Sample Question

Question: If the product of a single alpha decay is \( ^{206}\text{Pb} \) (Lead-206), what was the original nucleus?
A. \( ^{210}\text{Po} \)
B. \( ^{210}\text{Bi} \)
C. \( ^{206}\text{Th} \)
D. \( ^{202}\text{Hg} \)


1. Approach Normally:

  • First, determine the changes typically associated with alpha decay.
  • Alpha decay decreases the mass by 4 units and moves the element two places back on the periodic table.

2. Flip the Movement:

  • To find the original nucleus, reverse the changes from alpha decay.
  • Increase the mass by 4 units (from 206 to 210).
  • Move two places forward on the periodic table from Lead (Pb).

3. Answer: The original element before alpha decay, with a mass of 210 and two places ahead of Lead on the periodic table, is Polonium-210 (\( ^{210}\text{Po} \)). Therefore, the correct answer is A. \( ^{210}\text{Po} \).

Periodic Table Trick

When tackling radioactive decay questions you might encounter elements that are not commonly discussed. A lot of people are concerned about finding them quickly, but don’t worry; there’s a straightforward strategy to locate them on the periodic table. Start by considering the given mass number. For instance, if you come across Db-270, check the atomic weights around 270 on the periodic table. This will typically lead you to the lower middle section, where you can find Db (Dubnium) relatively easily. This method simplifies the process of identifying elements so you can get to actually answering decay questions.