Arrange the following radiations in order of increasing ionization of air. I. Alpha Il. Gamma IIl. Beta

Analyze the question: We need to rank alpha, beta and gamma radiation by how strongly they ionize air, from the weakest ionizer to the strongest.

Concept: Ionizing power depends on how strongly the radiation interacts with air molecules, which depends on its charge, mass and speed.

• Alpha (α): Helium nucleus with charge +2 and a large mass. It moves relatively slowly and interacts strongly with air molecules, so it has the highest ionizing power. It is stopped by a sheet of paper or a few cm of air.
• Beta (β): Fast electron with charge -1 and a small mass. It has moderate ionizing power and is stopped by a few mm of aluminium.
• Gamma (γ): Electromagnetic wave with no charge and no mass. It has the lowest ionizing power but the highest penetrating power. It is reduced by thick lead or concrete.

Solution:

Ionizing power: Alpha > Beta > Gamma

Arranging in increasing order:

Gamma (II) < Beta (III) < Alpha (I)

That is: II < III < I

Check against the options:

• A. I < III < II: this is the decreasing order, so it is wrong.
• B. II < I < III: this puts beta above alpha, so it is wrong.
• C. II < III < I: this matches our result.
• D. I < II: this is incomplete and wrong.

Exam tip: Ionizing power and penetrating power are opposites. Remember "A-B-G" for ionizing power going down (Alpha, Beta, Gamma), and "G-B-A" for penetrating power going down. Always check whether the question asks for increasing or decreasing order, because that is where students lose marks.