5. Radioactive decay

Beta Decay

Cartoon showing beta decay.

Cartoon showing beta decay.In beta decay, one of the neutrons in the nucleus suddenly changes into a proton, causing an increase in the atomic number of an element. Recall the name of an element is determined by its atomic number. Carbon is carbon because it has an atomic number of 6, while nitrogen is nitrogen because it has atomic number 7. That means that a reaction that changes the number of protons in the nucleus changes what element we actually consider the nucleus to be. This makes beta decay a great example of how nuclear reactions can eerily transform one substance into another.The product potassium chloride is commonly sold as a salt-substitute in grocery stores. This product contains trace amounts of potassium-40 (K), which tends to undergo beta decay into calcium-40 (Ca). Symbolically, this reaction looks like:

K→ Ca + e^-− + v

In addition to changing its atomic number, the nucleus creates and releases an electron (e-) from the atom that serves to counterbalance the positive charge it gained by transforming a neutron to a proton. These emitted, free electrons are the “radiation” associated with beta decay. The other released particle v is a mysterious particle called an antineutrino, which has no charge and barely any mass.

This means that if you were to go to the grocery store and buy a jar of potassium-40 isotopes (which are prone to beta decay) and then leave it sitting on your countertop for a couple of years, you would end up having less potassium than you started out with (calcium would take its place). This process happens incredibly slowly and in miniscule numbers for the potassium chloride available in the grocery store, and so the actual health risk posed by this radiation is nil.

A related type of beta decay actually decreases the atomic number of the nucleus when a proton becomes a neutron. Due to charge conservation, this type of beta decay involves the release of a charged particle called a “positron” that looks and acts like an electron but has a positive charge. Because this particle’s interactions with other tissues are easily identifiable, some medical imaging techniques involve purposefully injecting a patient with an element that beta decays into positrons, and then monitoring where the positrons are emitted. When beta decay creates a positron it’s called beta-plus decay, and when it creates an electron it’s called beta-minus decay.

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