17.3Types of Radioactivity: Alpha, Beta, and Gamma Decay

The idea

• Write alpha, beta, gamma, and positron equations.
• Balance both mass number and atomic number.

Alpha (α) radiation is a helium nucleus, ⁴₂He. The parent loses 4 from the mass number and 2 from the atomic number. Beta (β) radiation here means an electron from the nucleus, ⁰₋₁e, created as a neutron becomes a proton. Mass number stays the same and Z increases by 1. Gamma (γ) radiation is high-energy light, ⁰₀γ. It changes neither A nor Z; it often follows another decay that left the nucleus excited. A positron is ⁰₊₁e. Emitting one turns a proton into a neutron, so Z decreases by 1 and A stays the same.

Write the equation so superscripts balance and subscripts balance. ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He. ¹⁴₆C → ¹⁴₇N + ⁰₋₁e. If a problem gives the parent and the mode, you can name the daughter from the periodic table once you know the new Z. Penetration follows the particle: alpha is stopped by paper or skin, beta by plastic or metal sheet, gamma by thick lead or concrete. Damage if the source is inside the body is a different ranking, because alpha dumps all its energy in a tiny distance.

Worked path

Write the equation for positron emission from ¹¹₆C.

1. Observe

   A positron is ⁰₊₁e. A stays 11.

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