Common Half Life Mistakes to Avoid

Mistake One: Mixing Time Units

Entering a half life in years and an elapsed time in days. The arithmetic still produces a number, which is what makes this dangerous. Convert both to the same unit before entering anything.

Mistake Two: Assuming Linear Decay

Treating it as losing a fixed amount per half life rather than losing half of what remains. If 100 decays to 50, the next value is 25, not 40. The mistake compounds badly over several half lives.

Mistake Three: Expecting Zero

Expecting the sample to empty after a number of half lives. Exponential decay approaches zero asymptotically and never reaches it, so any question asking when it hits zero has the answer never.

Mistake Four: Using the Half Life as the Decay Constant

In the exponential form the rate constant is lambda, which equals ln(2) divided by the half life. Using the half life directly in place of lambda produces an answer wrong by a factor of about 0.693.

Mistake Five: Subtracting Half Lives From a Fixed Amount

Removing the same absolute quantity each half life rather than halving the remainder. Check that after each step the value is exactly half the one before, which catches this immediately.

Mistake Six: Quoting More Precision Than Exists

Reporting a carbon 14 age to the nearest year when the method realistically resolves to decades. The half life may be known precisely, but the measurement and the calibration curve are not.