2 half-lives elapsed, leaving 25% of the original sample.
Each half-life cuts the amount in half: after 1 you have 50%, after 2 just 25%, after 3 only 12.5%.
The decay constant λ and mean lifetime τ are both fixed properties of the nucleus. λ = ln(2) / T and τ = T / ln(2) ≈ 1.4427 T.
Decay follows N = N₀ × e^(-λt) equivalently, which is the basis of the exponential decay curve shown in the chart.
Radiocarbon dating uses the Carbon-14 half-life of 5,730 years: measuring the ratio of remaining C-14 to the original amount reveals the sample age via t = T × ln(N₀/N) / ln 2.
Next stepTo find the activity of a radioactive source in Becquerels, multiply the atom count by the decay constant: A = N × λ.