Which of the following statements correctly describe the characteristics of radioactive decay?

1. The decay constant $\lambda$ represents the probability of decay per unit time for an individual nucleus, and its value is unaffected by whether the isotope is in elemental form or within a chemical compound.
2. The total mass of a radioactive sample decreases by exactly 50% after a period of time equal to one half-life has elapsed.
3. A graph of the natural logarithm of the sample's activity, $\ln (A)$, against time, $t$, is a straight line with a gradient equal to $-\lambda$.