Define half-life of a radioactive isotope.
Radioactive isotope X decays to isotope Y. A sample contains only nuclei of X at time 
. The graph below shows the variation with 
of the numbers of nuclei of X and of Y as the sample decays.
State the name of the quantity represented by the magnitude of the gradient of line X in the graph above.
State three conclusions about X or Y that may be drawn from the graph. The conclusions may be qualitative or quantitative.
The mass of radioactive isotope X in the sample in (b) is 
at time . Determine the nucleon number of isotope X.
.
