Random nature of decay  constant decay probability for a given nucleus
Use of activity, A = λN
Questions may be set which require students to use
A = A_{0}e^{−λt}
Questions may also involve use of molar mass or the Avogadro constant
Halflife and decay constant and their determination from graphical decay data including decay curves and log graphs;
Applications, e.g. relevance to storage of waste radioactive materials; radioactive dating

Recall that radioactive activity is:
spontaneous (probability of a decay cannot be changed by changing pressure/temperature etc.)
random  direction that ray is emitted or which nucleus will decay when  is impossible to say! But when dealing with large numbers of atoms the mathematics of probability can be employed.
Recall that radioactive decay is an exponential process therefore probability is applicable!
You should be able to
sketch a graph of number of nuclei against time (marking on multiples of N_{o} and T (half) to show th exponential relationship)
recall that the rate of decayN/t (or Activity of the sample) is proportional to the number of atoms of the radioisotope present in the sample.
Recall that the decay constant () is the constant of proportionality between activity and sample size. It gives a measure of the probability that a particular nucleus in a sample will decay in a given time. It has units of time^{1}.(The negative sign before it is because the number present decreases with time  so that would lead to a negative constant!)
recall that the half life of a radioisotope is the time taken for half of the radioactive nuclei of that isotope substance in a sample to decay. This is constant for any given isotope. The units are those of time.
appreciate the variation in natural halflifes from nanoseconds to millions of years and the affect this has on uses and safeguards when disposing of waste.
know about carbon dating.
Now try some questions  I suggest Q4, Q6, Q8 and Q9 from this link
MS 1.3, 3.10, 3.11 / PS 3.1, 3.2 Investigate the decay equation using a variety of approaches (including the use of experimental data, dice simulations etc) and a variety of analytical methods. 