Carbon is one of the most important elements to life on planet Earth.
When 50g remain (and 50g have become something different), the amount of time that has passed is the half-life. That is a long time to wait for radioactive atoms to change, and many of the things that the original atoms change into are ALSO radioactive and dangerous!
Alpha particles are positively charged, beta particles are negatively charged, and gamma particles have no charge.
The radiations also have increasing levels of energy, first Alpha, then Beta, and finally Gamma, which is the most energetic of all these. When a radioactive nucleus changes, the remaining nucleus (and atom) is not the same as it was. The term half-life describes the time it takes for half of the atoms in a sample to change, and half to remain the same.
The total mass of the isotope is indicated by the numerical superscript.
While the lighter isotopes C has decayed that what remains can no longer be measured. In 5,730 years half of the C in the atmosphere, and therefore in plants and animals, has not always been constant.
Here is an easy-to understand analogy for your students: relative age dating is like saying that your grandfather is older than you.