Why are unstable isotopes useful for dating rocks
This may simply have to do with what the media is talking about.When there is a scientific discussion about the age of, say a meteorite or the Earth, the media just talks about the large numbers and not about the dating technique (e.g. On the other hand, when the media talk about "more recent events," ages that are more comprehendible, such as when early Man built a fire or even how old a painting is (or some ancient parchment), then we bring up the dating technique in order to better validate the findings.We have an activity in one of the PSI workshops "Exploring the Terrestrial Planets," that deals with this topic.So, you can use the radioactive elements to measure the age of rocks and minerals. Their useful range is from about 1/10 their half-life (the time it takes for half of the radioactive element/isotope-- the parent, to convert into a non-radioactive element/isotope-- the daughter) to 10 times their half-life. You can use this to measure the age of a rock from about 128 million years to more than 10 billion years (the Solar System is 4.56 billion years old).
The number of protons usually determines the element the atom belongs to and it is fixed for any particular element.These differing atoms are called isotopes and they are represented by the sum of protons and neutrons in the nucleus. Carbon has 6 protons in its nucleus, but the number of neutrons its nucleus can host range from 6 to 8.We thus have three different isotopes of carbon: Carbon-12 with 6 protons and 6 neutrons in the nucleus, Carbon-13 with 6 protons and 7 neutrons in the nucleus, Carbon-14 with 6 protons and 8 neutrons in the nucleus.We have rocks from the Moon (brought back), meteorites, and rocks that we know came from Mars.We can then use radioactive age dating in order to date the ages of the surfaces (when the rocks first formed, i.e. We also have meteorites from asteroids and can date them, too.