Scientists call these different variations of the same element isotopes of each other. Radioactive refers to the characteristic that these isotopes are unstable and tend to fall apart.
The Earth orbits the Sun in about one year's time, the Earth rotates on its axis every 24 hours, 60 ticks of the second hand on a clock indicates 1 minute has passed.To take it a step further, once only 1/4 of the original amount of K-40 isotopes are left (half of the half left over after 1.25 billion years), geologists can say that 2.5 billion years (double the half-life time) have gone by.Now, can you predict how much time has gone by if only 1/8 is left?Geologists who want to date objects are interested in the isotopes that change identity as they undergo radioactive decay.
In other words, they change their number of protons during radioactive decay and turn into a different element.
Geologists use those radioactive isotopes to date volcanic ash or granite formations like the giant Half Dome in Yosemite National Park.