Some of the atoms eventually change from one element to another by a process called radioactive decay.
Even though the Earth's age is never mentioned in the Bible, it is an issue because those who take a strictly literal view of the early chapters of Genesis can calculate an approximate date for the creation by adding up the life-spans of the people mentioned in the genealogies.
Assuming a strictly literal interpretation of the week of creation, even if some of the generations were left out of the genealogies, the Earth would be less than ten thousand years old.
Radiometric dating techniques indicate that the Earth is thousands of times older than that--approximately four and a half billion years old.
Many Christians accept this and interpret the Genesis account in less scientifically literal ways.
When the glass is turned over, sand runs from the top to the bottom.
Radioactive atoms are like individual grains of sand--radioactive decays are like the falling of grains from the top to the bottom of the glass.
It has become increasingly clear that these radiometric dating techniques agree with each other and as a whole, present a coherent picture in which the Earth was created a very long time ago.
Further evidence comes from the complete agreement between radiometric dates and other dating methods such as counting tree rings or glacier ice core layers.
Most processes that we are familiar with are like sand in an hourglass.
In exponential decay the amount of material decreases by half during each half-life.
Most of the elements in nature are stable and do not change.