From there, we can begin to calculate the age of the earth.
Let’s do a rough calculation to show how this works.
The number of parent atoms originally present is simply the number present now plus the number of daughter atoms formed by the decay, both of which are quantities that can be measured.
Samples for dating are selected carefully to avoid those that are altered, contaminated, or disturbed by later heating or chemical events.
Together with stratigraphic principles, radiometric dating methods are used in geochronology to establish the geological time scale.
For this particular element, the half-life is 4.47 billion years and uranium/lead dating is useful for rocks between 1 million and 4.5 billion years (as luck would have it! Other elements are also used (potassium/argon for example).
The choice of element depends on how widespread it is - if it isn't found in many rocks then it's not very useful.
And how long the half-life is - if the half life is shorter than the age of most rocks then its equally unuseful.
Radiometric dating can only be performed on igneous rocks.