Radiometric dating methods are the strongest direct evidence that geologists have for the age of the Earth.All these methods point to Earth being very, very old -- several billions of years old.

In electron absorption, a proton absorbs an electron to become a neutron.

In other words, electron absorption is the exact reverse of beta decay.

Some nuclides have very long half-lives, measured in billions or even trillions of years.

Others have extremely short half-lives, measured in tenths or hundredths of a second.

In alpha decay, the radioactive atom emits an alpha particle.

An alpha particle contains two protons and two neutrons.

Some, however, are unstable -- given time, they will spontaneously undergo one of the several kinds of radioactive decay, changing in the process into another element.

There are two common kinds of radioactive decay, alpha decay and beta decay.

I found several good sources, but none that seemed both complete enough to stand alone and simple enough for a What is radiometric dating?