## Relative vs absolute time

One of the biggest problems that geologists have when trying to piece together the history of our planet is figuring out how old different rocks are. Before the twentieth century, the best that they could do was to come up with a relative age date. A relative age date can help estimate which rock is older when it is compared with another, but it does not tell how old the rock actually is.

To understand how radioactivity can change one type of atom into another, we must first look at what an atom is made of. Atoms have two main parts. First, in the center of the atom, is the nucleus. This is where most of the mass of an atom is located. The nucleus is made up of two different particles: protons, which have a positive charge, and neutrons, which have no charge. The second main part contains even smaller particles that move around the outside of the nucleus: These particles are called electrons. Electrons have a negative charge, and they produce electricity.

Each type of element has its own special atom with a different number of protons in the nucleus. For example, an oxygen atom has eight protons in its nucleus, while a helium nucleus has only two protons. Scientists have discovered over 100 different elements, each with a different number of protons in the nucleus. As it turns out, many of the elements that have the largest atoms are radioactive. For example, the most common form of the element uranium is called U-238. It has 92 protons and 146 neutrons in its nucleus. All of these particles jammed together in such a small space means that the uranium nucleus is very unstable.

Every so often, some of the protons and neutrons break up and the nucleus will eject the particles as different types of radiation. When this happens, scientists say that the element undergoes radioactive decay. This happens when a neutron of an atom changes to become a proton. As a result, the atom changes into the atom of a different element. A neutron of an atom can also change to become a proton. These types of changes are called beta decay.

Most radioactive elements actually decay several times, changing in a step-like fashion until they finally turn into an atom that is stable. This is called a decay series and it happens in a very predictable manner.

Using the principles described by Charles Lyell and several other geologists, scientists working in the eighteenth and nineteenth centuries did some remarkable work. By using fossils to link up