Absolute dating

Isotopes Most atoms are stable and do not change. Some are unstable, constantly releasing heat as their nuclei break apart or decay. Isotopes are atoms of the same element that have different mass numbers but the same atomic number. Review atomic structure.

Radioactivity The forces that bind neutrons and protons is very strong. Some isotopes have unstable nuclei; the forces that bind protons and neutrons is not as strong. As a result, the nuclei spontaneously break apart or decay in a process known as radioactivity.

What happens when they break apart? Alpha particles may be emitted from the nucleus. An alpha particle is composed of 2 protons and 2 neutrons. This reduces the mass number by 4, and the atomic number by 2. When a beta particle, or electron, is given off from a nucleus, the mass number remains the same. Since the electron must have come from a neutron (a neutron is a combination of a proton and an electron), the nucleus contains one more proton than before, therefore the atomic number increases by one. Diagram of alpha and beta particles being emitted.

Why is radioactive dating reliable? Radiometric dating is reliable to calculate ages of rocks/minerals which contain radioactive isotpoes because of the rate at which radioactive isotopes decay is constant and unaffected by any physical or chemical processes.

Half- life The amount of time required for one-half of the nuclei in a sample to decay (it’s half-life) is a common way of expressing radioactive degeneration. eg – if we began with 1kg of radioactive material, ½kg would decay after one half-life, half of the remaining amount would decay after another half-life etc. Diagram of half-life and table of frequently used isotopes.

Radiocarbon dating To date more recent events, Carbon-14 (radiocarbon) is used. It has a half-life of 5730 years. C-14 is continuously produced in the atmosphere as a consequence of cosmic ray bombardment (cosmic rays shatter the nuclei of gases to release neutrons). The neutrons are absorbed by Nitrogen (atomic number 7, mass number 14) causing the nucleus to emit a proton, therefore the atomic number drops by one (to 6) and C-14 is formed (a new element).

How is C-14 useful? The isotope is quickly incorporated into carbon dioxide, circulates in the atmosphere, and is absorbed by living matter. As a result, all organisms contain a small amount of C-14. While the organism is alive, the decaying radiocarbon is continually replaced. As a result, the C-14 to C-12 ratio remains constant. When an organism dies, the amount of C-14 gradually decreases as it decays to N-14 by beta emission. Therefore, by comparing the proportions of C-14 and C-12 in a sample, radiocarbon dates can be determined. C-14 decay

Absolute dates Radiometric dating results in specific dates for rock units which represents various events in the earth’s distant past. We can now state with some confidence that particular geologic events took place a certain number of years ago. Such dates are referred to as absolute dates.

Relative dating This refers to putting rocks into their proper sequence. It doesn’t tell us how long ago something happened, just that it followed one event and preceded another. Absolute dating and relative dating supplement each other.