It is a colorless, odorless, tasteless, non-toxic, inert , monatomic gas , the first in the noble gas group in the periodic table. Helium is the second lightest and second most abundant element in the observable universe hydrogen is the lightest and most abundant. Its abundance is similar to this in both the Sun and in Jupiter. This is due to the very high nuclear binding energy per nucleon of helium-4 , with respect to the next three elements after helium.
Isotopes are various forms of an element that have the same number of protons but a different number of neutrons. Some elements, such as carbon, potassium, and uranium, have multiple naturally-occurring isotopes. Isotopes are defined first by their element and then by the sum of the protons and neutrons present. While the mass of individual isotopes is different, their physical and chemical properties remain mostly unchanged. Isotopes do differ in their stability. Carbon 12 C is the most abundant of the carbon isotopes, accounting for Carbon 14 C is unstable and only occurs in trace amounts.
Environmental radioactivity is produced by radioactive materials in the human environment. While some radioisotopes , such as strontium 90 Sr and technetium 99 Tc , are only found on Earth as a result of human activity, and some, like potassium 40 K , are only present due to natural processes, a few isotopes, e. The concentration and location of some natural isotopes, particularly uranium U , can be affected by human activity.
Radiometric dating , radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon , in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay. Together with stratigraphic principles , radiometric dating methods are used in geochronology to establish the geologic time scale. By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change.