Helium: abundance 116 properties

Tlie most abundant helium atoms. 4He. are bosons, but the He atoms are fermions. This has a consequence that liquid He docs not show superlluidiiy—a property very probably connected with the Bose-Einstcin statistics obeyed by the 4 He atoms. 

Tritium (tm = 12.3 y) is used to date relatively recent samples in a technique similar to carbon-14 dating. For example, tritium is used to determine the age of groundwater. The ratio of and 3H abundances in rain and snow is stable over time. However, once the water seeps into the ground, it cannot mix with the water in the atmosphere, and the tritium nuclei that decay are not replaced. To determine the age of groundwater, we can measure the concentrations of both tritium and its daughter nuclide, helium-3. The age of the groundwater can help determine properties such as the depth of its source. 

Fig. 13. Energy levels of nuclei participating in thermonuclear reactions during the helium burning stage in red giant stars (after [13]). Survival of both 12C and 16O in red giants, from which terrestrial abundances result, depends upon the fortuitous circumstances of nuclear level structures and other properties of these nuclei...

The number of protons is fixed for each given element and determines what element a nucleus forms. For example, helium [He] always has two protons (Fig. 1.1), lead [Pb] always has 82 protons, and uranium [U] always has 92 protons. In contrast, the number of neutrons in the nucleus of an element can vary. Atoms of a given element with different numbers of neutrons are called isotopes. Because neutrons have mass, the atomic weight of isotopes of the same element varies. Even so, isotopes have the same chemical properties as the base element. For example, in lead there are four common isotopes. In order of abundance, the common isotopes of lead are [the superscript indicates that overall atomic weight (protons plus neutrons) of the atoms] Pb (52.4%), Pb e (24.1 %), Pb o (22.1 %), Pb O (1.4%). All Pb atoms have 82 protons, but Pb ° has 126 neutrons in the nucleus, while Pb ° has 122 neutrons in its nucleus. Despite having different atomic weights, the chemical properties of all lead isotopes are identical. Thus, a solid block of Pb ° would be chemically indistinguishable from a block of Pb °. ... 

---