Recap - Helium from Rocks

Helium: the gas that comes from rocks

Helium is the second element of the periodic table: every helium atom has 2 protons and 2 electrons. It is the second lightest element, and in the universe it is the second most common, after hydrogen. On Earth, though, it is rare, and almost all of it comes from inside rocks.

Rocks hold small amounts of uranium and thorium. Their nuclei are unstable and decay by throwing out α particles. An α particle is a helium nucleus: 2 protons and 2 neutrons, with no electrons. Trapped in the rock, each α particle soon picks up two electrons and becomes a helium atom. Over millions of years this helium seeps through the rock and collects underground, often together with natural gas, which is where we take it from.

The two isotopes of helium: one full shell of 2 electrons

Helium does not react with anything: its two electrons fill its only shell. It fills party balloons and weather balloons, and liquid helium, colder than any other liquid, cools the magnets of hospital MRI scanners.

Thorium-232, one of the sources of Earth's helium, decays by giving out one α particle:

$${}^{232}_{90}\mathrm{Th} \to \mathrm{X} + {}^{4}_{2}\mathrm{He}$$

Build a neutral atom of X.