Radium spontaneously decays into helium and radon. Why is radium regarded as an element rather than a chemical compound of helium and radon?

Answer 1

Single Nucleus

The reason we don't see radium as a compound is because in radium (226Ra) where everything starts, all the protons and neutrons are bound into one nucleus - the 226Ra nucleus. There are no distinct atomic identities - just the one nucleus. It is an unstable nucleus, but everything is stuck together (with binding energy or nuclear glue) until the instability "wins" and the nucleus decays. It is at that time that those two individual identities appear. The alpha particle, which is He-4 with its two protons and two neutrons, breaks out of the nucleus and leaves 222Rn behind. Alpha decay of 226Ra has occurred.

Radium is not considered a chemical compound of helium and radon because the reaction that splits radium into helium and radon is a radioactive decay event . The nucleus of the radium atom splits into two chunks, one little chunk with 2 protons (the helium) and one with 86 protons (the radon). Additionally, a chemical compound is formed by chemical bonds, and not by "nuclear stuff" as is the case here. In chemical compounds, the atomic nuclei of the component atoms remain distinct features of the molecule formed by the chemical bond or bonds.

History and etymologyThe words "atom" (Greek for "uncuttable"), and the use of "element" to mean a chemical with one kind of atom only, date back long before we knew atoms had internal structure. For the purposes of nuclear physics it is no longer appropriate to call them "uncuttable", but the name has stuck. Besides, for the purposes of chemistry and most of physics, atoms are uncuttable. It takes a lot of energy to break them. Another aspect to the decayRadium does not always decay into helium and radon, which it does by alpha decay. It can also undergo negative beta decay, in which case the daughter is actinium and a negative beta particle is emitted. And it can also decay by positive beta decay, in which case the daughter is francium, and a positive beta particle is emitted along with an electron antineutrino.

The point, of course, is that the nucleus of an atom cannot be viewed in terms of combinations of other nuclei in a way analogous to chemical compounds.