Gold and lead are both forged in supernovae and are but a few subatomic particles apart in the periodic table. So why is there so much more of one than the other?
• There are several reasons. First, atomic nuclei are bombarded with neutrons inside supernovae, causing the nuclei to grow bigger and bigger. Some of the neutrons turn into protons, giving rise to elements higher and higher in the periodic table. Four of the stable nuclei are lead (Pb), but only one is gold (Au).
Furthermore, nuclei with an even number of protons and an even number of neutrons are more stable than those with an odd number of one or the other. Lead has 82 protons, whereas gold has 79. The lead isotopes Pb-204, Pb-206, and Pb-208 also have even numbers of neutrons. That means their potential energy is lower than one would otherwise expect, and therefore more of those isotopes get produced than those with an odd number of protons or neutrons, like the stable isotope of gold, Au-197.
Advertisement
In addition, many elements heavier than lead are produced in supernovae, but then decay into stable Pb-206, Pb-207 or Pb-208 – creating even more lead.
All of these explain why there is a lot more lead than gold in the solar system. In Earth’s crust, there is another factor. Gold is a siderophile, meaning it associates with iron. When our planet was a largely molten mass, the gold mostly went with the iron and settled into Earth’s core.
Eric Kvaalen, Les Essarts-le-Roi, France
• Supernovae aren’t the only source of lead in the universe, for this reason: lead is the last stable element in the periodic table. This means that all elements heavier than lead will undergo radioactive decay. There are various chains in which an atom of one element will release radiation and become another (radium can emit an alpha particle and become radon, for example). The most common of these chains are the thorium, radium and actinium series, all ending with stable, unreactive and frankly boring lead.
These decay chains occur constantly in Earth’s crust, continuously replenishing reservoirs of lead. This helps explains why it is so much more abundant than gold.
Incidentally, this transmutation of elements is precisely the sort of thing sought by alchemists of past centuries. Unfortunately, instead of expensive, shiny gold, it gives us dull, cheap lead, which doesn’t exactly gleam on the finger of those to be betrothed.
Bradley Clarke, Brighton, UK
• For a long time, astrophysicists believed that heavy metals such as gold or lead were forged in exploding supernovae. However, observations of gamma-ray bursts by a NASA telescope have now confirmed an entirely different theory, which is that our universe’s metals are created when large, extremely dense neutron stars in orbit around each other collide.
It takes billions of years for such collisions to occur, but when they do, they release huge amounts of energy and create vast quantities of gold, lead and platinum, with platinum in larger quantities than lead.
Nadia Chigmaroff, West Vancouver, British Columbia, Canada
We pay £25 for every answer published in ¿ìè¶ÌÊÓƵ. To answer this question – or ask a new one – email lastword@newscientist.com.
Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.
¿ìè¶ÌÊÓƵ retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.
You can also submit answers by post to: The Last Word, ¿ìè¶ÌÊÓƵ, 25 Bedford Street, London WC2E 9ES.