Gold Part Two…

[This is follow up to Gold Part One.]

A pupil asked…

Will the earth naturally produce more gold for us to extract? How long will this take? How long is the half-life of gold and what does it decompose into? Would it be possible to industrially produce gold from different atoms?

A. Pupil Esq.

When you understand where the gold on Earth came from, these questions can answer themselves. To explain this we need to go a little way back in time, to the Big Bang, about 13.8 billion years ago. From about 3 seconds after the initial expansion of space and time, and until stars formed some 300 million years later, the only atoms in the Universe were hydrogen, helium and lithium; the three simplest elements on the Periodic Table.

After this time gravity was able to cause hydrogen atoms to clump together and the first stars formed. In a star, hydrogen atoms are fused into helium atoms releasing great heat and light. This heat fuels the continued nuclear fusion and leaves some spare energy to radiate out into space. A sun that is fusing hydrogen into helium is called a main sequence star.

Eventually the hydrogen in the star runs out, the temperature increases and helium atoms start to fuse together as the star expands into a red giant. This marks the beginning of the end for the star. What happens next depends on how massive the star is. Small stars like our sun will burn up the available helium and collapse into a white dwarf; a star about the size of Earth. It’s core will be mostly carbon but it will not be hot enough to fuse that carbon and continue the process.

A sufficiently massive sun will be able to produce more heat by fusing carbon atoms to make iron. The iron nucleus is very stable so the process must stop here. Depending upon the mass of the star at this point, and its surrounding environment, it may go supernova. In this enormous explosion all the other naturally occurring elements above iron in the periodic table are produced and flung out into space as part of a vast dusty cloud. It is from within this cloud that planets like Earth condense and why we find such a rich variety of different elements. All the gold found on Earth was produced during a supernova explosion, or by more than one supernova. The atoms joined others such as oxygen, silicon, iron and aluminium and were drawn together by gravity to be washed or dug out of the ground billions of years later.

People called alchemists believed that it was possible “to turn base metals into gold“. We can fuse hydrogen atoms to make helium and lots of work is being done to try to make electricity from nuclear fusion a reality. At the other end of the periodic table, we have been able to produce a small number of radioactive elements by bombarding atomic nuclei together in particle accelerators. They decay again in seconds but their presence can be detected in the debris produced in the collision. Gold can be produced in small quantities by particle accelerators colliding mercury atoms, but the process is many times more expensive to do than the value of the gold produced.

Gold is not radioactive so it does not have a half-life. The half-life of an element is the time it takes for half of its atoms to have decayed into a different element. This only applies to radioactive elements. Gold has one stable isotope called ¹⁹⁷Au (a gold atom with a mass 197 times greater than hydrogen). It does not decay. There are some radioactive forms of gold, such as ¹⁹⁵Au, which has a half-life of 186 days.

Questions…

1. The Big Bang was (a) an explosion of stuff (b) a rapid expansion of spacetime or (c) when something created the Universe. Which do you think is correct?
2. Gold is a metal. Suggest two physical properties of a typical metal.
3. Hydrogen is a non-metal. What substance is formed when hydrogen oxidises?
4. Roughly how far away is the Sun from earth in kilometres?
5. What word means (a) changing from a gas to a liquid (b) changing from a solid straight into a gas and (c) changing from a gas back into a solid?