Under a starry night, in ancient Greece, philosophers looked to the heavens and were inspired to develop a cosmology. During the day, they scoured the Earth to gain an understanding of natural philosophy - what we would call chemistry, physics, and biology.
While this was a good start in the long line of events and thinking that eventually got us to our modern world, there is much that they got wrong. But there is also much that they got right - or, at least, led us in the right direction.
In particular, ancient Greeks believed that the world was made up of four essential elements - earth, wind, water, and fire. This philosophy explained that everything - from a rock to a tree to you and me - was made up of varying components of these four essential elements.
More to the point, they believed that transmutation could occur by adjusting the relative levels of these elements in any material. For example, lead could be turned to gold by increasing the level of fire. After all, fire shines and gold is shinier than dull lead.
As modern chemistry developed, scientists realized that the notion of four essential elements was incorrect. There are many more than four and substances such as water, air, and earth are composed of molecules generated by combining different elements. It took almost 200 years, but from the early 1700s to the late 1800s, chemists filled in most of the known elements.
Indeed, enough was known about the physical and chemical properties of the elements by the mid-1800s, that Mendeleev was able to synthesize the information into a periodic table of elements. He even recognized that there were elements missing and left blanks in his table for when the new elements would be discovered.
The validity of his approach was assured when elements bearing the chemical and physical characteristics that he had predicted were found and the blanks in the table were filled in. His periodic table even withstood the addition of an entire new group of elements in the 1890s as the Noble gases were isolated.
By the turn of the 20th century, there was a feeling that the periodic table provided a good picture of the known elements and any that were not known would soon be discovered. But the shape of the periodic table was tantalizing.
Why were there only two elements in the first row but eight in the second? Why do the elements in a group bear similar chemical properties but still differ enough that they can be separated one from another? What was the underlying basis?
Further, in the 1890s, Henri Becquerel, Pierre and Marie Curie, and others discovered that the elements could undergo spontaneous transmutation through radioactivity. Naturally occurring isotopes of uranium, thorium, and even radium spontaneously disintegrated at measureable rates. The heavy elements were unstable.
With the discovery of the nucleus by Rutherford, the arrangement of the elements based on the number of protons in the nucleus as determined by Moseley, and the eventual detection of the neutron by Chadwick, a theory of nuclear structure began to emerge.
All elements except hydrogen have nuclei consisting of protons and neutrons. Indeed, two of the isotopes of hydrogen - named deuterium and tritium - contain one and two neutrons in their nucleus, respectively. The extra neutron is what makes D2O heavy water compared to H2O.
The rest of the elements in the periodic table have two or more protons in their nucleus. It is the number of protons that determines the identity of each element. Oxygen, for example, is oxygen because it has 8 protons. All oxygen atoms have exactly 8 protons.
But the number of neutrons can vary. Most oxygen atoms have 8 neutrons, giving rise to oxygen-16 as the most common isotope for oxygen. But there are also oxygen-15, oxygen-17, and oxygen-18 isotopes with, respectively, 7, 9, and 10 neutrons.
As we progress down the periodic table or to more protons, the force of repulsion between the protons in the nucleus increases and it takes more and more neutrons to generate a stable isotope. Iron has 26 protons but 30 neutrons in its most stable isotope. Iodine-126 has 53 protons but it requires 73 neutrons to hold the nucleus together.
By the time that we get to uranium-238, the number of protons (92) is swamped by the number of neutrons (146). While this makes the atom incredibly heavy, it also makes the nucleus unstable with the result that the element undergoes natural radioactive decay.
The result is a series of daughter elements, each generated at a characteristic rate in a radioactive decay sequence, and eventually in the production of lead. It is hard to turn lead into gold but uranium turning into lead is a naturally occurring process.
Just not one that involves earth, air, water, or fire.
