Hi, I'm Donald Sinclair. I'm a science teacher with Greater London Tutors and today, we're going to be looking at a few topics in Chemistry. The Periodic Table is one of the most famous charts in history, certainly in scientific history. You can think of it as an ingredient list for the universe. Everything that exists, or has existed, or will ever exist, consists of atoms taken from the periodic table. The Periodic Table is made of elements. The idea of elements traces back to ancient Greece. Aristotle proposed that there are four elements that make up everything on earth: Earth, Fire, Wind and Water, as well as a fifth element, the ether, of which supposedly the heavens were made. In 1789, the French scientist Antoine Le Voissier came up with a list of 33 elements. These elements were categorised according to a new definition of element. An element then was thought of as any substance that could not be broken down into a simpler substance. Water for example, can be broken down into hydrogen and oxygen and therefore, was not an element. Hydrogen and oxygen were because they could not be broken down any further into simpler substances. In 1858, the German scientist August de Coolie noticed that set amounts of certain chemicals reacted with set proportions of other chemicals. For example, carbon would react with four times as much hydrogen. This idea of proportions of chemicals reacting with each other, eventually gave rise to the idea of the valency of certain chemical elements. In 1864, another German scientist, Julius Loath Armeer, arranged all the known chemical elements in order of valency. It was found that chemicals with similar valencies also had very similar chemical properties. This turned out to be the basis of modern chemistry, the idea that valency dictates chemical properties. This apparent periodicity led an English Scientist John Newlands to suggest the idea of octaves. Similar to a piano keyboard where seven notes repeat themselves over and over, he suggest that the elements would repeat themselves in set periods. This idea was roundly mocked when he brought in the idea of octaves, with one fellow scientist asking him whether he could play his elements like a piano. The periodic table as we would recognise it nowadays was first drawn up in 1869, by the Russian scientist Dmitri Mendeleev. He had two strokes of genius which allowed the periodic table to exist in a form which you and I would recognise. The first one was that he left gaps in the table for as yet undiscovered elements. He was even able to predict the chemical properties of these elements when they were discovered and that led to a great deal of new discoveries. The second, was that he rearranged the order of the elements slightly. Instead of putting them purely in order of atomic weight, he moved the elements one space, or two spaces to either side, so that their chemical properties would coincide with the other elements in this group. Without knowing, he was arranging all of the elements in order of atomic number, which is how we arrange the elements in the Periodic Table today. The Periodic Table continues to grow. New elements are being synthesized in laboratories. The last gap to be filled in which Dmitri Mendeev left, was francium, which was finally discovered in 1939. The first synthetic element neptunium was created by bombarding uranium with neutrons in 1939. The periodic table is a great example of the power of science to make prediction based on deductions and observations. .