Lesson 1: Alchemy and Atomic Models Key Points: 1. Understand how theories of the structure of the atom were developed. 2. Learn the parts of the atom and their characteristics. Discovery and Development of the Atomic Theory Part I: Alchemy and the birth of Chemistry Long before the science of chemistry existed, people made use of chemical reactions to dye cloth, tan leather, and prepare foods. Eventually people began to search for explanations for the structure and behavior of matter. Discovery and Development of the Atomic Theory One of the earliest “Atomists” was the Greek philosopher Democritus, he proposed that matter was made of indivisible particles called atoms. His ideas were overshadowed however by a more famous philosopher, Aristotle, who proposed that all matter was made of four elements, earth, air, water and fire. As a result the idea of atoms was not discussed seriously for 2000 years. Discovery and Development of the Atomic Theory During the middle ages chemistry reappeared because of the rise of a group called the alchemists, whose goal was to change common substances into gold. Although they were never successful in this quest they did make many important discoveries that helped to develop science. They produced many experimental procedures and laboratory apparatus. Their experiments yielded a wealth of knowledge about the characteristics of substances. Discovery and Development of the Atomic Theory Later the contributions of Roger Bacon and Antione Lavoisier changed chemistry from a trial and error process of observation to a science of measurement, and paved the way for modern chemistry and the atomic theory. Discovery and Development of the Atomic Theory Suppose we take a small cube of the element lead and cut it into smaller and smaller pieces. As the soft grey metallic pieces get smaller and smaller they still retain the properties of lead. Eventually we would reach a point when the particle of lead could no longer be divided and still retain its properties. This particle is an atom the smallest particle of an element that retains the properties of that element. • It was not until the late 1700s that chemists were able to relate chemical changes to events at the level of individual atoms. At that time the English chemist John Dalton first stated his atomic theory. Dalton’s Atomic Theory included the following ideas: 1. All elements are composed of tiny indivisible particles called atoms. 2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. 3. Atoms of different elements can combine with one another in simple whole number ratios to form compounds. 4. Chemical reactions occur when atoms are separated, joined, or rearranged. However atoms of one element are not changed into atoms of another by a chemical reaction. This theory has been generally accepted over the years although we now know that an atom can be broken down into smaller particles. Dalton believed in the “solid sphere model” which said that an atom is nothing more than an infinitely small solid sphere. Later in 1897 J.J. Thomson discovered that atoms contained small negatively charged particles called electrons. So he modified Dalton’s model into what he called the “Plum Pudding model” which said that an atom was a solid sphere with small negatively charged particles embedded in it like plum pudding had pieces of plum in it. Shortly thereafter, Protons were discovered, this was due to the fact that all chemists agreed that atoms had to be electrically neutral, so if there was a small negatively charged particle there had to be a positively charged particle as well, this positively charged particle is many times larger than the electron and is called the Proton. In 1932 James Chadwick discovered that Protons were not the only large particle in the atom. He found that the mass of many elements was too large to be accounted for by only protons and electrons. His experiments proved the existence of the third and final subatomic particle the Neutron which carries no charge or is neutral. Further to this discovery a scientist named Ernest Rutherford felt that atoms could not have all these particles floating around randomly, and that the plum pudding and solid sphere models were not accurately explaining the findings of later experiments. So he conducted an experiment where he bombarded a thin sheet of gold foil with alpha particles. If the protons, neutrons and electrons were simply floating about (which was the current theory) then the alpha particles should pass through the foil unhindered. To the shock of Rutherford and his associates many of the alpha particles bounced back. Rutherford was so impressed that he was quoted as saying “it was like firing a 15inch shell at a piece of tissue paper and having it bounce back at you!”. This led to the development of the Rutherford model of the atom which said the atom was made of a dense central nucleus where the heavy particles (protons and neutrons) were located and a the electrons surrounded this nucleus. Later experiments by Neils Bohr showed this to be mostly correct except that the electrons are not positioned randomly they can be found orbiting the nucleus at fixed distances or energy levels. Further to this Louis de Broglie surmised that electrons could not be found in fixed positions, but did show evidence of distinct energy levels, so he combined the Rutherford nuclear modeland the Bohr model into what is known as the “electron cloud model”. In de Broglie’s model the electrons can jump or fall from one level to another depending on the energy of the atom. This is what allows for “glow in the dark” fluorescent type materials, electrons in the atom gain energy from light and are excited to a new level. When the light is turned off they slowly return to their previous state, releasing their energy in the form of light. The de Broglie model is currently the accepted model. The structure of the atom, as we currently understand it is that there is a dense positively charged nucleus that contains the neutrons and protons, and there is a large volume of space surrounding the nucleus in which the electrons can be found. So in fact atoms are made up of a lot of empty space! So if all atoms are made of protons neutrons and electrons, how can the atoms of one element differ from another? The answer is that all elements have different numbers of protons in their nucleus, Carbon atoms, for example, have 6 protons while lead atoms have 82 protons, and fluorine atoms have 9 protons. These differing numbers of subatomic particles influence the properties of the different elements.