The elements The Greeks were the first to explain why chemical changes occur. They proposed that all matter was composed of four substances: fire, water, earth, and air. All material on earth can be broken down into 100 different elements. 18 th Century Thoughts 1) Most natural materials are mixtures of pure substances 2) pure substances are either elements or a combo of elements called compounds 3) a given compound always contains the same proportion of elements Early Models of the Atom By the late 1700s, experiments carried out by French chemists Proust, Priestley, and Lavoisier suggested that matter might be made of atoms after all. After very careful experiments with precise measurements, English school teacher John Dalton combined his own ideas with those of the French chemists and proposed the first atomic theory of matter based on experimental results in 1803. Daltons Atomic Theory 1. Matter is composed of indivisible particles called atoms. Atoms can be divided. 2. All atoms of a particular element are identical. Does not account for isotopes. 3. Different elements have different atoms. YES! 4. Atoms combine in certain whole-number ratios. YES! Called the Law of Definite Proportions 5. In a chemical reaction, atoms are merely rearranged to form new compounds; they are not created, destroyed, or changed into atoms of any other elements. Yes, This is called the Law of Conservation of Mass. Exception- nuclear reactions that can change atoms of one element to a different element Law of Constant Composition The law of constant composition states that the composition of a substance is always the same, regardless of how the substance was made or where the substance is found. So, whenever we talk about water, we know that there are 2 atoms of hydrogen and 1 atom of oxygen in a molecule of water. As soon as the composition of a molecule changes, then you have a different substance with different properties. Daltons Impact For a century after Dalton made his proposal, it remained hypothetical. Many scientists doubted the existence of atoms at all. It was Einstein who observed Brownian motion in 1905 thus proved atoms existed J.J. Thomson In the late 1890s, J.J. Thomson used a cathode ray tube to show that atoms of any element can be made to emit tiny negative particles. He discovered Electrons. The Structure of the Atom The Plum Pudding Model - By J.J. Thomson Ernest Rutherford In 1910, Rutherford fired ionized helium atoms, or alpha particles, at a sheet of gold foil. To Rutherfords astonishment, some of the particles bounced back. Explanation: The particles that bounced back were striking something small and dense at the heart of the atom. The nucleus. He said positive protons were in the nucleus. Rutherfords Experiment Bohr Model In 1913, the Danish scientist Niels Bohr proposed an improvement on Rutherfords model. In his model, he placed each electron in a specific energy level. Bohr Model Discovery of the Neutron In 1935, James Chadwick received the Nobel Prize in Physics for finding neutrons. When Rutherford discovered the proton, he figured that there must be another particle making up the mass of the nucleus. The Curies (of radiation fame) also worked to find the neutron but misread their results and missed the discovery. (they won the Nobel Prize in Chemistry in 1935 so dont feel so bad.) Quantum Mechanical Model Erwin Schrodinger Austrian Physicist Used mathematical equations to describe likelihood of finding an electron in a certain position. Does not define exact path but predicts odds of the location of an electron Electron Cloud Model A space in which electrons are likely to be found. Electrons whirl about the nucleus billions of times per second Location of electrons depends upon how much energy the electron has. Electrons with the lowest energy are found in the energy level closest to the nucleus Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus.