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14.1 An early model !In 1897 English physicist J. J. Thomson discovered that electricity passing through a gas caused the gas to give off particles that were too small to be atoms. !These negative particles were eventually called “electrons.” The Plum Pudding model Ernest Rutherford and the Gold Foil Experiment J.J. Thompsonʼs “Plum Pudding Model” E. Rutherfordʼs “Nuclear model” 14.1 The nuclear model !In 1911, Ernest Rutherford, Hans Geiger, and Ernest Marsden did a clever experiment to test Thomson’s model. !We now know that every atom has a tiny nucleus, which contains more than 99% of the atom’s mass. Parts of the atom !The Nucleus: contains sub-atomic particles called PROTONS and NEUTRONS. !Protons: Have a positive charge (+) !Neutrons: Have a neutral charge (0) !Both Protons and Neutrons: have a mass of 1 AMU (atomic mass unit). Parts of the atom !The Electron Cloud: contains subatomic particles called ELECTRONS. !Electrons: Have a negative charge (-) Have almost no mass (1/1836 AMU) Orbit around the nucleus in 3D. Only a set number of electrons can fit in each orbital 14.1 Electric charge in matter !We say an object is electrically neutral when its total electric charge is zero. 14.1 Inside an atom !The mass of the nucleus determines the mass of an atom because protons and neutrons are much larger and more massive than electrons. !The number of Protons = ATOMIC NUMBER !The number of Protons + Neutrons = MASS NUMBER 14.1 Force inside atoms !In order to understand atoms, we need to understand the idea of electric charge. !Like charges repel one another (like 2 protons) !ELECTRIC FORCE 14.1 Forces inside atoms !Electrons are bound to the nucleus by the attractive force between electrons (-) and protons (+). !ELECTROMAGNETIC FORCE 14.1 Force inside atoms !What holds the nucleus together? !There is another force that is even stronger than the electric force. !We call it the strong nuclear force. 14.1 Force inside atoms !The WEAK FORCE sometimes causes neutrons to break apart into a Proton and an Electron. !Usually not important in stable atoms, but for radioactive atoms. 14.1 How atoms of various elements are different !The atoms of different elements contain different numbers of protons in the nucleus. 14.1 How atoms of various elements are different !Isotopes are atoms of the same element that have different numbers of neutrons. How are these carbon isotopes different? !The mass number of an isotope tells you the number of protons plus the number of neutrons. 14.1 Radioactivity !Almost all elements have one or more isotopes that are stable. !“Stable” means the nucleus stays together. !Carbon-14 is radioactive because it has an unstable nucleus. Solving Problems !How many neutrons are present in an aluminum atom that has an atomic number of 13 and a mass number of 27? Solving Problems 1. Looking for: ! …number of neutrons in aluminum-27 2. Given ! … atomic no. = 13; mass no. = 27 3. Relationships: ! Periodic table says atomic no. = proton no. ! protons + neutrons = mass no. 4. Solution ! neutrons = mass no. – protons ! neutrons = 27 – 13 = 14