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Lecture 3 UNIT CONVERSION Atomic Theory • • • • Each element is made up of tiny indivisible particles All atoms of a given element are identical Chemical compunds are formed when different atoms combine Chemical reactions involve atom reorginisation – changes in the way they are bound together. LAW OF CONSERVATION OF MASS – confirmed by Antoine Lavoisier by identifying the role of oxygen in combustion. “Mass is neither created nor destroyed during chemical reactions” LAW OF CONSTANT COMPOSITION “A given compound always contains exactly the same proportion of elements by mass” J.J. Thomson (1897) • Suggested that electrons exist in all atoms • The electron swarm atom with uniform positive sphere was the model he came up with (plum pudding model). Robert Millikan – determined the charge of an electron in 1909. Charge of electrically charge plates had effect on small drops of oil. Charge of electron = 1.60217733x10-­‐19 Coulomb. Experiments with radioactive Rutherford – revealed 3 types of radiation. Alpha, gamma and beta. Tested Thomson’s model of uniform atomic structure. Revealed that there is a lot of empty space MODERN VIEW OF ATOMIC STRUCTURE • Atoms are complex with many subatomic particles • A simplified view is: o A dense nucleus (protons and neutrons) surrounded by a cloud of electrons • Quantum mechanical description. • Protons are positive, neutrons are neutral and electrons are negative. • Overall charge on an atom is zero. • The nucleus o Contains protons and neutrons o Held together by stronf nuclear force o Unchanged in chemical reactions (but is in nuclear) • The Electrons o Held around the nucleus by their attraction to it and the forces arising from thei movement o Most of the atomic volume o Mass is so small we ignore o Plays a major role in chemical reactions ELEMENTS • Substances that can’t be decomposed or made by chemical reactions • All atoms of the same element behave the same chemically and have the same number of protons and electrons. • Atoms of different elements has different atomic numbers.