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Chemistry for Changing Times 10th edition Hill/Kolb Chapter 2 Atoms: Are They for Real? Atoms • Very small • 1022 atoms in 1 penny • Equivalent to 1 grain of sand in a sandbox the size of Texas Properties of Atoms • Smallest characteristic of a given element • Make up everything • Different for each element – Only 90 elements occur in nature Ancient Greeks and the Atom • Leucippus and Democritus postulated tiny particles of water that could not be subdivided further – Called them atomos • Each type of atom has a distinct shape and size Four Elements of Ancient Greece Aristotle • Thought everything was continuous Atoms didn’t exist • Ancient Greeks could not test either model • Aristotle’s view prevailed for >2000 years Lavoisier • Father of modern chemistry • Accurately weighed starting materials and products of reactions Law of Conservation of Mass • Mass is neither created nor destroyed during chemical reactions • Make new materials by rearranging atoms • Basis for chemical calculations Law of Definite Proportions • Also called law of constant composition • A compound has the same elements in a certain definite proportion and no other combinations • Also implies compounds have constant properties • Possible to tell when reaction is complete or when too much starting material is present End of Ancient Greek View • Electrolysis of water was death blow to the ancient Greeks' view of water as an element Could separate water into simpler things: hydrogen and oxygen Law of Multiple Proportions • Elements might combine in more than one set of proportions – Each set makes up a new compound Law of Multiple Proportions Mass Ratios Example: The gas methane (CH4) can be decomposed to give carbon and hydrogen in a ratio of 3.00 g of carbon to 1.00 g of hydrogen. How much hydrogen can be made from 90.0 g CH4? Atom Ratios Dalton’s Atomic Theory 1. All matter is composed of extremely small particles – Atoms are indivisible 2. Atoms of a given element are alike but different from atoms of any other element – Atoms for any element have identical chemical and physical properties 3. Compounds are formed when different elements combine in fixed proportions – Typically written with smallest whole numbers 4. A chemical reaction involves only a rearrangement of atoms Mendeleev’s Periodic Table • Arranged in order of increasing atomic mass and by chemical property – Some elements don’t fit where their atomic mass suggests Data from Periodic Table • Each element is represented by a box Molecules • • • • • Groups of atoms chemically bonded together H represents a hydrogen atom H2 represents a hydrogen molecule How many atoms of O are in H2O2? Be careful when writing formulas for molecules • H2O2 H2O • Remember the law of definite proportions!