Chapter 2 Atoms: Are They for Real? Chemistry for Changing Times 10 th edition Hill/Kolb

Atoms Very small 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 (CH 4 ) 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 CH 4 ?

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 H 2 represents a hydrogen molecule How many atoms of O are in H 2 O 2 ? Be careful when writing formulas for molecules H 2 O 2  H 2 O Remember the law of definite proportions!