Chapter 2 - Chapter 2 Atoms Molecules and Ions A little...

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Chapter 2: Atoms, Molecules, and Ions 1 A little history : Greek Philosophers led to fundamental substances called elements . Asked question: Can you continue dividing elements into smaller and smaller pieces indefinitely? Plato and Aristotle said “Yes” Democritus (460 – 370 B. C.) said “No” Elements were composed of tiny particles that couldn’t be further divided. These particles were called atoms (atomos=indivisible) Robert Boyle (1627 – 1691) Showed matter is made up of atoms (mostly from studies of air and gases) and elements were pure substances that couldn’t be broken down any further. Joseph Priestly (1733 – 1804) Heated mercury (II) oxide to produce oxygen (1774) 2HgO 2Hg + O 2 Antoine Lavoisier (1743 – 1794) Showed oxygen key substance in combustion. Showed in combustion reaction mass products = mass reactants. Law of Mass Conservation – Mass is neither created nor destroyed in a chemical reaction. Burning of alcohol C 2 H 5 OH + 3O 2 2CO 2 + 3H 2 O 4.607 g 9.600 g 8.802 g 5.405 g start out with a total of 14.207 g – C 2 H 5 OH and O 2 end up with a total of 14.207 g – CO 2 and H 2 O Joseph Proust (1754 – 1826) Law of Definite Proportions Different samples of a pure chemical substance always contain the same proportion of elements by mass. All CO 2 contains 1 part C and 2.7 parts O by mass All H 2 O contains 1 part H and 7.9 parts O by mass All alcohol (ethyl) contains 1 part H, 4 parts C, 2.6 parts O by mass Dalton’s Atomic Theory – John Dalton (1766 – 1844) Elements are made of tiny particles called atoms. Atoms of the same element have the same mass; atoms of different elements have different masses – Explained how each element’s atoms were unique. Law of definite proportions showed that elements always combined in same mass ratio to make a certain compound; so Dalton thought what made atoms of different elements different was mass. Chemical combinations of elements to make different substances occur when atoms join together in small, whole number ratios. – Only if whole numbers of atoms combine
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Chapter 2: Atoms, Molecules, and Ions 2 will different samples of a pure substance always have the same ratio of elements by mass. (Law of Definite Proportions) Fractions of atoms aren’t involved in chemical reactions All chemical reactions only rearrange the way the atoms are combined; the atoms themselves are unchanged. – Atoms are chemically indestructible; they can neither be created nor destroyed. This explains how masses of reactants and products have to be the same. (Law of Mass Conservation) If the same number and type of atoms are present in both reactants and products, then the masses will be the same. Dalton’s Atomic Theory predicts:
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This note was uploaded on 04/10/2008 for the course CHEMISTRY 101 taught by Professor Shalvoy during the Fall '07 term at Quinnipiac.

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Chapter 2 - Chapter 2 Atoms Molecules and Ions A little...

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