Distinguish between elements and compounds. Section 3-4 Section 3.4 Elements and Compounds Distinguish between elements and compounds. Describe the organization of elements in the periodic table. Explain how all compounds obey the laws of definite and multiple proportions.

proportion: the relation of one part to another or to the whole with respect to quantity

law of definite proportions percent by mass Section 3-4 Section 3.4 Elements and Compounds (cont.) element periodic table compound law of definite proportions percent by mass law of multiple proportions A compound is a combination of two or more elements.

92 elements occur naturally on Earth. Section 3-4 Elements An element is a pure substance that cannot be separated into simpler substances by physical or chemical means. 92 elements occur naturally on Earth. Each element has a unique name and a one, two, or three-letter symbol. The periodic table organizes the elements into a grid of horizontal rows called periods and vertical columns called groups.

A compound is a made up of two or more elements combined chemically. Section 3-4 Compounds A compound is a made up of two or more elements combined chemically. Most of the matter in the universe exists as compounds. Table salt, NaCl, and water, H2O, are compounds.

Elements can never be separated. Section 3-4 Compounds (cont.) Elements can never be separated. Compounds can be broken into components by chemical means.

This figure shows electrolysis of water to form hydrogen and oxygen. Section 3-4 Compounds (cont.) This figure shows electrolysis of water to form hydrogen and oxygen.

Section 3-4 Compounds (cont.) The properties of a compound are different from its component elements.

Section 3-4 Law of Definite Proportions The law of definite proportions states that a compound is always composed of the same elements in the same proportion by mass, no matter how large or small the sample.

Section 3-4 Law of Definite Proportions (cont.) The relative amounts are expressed as percent by mass, the ratio of the mass of each element to the total mass of the compound expressed as a percentage.

Section 3-4 Law of Definite Proportions (cont.) This table demonstrates that the percentages of elements in sucrose remain the same despite differences in sample amount.

Copper(I) chloride and copper(II) chloride Section 3-4 Law of Multiple Proportions The law of multiple proportions states that when different compounds are formed by a combination of the same elements, different masses of one element combine with the same relative mass of the other element in whole number ratios. H2O2 and H2O Copper(I) chloride and copper(II) chloride

Section 3-4 Law of Multiple Proportions (cont.)

Practice Problems: 76 (20-22) 20. A 78.0 g sample of an unknown compound contains 14.4 g of hydrogen. What is the percent by mass of hydrogen in the compound? % H = 12.9 x 100 = 15.9 % 78.0

Practice Problems: 76 (20-22) 21. If 1.0 g of an hydrogen reacts completely with 19.0 g of fluorine, what is the percent by mass of hydrogen in the compound that is formed? Compound Mass = 1.0 + 19.0 = 20.0 g % H = 1.0 x 100 = 5.0 % 20.0

Practice Problems: 76 (20-22) 22. If 3.5 g X reacts with 10.5 g of Y to form the compound XY, what is the percent by mass of X in the compound? the percent by mass of Y?

Compound Mass = 3.5 + 10.5 = 14.0 g % X = 3.5x 100 = 25.0 % 14.0 % Y = 10.5 x 100 = 75.0 %