A compound is a combination of two or more elements. Section 4: Elements and Compounds K What I Know W What I Want to Find Out L What I Learned

4(D) Classify matter as pure substances or mixtures through investigation of their properties. 2(G) Express and manipulate chemical quantities using scientific conventions and mathematical procedures, including dimensional analysis, scientific notation, and significant figures. 5(A) Communicate valid conclusions supported by the data through methods such as lab reports, labeled drawings, graphs, journals, summaries, oral reports, and technology–based reports. Elements and Compounds Copyright © McGraw-Hill Education

Essential Questions What distinguishes elements from compounds? How is the periodic table organized? What are the laws of definite and multiple proportions and why are they important? Elements and Compounds Copyright © McGraw-Hill Education

Review proportion Elements and Compounds Copyright © McGraw-Hill Education Vocabulary New element periodic table compound law of definite proportions percent by mass law of multiple proportions

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. Elements in the same group have similar chemical and physical properties. The table is called periodic because the pattern of similar properties repeats from period to period. Elements and Compounds Copyright © McGraw-Hill Education

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, H 2 O, are compounds. Unlike elements, compounds can be broken into smaller components by chemical means. Elements and Compounds Copyright © McGraw-Hill Education

Compounds Separating a compounds into its elements often requires external energy, such as heat or electricity. This figure shows electrolysis of water to form hydrogen and oxygen gas. Elements and Compounds Copyright © McGraw-Hill Education

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. Example: Water is always composed of 2 Hydrogen to 1 Oxygen 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. Elements and Compounds Copyright © McGraw-Hill Education

Law of Definite Proportions This table demonstrates that the percentages of elements in sucrose remain the same despite differences in sample amount. Elements and Compounds Copyright © McGraw-Hill Education

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. Example: Peroxide, H 2 O 2, and water, H 2 O Different compounds formed from the same elements. Hydrogen mass the same in both compounds but oxygen mass is a 2:1 ratio in peroxide to water. Elements and Compounds Copyright © McGraw-Hill Education

Law of Multiple Proportions Elements and Compounds Copyright © McGraw-Hill Education

Elements and Compounds Copyright © McGraw-Hill Education Review Essential Questions What distinguishes elements from compounds? How is the periodic table organized? What are the laws of definite and multiple proportions and why are they important? Vocabulary element periodic table compound law of definite proportions percent by mass law of multiple proportions