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Elements and Compounds: Organization and Proportions

Learn about the distinction between elements and compounds, the organization of elements on the periodic table, and how compounds follow the laws of definite and multiple proportions.

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Elements and Compounds: Organization and Proportions

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  1. Chapter 3 Sec. 3.4: Elements & Compounds

  2. Objectives • Distinguish between elements and compounds. • Describe the organization of elements on the periodic table. • Explain how all compounds obey the laws of definite and multiple proportions.

  3. Matter Substance Mixture Element Compound Homogeneous Heterogeneous Recall . . . Anything that has mass and takes up space Matter that has a uniform, unchanging composition Physical combination of 2 or more substances • Mixture that is not blended smoothly throughout. Mixture with a constant composition throughout

  4. Elements • An element is a pure substance that cannot be separated into simpler substances by physical or chemical means. • They are considered to be the basic building blocks of all matter. • 91 occur naturally; the rest of the elements are synthetic. • Each element has a unique symbol consisting of 2-3 letters: the first is always uppercase & the rest are always lowercase.

  5. Stands for the element carbon Stands for the element lead Stands for the element thorium

  6. The Periodic Table • The periodic table organizes the elements into a grid. • The vertical columns in the grid are called groups or families. • Elements in the same group have similar chemical and physical properties.

  7. The Periodic Table • The horizontal rows in the periodic table are called periods. • The table is called periodic because the pattern of similar properties repeats from period to period.

  8. Compounds - 4 part Definition • A compound is a chemical combination of 2 or more elements. (There are chemical bonds between the elements - a reaction has occurred between them.) Elemental symbols make it easy to write the formula for a compound. Butane, pictured here, has 4 carbon atoms and 10 hydrogen atoms. Its formula is C4H10.

  9. Compounds - 4 part Definition • The properties of a compound are different from those of its component elements. Sodium chloride

  10. Compounds - 4 part Definition • The composition of a compound is fixed. (The elemental components of a compound are always in the same ratio. Water, for example, always has 2 hydrogen atoms and 1 oxygen atom.)

  11. Compounds - 4 part Definition • Compounds can only be broken down into their component elements by chemical means. (The bonds between the elements must be broken by the addition of external energy.) The electrolysis of water

  12. Law of Definite Proportions(also called the Law of Constant Composition) • Recall that the elemental components of a compound are always in the same, fixed ratio. • This means that the elements are in a definite proportion by mass. • Summarized as the Law of Definite Proportions, scientists observe that a compound is always composed of the same elements in the same proportion by mass.

  13. Percent Composition • When the law is applied to compounds, the sum of the masses of the elements that make up the compound is equal to the mass of the compound. • Water, H2O, contains 2 H atoms (each has a mass of 1.008 amu) and 1 O atom (with a mass of 15.999 amu); the mass of 1 molecule of water is, therefore, 18.015 amu.

  14. Percent Composition • The law also shows us that the ratio of the mass of each element in a compound to the total mass of the compound will never change. • The ratio described above is called the percent by mass or percent composition of a compound. It is summarized by the formula: percent by mass = mass of element x 100 mass of compound

  15. Samples of water from any source will always have the same mass proportions. A different proportion means it’s a different compound!

  16. Practice Problems • If 3.5 g of X reacts with 10.5 g of Y to form XY, what is the percent by mass of X in the compound? Of Y? • If 150 g of XY are obtained, how many grams are X? How many grams are Y? • Compound 1 contains 15.0 g of hydrogen and 120.0 g of oxygen. Compound 2 contains 2.0 g of hydrogen and 32.0 g of oxygen. Are the compounds the same?

  17. Law of Multiple Proportions • Compounds composed of the same elements CAN be different compounds IF they have different mass compositions. • The law of multiple proportions says 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 a ratio of small whole numbers.

  18. Law of Multiple Proportions • H2O (water) and H2O2 (hydrogen peroxide) both contain H and O. • In water, the ratio of hydrogen to oxygen is 2/1; in hydrogen peroxide, the ratio is 2/2. • The mass ratio, then, of water to hydrogen peroxide is 2/1 or 2/1. This is a small, whole 2/2 number ratio!

  19. Another example • Compound 1 contains 64.2% Cu and 35.8% Cl; compound 2 contains 47.27% Cu and 52.73% Cl. • In compound 1, the Cu/Cl ratio is 64.2 or 35.8 1.79; in compound 2, it is 0.8965. • The mass ratio of Cu to Cl in compound 1 to compound 2 is 1.79/0.8965 or 2.00. Again, this is a small, whole number ratio!

  20. Practice Problem Complete the table and then analyze the data to determine if • Compounds 1 and 2 are the same compound • If they are not, find the small whole number ratio that iron combines to the same relative mass of oxygen.

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