The Laws Governing How Compounds Form

Chapter 9Chemical Names and Formulas

Section 5

2

Vocabulary

• Law of Definite Proportions– In samples of any chemical compound, the

masses of the elements are always in the same proportion.

• Law of Multiple Proportions– Whenever two elements form more than one

compound, the different masses of one element that combine with the same mass of the other element are in the ration of small whole numbers.

3

State Standards

• 2.3• Interpret and apply the laws of conservation

of mass, constant composition (definite proportions), and multiple proportions.

4

Key Questions

• How is the law of definite proportions consistent wit Dalton’s atomic theory?

• What general guidelines can help you write the name and formula of a chemical compound?

5

Law of Definite Proportions

• Dalton postulated that atoms combine in simple whole-number ratios. If the ratio of atoms of each element in a compound is fixed then it follows that the ratio of their masses is also fixed.

6

Law of Multiple Proportions

• Dalton stated the law of multiple proportions. Whenever two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ration of small whole numbers.

• H2O and H2O2

• CuCl2 and CuCl

7

Guidelines for Naming Chemical Formulas

1. Follow the rules for naming acids when H is the first element in the formula.

2. If the compound is binary, generally the name ends with the suffix –ide. If the compound is a molecular binary compound, use prefixes to indicate the number of atoms.

3. When a polyatomic ion that includes oxygen is in the formula, the compound name generally ends in –ite or –ate.

8

Guidelines for Naming Chemical Formulas

4. If the compound contains a metallic cation that can have different ionic charges, use a Roman numeral to indicate the numerical value of the ionic charge in the compound.

9

10

Writing Chemical Formulas

1. An –ide ending generally indicates a binary compound.

2. An –ite or –ate ending means a polyatomic ion that includes oxygen is in the formula.

3. Prefixes in a name generally indicate that the compound is molecular.

4. A Roman numeral after the name of a cation shows the ionic charge of the cation.

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12

Summary

• If the ratio of atoms of each element in a compound is fixed, then the ratio of their masses is also fixed.

• Follow the rules for naming acids when H is the first element. If the compound is binary, generally the name ends with –ide.

• For a molecular binary compound, use prefixes to indicate the number of atoms.

• When a polyatomic ion with oxygen is in the formula, the compound names ends in –ite or –ate.

13

Summary

• If the compound contains a metallic cation that change have different ionic charges, use a Roman numeral to indicate the ionic charge.

• An –ide ending usually indicate a binary compound. An –ite or –ate ending indicates a polyatomic ion with oxygen. Prefixes usually indicate a molecular compound. A Roman numeral after the name of a cation shows the ionic charge of the cation.