Laws of Chemical Combination & Dalton's Theory

Laws of Chemical Combination

• Elements are made up of tiny particles called atoms.
• All atoms of an element are identical in mass and size, differing from atoms of other elements.
• Atoms of different elements have different shapes/figures that represent them.
• Compounds are formed from atoms of different elements in definite proportions with whole-number ratios.
• During a chemical reaction, atoms are rearranged, combining or separating but never created or destroyed.

Dalton's Atomic Theory

• Elements are composed of atoms.
• All atoms of a given element are identical in mass and size, differentiating them from atoms of another element.
• Compounds are formed by atoms of different elements combining in definite whole number ratios.
• During a chemical reaction atoms combine, separate, or rearrange. No atoms are created or destroyed.

Introduction

• In studying matter, we observe phenomena that can be reproduced at will.
• Scientific facts are observed phenomena that can be reproduced.
• Scientific laws are summaries of numerous scientific facts that are expressed as broad statements.

Laws of Chemical Combination

• Law of Conservation of Mass: In an ordinary chemical reaction, there is no noticeable change in the total mass of substances involved before and after the reaction. Mass of reactants equals mass of products.

  • Lavoisier's experiment with mercuric oxide illustrates this law.
  • A sample problem demonstrating this law is provided.

• Law of Definite Proportions: A given chemical compound always contains its constituent elements in a fixed ratio (by mass). The ratio doesn't depend on the source or preparation method.

  • A sample problem illustrating the law is given.

• Law of Multiple Proportions: When two elements form more than one compound the ratio of the masses of one element that combine with a fixed mass of the other element is a ratio of small whole numbers.

  • A sample problem about carbon and oxygen illustrates this law.

Atoms, Molecules, and Ions

• Atoms combine to form molecules.
• Atoms lose or gain electrons to form ions.
• Positive ions are called cations.
• Negative ions are called anions.
• Ions can be formed by a single atom (monoatomic) or multiple atoms (polyatomic) .

Naming Monoatomic Ions

• Cations: The name of the element remains unchanged. If multiple ions are possible, a suffix -ic (higher charge) or -ous (lower charge) is added.
• Anions: The name of the element is modified with the suffix -ide.

Naming Cations

• Examples of cations include zinc ion, magnesium ion, potassium ion, ferrous ion (or iron(II)) ion and ferric ion (or iron(III)) ion.

Naming Anions

• Examples of anions include bromide ion, sulfide ion, oxide ion, and iodide ion.

Polyatomic Ions

• Several anions are polyatomic (composed of more than one atom).

• Polyatomic anions are named using the root word of the central atom and using suffixes -ate (higher oxygen content) or -ite (lower oxygen content).

• Examples include hydroxide ion, cyanide ion, nitrate ion, nitrite ion, sulfite ion, sulfate ion, phosphate ion.

• Some anions have common names ending with the suffix -ate. Examples include acetate ion, oxalate ion.