Atoms and Elements

Chapter 2 – Atoms and Elements

Laws of Chemical Combination

1) Lavoisier The law of Conservation of Mass

The total mass remains constant during a chemical reaction.

The products of a chemical reaction have the same total mass as the starting reactants.

Matter is not created or destroyed during a chemical reaction.

2) Proust The Law of Definite (Constant) Proportions

All samples of a compound have the same composition, no matter what it source.

A compound contains fixed proportions, by mass, of its elements.

A compound not only has a fixed and constant composition; it also has a fixed properties.

The properties of a compound are not a matter of chance. The physical and chemical properties of a compound directly depend on its composition.

3) Dalton The Law of Multiple Proportions

John Dalton proposed a theory to combine and explain the laws of conservation of mass and constant composition.

Two or more compounds may form of the same two elements.

The masses of one element that combined with a fixed mass of the second element are in the ratio of small whole numbers.

Dalton’s Atomic Theory

• All mater is composed of extremely small, indivisible particles called Atoms.

• Atoms of an element have identical mass and properties; atoms of different elements have different masses and properties.

• Compounds are formed when atoms of different elements unite in fixed proportions. The relative numbers of each kind of atom in a compound form a simple ratio.

• A chemical reaction involves a rearrangement of atoms to produce new compounds. No atoms are created, destroyed, or broken apart in a chemical reaction, thus the total mass remains unchanged.

The Divisible Atom

Towards the end of nineteenth century, it was revealed that atoms are made up of smaller parts. Of the dozens of subatomic particles now known, three are of special importance in the study of chemistry.

Subatomic...