Section 2.3: Modern Atomic Theory and Laws That Led to It

Atomic Theory

the idea that all matter is composed of atoms.

1. Each element is composed of tiny, indestructible particles called atoms.

2. All atoms of a given element have the same mass and other properties that distinguish them from the atoms of other elements.

3. Atoms combine in simple, whole-number ratios to form compounds.

4. Atoms of one element cannot change into atoms of another element. In a chemical reaction, atoms only change the way they are bound together with other atoms.

Law of Conservation of Mass

In a chemical reaction, matter is neither created nor destroyed. (When a chemical reaction occurs, the total mass of the substances involved in the reaction does not change. The particles rearrange during a chemical reaction, but the amount of matter is conserved because the particles themselves are indestructible (at least by chemical means). (See image)

Law of Definite Proportions

All samples of a given compound, regardless of their source or how they were prepared, have the same proportions of their constituent elements. EX: In 18.0g of water, there are 16.0g of Oxygen and 2.0g of hydrogen. The Oxygen-to-Hydrogen ratio is 16.0gO/2.0gH = 8.0 or 8:1 ratio. (See image).

Law of Multiple Proportions

When two elements can combine to make more than one compound, they always do it in simple whole-number ratios. EX: Imagine carbon and oxygen are building with lego blocks. In carbon monoxide (CO): 1 carbon block + 1 oxygen block. In carbon dioxide (CO₂): 1 carbon block + 2 oxygen blocks. If you keep the amount of carbon the same, the amount of oxygen that combines with it is in a simple ratio: 1:2. (When the same elements make different compounds, the masses of one element that combine with a fixed mass of the other are in small whole-number ratios (like 1:2, 2:3, 3:4 — not fractions).