BackFundamental Chemical Laws: Atoms, Molecules, and Ions
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Atoms, Molecules, and Ions
Fundamental Chemical Laws
Understanding the basic laws that govern chemical reactions is essential in General Chemistry. These laws describe how matter behaves during chemical changes and form the foundation for the study of atoms, molecules, and ions.
1. The Law of Conservation of Mass
Definition: The Law of Conservation of Mass states that matter is neither created nor destroyed in a chemical reaction. This principle was first established by Antoine Lavoisier (1743–1794).
Implication: The total mass of reactants equals the total mass of products in a chemical reaction.
Example: The decomposition of water:
On both sides of the equation, there are 4 hydrogen atoms and 2 oxygen atoms.
Key Point: The number and types of atoms (elements) are the same on both sides of the equation.
2. The Law of Definite Proportions
Definition: The Law of Definite Proportions states that a given chemical compound always contains exactly the same proportion (ratio) of elements by mass, regardless of the sample source or size.
Example: Water (H2O) always consists of hydrogen and oxygen in a fixed mass ratio.
Element | Number of Atoms | Atomic Mass (u) | Total Mass (u) | Percent by Mass |
|---|---|---|---|---|
Hydrogen (H) | 2 | 1.01 | 2.02 | 11.2% |
Oxygen (O) | 1 | 16.00 | 16.00 | 88.8% |
Total | 18.02 | 100% |
Key Point: The mass ratio of hydrogen to oxygen in water is always 2.02:16.00, or approximately 1:8 by mass.
In other words: The ratio of H:O atoms in water is always 2:1 within the single, definite compound H2O.
Additional info: This law is also known as the Law of Constant Composition.
3. Historical Context: Discovery of the Atom
While investigating the composition of compounds, John Dalton formulated the atomic theory, which further explained these fundamental laws. His work led to the next major chemical law (not detailed on this page).
