BackChapter 4: Molecules and Compounds – Study Notes
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Molecules and Compounds
Introduction to Molecules
When two or more elements combine, they form a molecule. Molecules can consist of the same type of element or different elements. The formation of molecules is fundamental to the diversity of substances found in nature.
Molecular compound of oxygen (O2): Example of a molecule formed from the same element.
Molecular compound of water (H2O): Example of a molecule formed from different elements.
All molecules are compounds, but not all compounds are molecules.
Key Point: The ability of elements to form compounds leads to the vast diversity of substances in nature.
Elements vs. Compounds: Hydrogen, Oxygen, and Water
The properties of compounds are often dramatically different from the properties of the elements that compose them. For example, hydrogen and oxygen are both gases with distinct properties, but when combined to form water, the resulting compound has entirely new characteristics.
Selected Properties | Hydrogen | Oxygen | Water |
|---|---|---|---|
Boiling Point | -253 °C | -183 °C | 100 °C |
State at Room Temperature | Gas | Gas | Liquid |
Flammability | Explosive | Necessary for combustion | Used to extinguish flame |
Example: Water (H2O) is a liquid at room temperature and is used to extinguish flames, whereas hydrogen is explosive and oxygen is necessary for combustion.
Law of Definite Proportion
The Law of Definite Proportion states that a chemical compound always contains exactly the same proportion of elements by mass. For example, a mixture of hydrogen and oxygen can have any ratio, but water (H2O) always has a fixed ratio of two hydrogen atoms to one oxygen atom.
Mixtures: Variable proportions of hydrogen and oxygen.
Compounds: Definite proportions; water always has a 2:1 ratio of hydrogen to oxygen atoms.
Formula: For water, the ratio is always:
Example: In a water molecule, there are always two hydrogen atoms for every one oxygen atom.
Additional info: These slides introduce foundational concepts in chemical bonding and composition, which are essential for understanding chemical reactions and properties of substances in General Chemistry.
