Chemical Properties:
* New Substances Formed: The most fundamental change is that a new substance is created with entirely different properties than the original elements. For example, sodium (a highly reactive metal) and chlorine (a toxic gas) combine to form sodium chloride (table salt), a stable and edible compound.
* Bonding: Elements share or transfer electrons to form chemical bonds. The type of bond (ionic, covalent, metallic) determines the compound's properties.
* Reactivity: Compounds often exhibit drastically different reactivities compared to their constituent elements. For instance, sodium reacts violently with water, while sodium chloride is unreactive in water.
Physical Properties:
* Appearance: The appearance of a compound is often completely different from the elements it's made of. Consider how sodium (silvery metal) and chlorine (greenish gas) form white crystalline table salt.
* Melting Point and Boiling Point: Compounds typically have melting and boiling points that are different from the elements they contain.
* Density: The density of a compound is usually not simply the average of the densities of its constituent elements.
* Solubility: Compounds can have vastly different solubilities in various solvents compared to the elements they're composed of. For example, sodium chloride dissolves readily in water, while sodium is insoluble in water.
* Conductivity: Electrical and thermal conductivity can be greatly altered when elements combine to form compounds.
Key Points:
* Atoms are rearranged: The atoms of the elements are not just mixed together in a compound; they are rearranged and bonded in a specific way.
* New properties emerge: The properties of a compound are not simply the sum of the properties of the elements. New properties emerge from the interactions between the atoms in the compound.
Examples:
* Water (H2O): Hydrogen is a flammable gas, and oxygen supports combustion. However, water is a liquid that extinguishes fire.
* Carbon Dioxide (CO2): Carbon is a solid element, and oxygen is a gas. Carbon dioxide is a colorless gas that is essential for plant life.
In summary, the formation of a compound involves a fundamental change in the chemical and physical properties of the elements, resulting in a new substance with unique characteristics.
