Compounds as Substances

Understanding Compounds

Compounds are substances formed when two or more elements chemically combine.
Fixed Proportions: In a pure compound, the elements are combined in definite, fixed proportions by mass.
The properties of a compound are different from the original elements, showing that a chemical change has occurred.
Compounds are represented by chemical formulas. These show the type and number of atoms in a compound.

Compounds are formed through chemical reactions. Elements react to form compounds when their atoms join together.
Chemical bonds, which hold the atoms together, involve the exchange or sharing of electrons.
The breakdown of compounds back into their constituent elements also involves a chemical reaction. This is often hard to do as it requires energy.
Chemical reactions that form compounds are generally exothermic (they release energy), while the breakdown of compounds is generally endothermic (they absorb energy).

Molecular compounds consist of nonmetal elements. They are usually gases, liquids or solids with low melting points.
Ionic compounds contain metals and nonmetals. They are usually solid and have high melting and boiling points due to the strong bonds between ions.
In covalent compounds two non-metals share electron pairs, these are common in biological systems and many are gases, liquids or solids with low melting points.
Metallic compounds contain metal and nonmetal elements. They are generally solid with high melting and boiling points and can conduct electricity.

The naming of compounds is a standardised system known as IUPAC nomenclature. Usually the element with the positive oxidation number is first, followed by the element with the negative oxidation number.
In molecular and covalent compounds, prefixes indicate the number of atoms of each element. For instance, “carbon dioxide” contains one carbon atom and two oxygen atoms (CO2).
Ionic compounds do not use prefixes in their names as their ratios are not fixed but based on ion charge balance. For instance, “sodium chloride” (table salt) is NaCl.