Compounds as Substances

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.

Formation and Breakdown of Compounds

  • 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).

Types of Compounds

  • 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.

Naming Compounds

  • 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.