An element is a pure substance. It cannot be decomposed into simpler substances by chemical means. It contains only one type of atom. Over 100 known elements are listed in the Periodic Table. Sometimes found free but more oft en occur combined with other elements as compounds. Elements can be classified as metals, non-metals & semi- metals (metalloids).
Metals & Non-metals
Metals
Metals have a shiny appearance, are good conductors of electricity and heat. They can be drawn into wires (ductile) and can be hammered into different shapes (malleable).
Non-metals
Non-metals have a dull appearance. They are poor conductors of electricity and heat. They cannot be drawn into wires or hammered into different shapes (brittle).
Compound
A compound is a pure substance made up of two or more elements combined in fixed proportions which contains more than one type of atom and can only be broken down to simpler substances by chemical means. A compound may contain charged particles (ions) or groups of atoms (molecules).
E.g. Sodium chloride is a naturally occurring compound quite different to its constituent elements.
Mixture
A mixture contains two or more substances; which could be elements or compounds and has variable composition. A mixture can be separated into its components by physical means.
E.g. sea water, on average has about 3.5% sodium chloride, but can be more or less in the same amount of water. The sea near a river mouth will be less salty, but the seawater in the Dead Sea is very salty. Also contains other substances such as magnesium, sulphur and iodine.
