How to Tell Elements and Compounds Apart Instantly

An element is a pure substance made of only one kind of atom, while a compound is a pure substance formed when two or more elements combine chemically in a fixed ratio. Elements cannot be broken down into simpler substances by chemical means, but compounds can be broken down into their elements by chemical reactions. Elements have unique properties, but the properties of a compound are usually very different from the properties of the elements it is made from. For example, iron is an element and is a grey, hard metal, while sulphur is a yellow, brittle non-metal. When they combine chemically, they form iron sulphide, a black compound with properties different from both iron and sulphur. Many students think that mixing two elements always forms a compound, but a true compound only forms when a chemical reaction takes place and a new substance is created.

Elements are classified as metals, non-metals, metalloids, and noble (inert) gases. Metals are generally shiny (lustrous), can be hammered into thin sheets (malleable), drawn into wires (ductile), and are good conductors of heat and electricity. Most metals are solid at room temperature (except mercury, which is liquid). Non-metals, on the other hand, are usually dull, brittle (break easily), not malleable or ductile, and poor conductors of heat and electricity (except graphite, a form of carbon). Non-metals can be solids, liquids, or gases at room temperature. Metalloids have properties intermediate between metals and non-metals. Noble gases, like helium and neon, are very unreactive and exist as gases. For example, copper (a metal) is shiny and conducts electricity, while sulphur (a non-metal) is dull and does not conduct electricity. Students often confuse malleability and ductility, so remember: malleable means can be beaten into sheets, ductile means can be drawn into wires.

An atom is the smallest particle of an element that takes part in a chemical change. It cannot exist freely for most elements, but it retains the properties of the element. A molecule is the smallest particle of a substance (element or compound) that can exist independently and shows all the properties of that substance. Molecules can be made up of one kind of atom (as in elements) or different kinds of atoms (as in compounds). For example, an oxygen atom (O) does not exist freely in nature; instead, two oxygen atoms combine to form an oxygen molecule (O2), which we breathe. In real life, water is made up of molecules, each containing two hydrogen atoms and one oxygen atom (H2O). A common mistake is to think that atoms can always exist alone, but in reality, most atoms are found combined as molecules.

Molecules of elements contain only one type of atom, while molecules of compounds contain atoms of two or more different elements chemically combined. The number of atoms present in a molecule is called its atomicity. For example, helium (He) exists as single atoms, so its atomicity is 1 (monoatomic). Hydrogen (H2) molecules have two atoms, so their atomicity is 2 (diatomic). Ozone (O3) molecules have three oxygen atoms (atomicity 3), and phosphorus (P4) molecules have four phosphorus atoms (atomicity 4). In real life, the oxygen we breathe is O2 (diatomic), while the gas in helium balloons is monoatomic He. Students sometimes think all molecules are diatomic, but atomicity can vary, especially among elements.