An element is a pure substance made of only one type of atom. A compound is a pure substance made of two or more different types of atoms chemically bonded together in a fixed ratio. Water is a compound because every water molecule contains oxygen and hydrogen atoms bonded together. You cannot separate a compound into its elements by physical means like filtering or boiling — it takes a chemical change to break those bonds.
Compare real examples side by side: a piece of pure copper (element) versus table salt (compound of sodium and chlorine). Discuss how the compound's properties differ completely from the properties of the elements that make it up. Use molecular models or diagrams to show the difference visually.
You already know that pure substances are different from mixtures, and that everything is made of atoms. Now it is time to divide pure substances into two important categories: elements and compounds.
An element is the simplest type of pure substance. It is made entirely of one kind of atom. A bar of pure iron contains only iron atoms. A tank of pure oxygen gas contains only oxygen atoms (paired up as O2 molecules, but still only oxygen). There are about 118 known elements, and they are all listed on the periodic table. No matter how you try — heating, filtering, dissolving — you cannot break an element down into simpler substances by ordinary chemical means.
A compound is a pure substance made of two or more different elements chemically bonded together. The key word is "chemically" — the atoms are not just sitting next to each other; they are connected by chemical bonds in a fixed ratio. Water (H2O) always has two hydrogen atoms for every one oxygen atom. Carbon dioxide (CO2) always has one carbon atom for every two oxygen atoms. That fixed ratio is what makes a compound a compound.
One of the most surprising things about compounds is that their properties are usually nothing like the properties of the elements that form them. Consider table salt (sodium chloride, NaCl). Sodium is a soft, silvery metal that reacts violently with water. Chlorine is a yellow-green toxic gas. But when sodium and chlorine atoms bond together, they form white, crystalline table salt — something you sprinkle on your food. The chemical bond creates an entirely new substance with its own unique set of properties.
It is important not to confuse compounds with mixtures. A mixture of iron filings and sulfur powder can be separated with a magnet because the iron and sulfur are not chemically bonded. But if you heat that mixture enough to cause a chemical reaction, you get iron sulfide — a compound that cannot be pulled apart with a magnet. The difference is the chemical bond. Compounds require chemical reactions to form and chemical reactions to break apart; mixtures can be separated by physical means.