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Elements, Atoms, and Molecules

For a more advanced list of resources on atoms, elements and compounds

Elements are substances that cannot be separated into simpler substances. Salt is made up of the elements sodium and chloride. Water is made up of the elements hydrogen and oxygen.

The smallest particles of matter are called atoms. Remember the carrot from the other chapter. If you continue to chop a carrot into smaller and smaller pieces, eventually you would reach a point where you could not cut up the carrot anymore, but still have carrot. You would then have molecules of carrot. The same applies to elements. If you continually cut up a piece of aluminum, you will reach a point that you could no longer divide it. These are aluminum atoms. An atom is the smallest particle of an element that has the properties of that element. Some properties of aluminum are: shiny, silver colored, fragile, and thin. Each element has its own type of properties.

Chemists use symbols to represent elements. A symbol is a letter or picture used to represent something. Chemists use one or two lettters to represent elements. The symbol for aluminum is Al. The symbol for oxygen is O.

A model of an oxygen molecule.

The symbol for oxygen is O. "O" stands for one atom of oxygen. Oxygen stoms are joined in pairs. To write a pair of oxygen atoms using symbols, we use the symbol O and the number 2. Oxygen would be (O2). The 2 is a subscript. "Sub" means "below". The 2 is written to the right of and below the O. A pair of oxygen atoms is a molecule of oxygen. A molecule is the smallest particle of a substance that exists independently. Molecules of most elements are made up of only one of atom of that element. Oxygen, along with nitrogen, hydrogen, and chlorine are made up of two atoms. Look at the model of oxygen above. The two balls represents the two oxygen molecules. The oxygen molecules are bonded or stuck together. We will learn about bonds later.


A compound is a substance formed when two or more elements are chemically joined. Water, salt, and sugar are examples of compounds. When the elements are joined, the atoms lose their individual properties and have different properties from the elements they are composed of. A chemical formula is used a quick way to show the composition of compounds. Letters, numbers, and symbols are used to represent elements and the number of elements in each compound.


Mixtures are two or more substances that are mixed together but not chemically joined. A good example of a mixture is a salad. There are tomatoes, lettuce, cucumbers, and salad dressing all mixed together. No chemical reactions occur between the vegetables and the dressing. You can separate each of the vegetables from each other. When spices are added to a cake before baking, do you think they form compounds or mixtures? Why?