a substance that cannot be broken down into other substances is what?  

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Atoms cannot be broken down into other substances. Atoms are the smallest units of matter that have the properties of matter.

For example, imagine you want to break down a sample of water. Water has specific properties (boiling point, freezing point, density, and so on) that are all its own. If you divide your sample in two, you will have two smaller amounts of water, but the properties will still remain the same. Divide again, the same thing happens, and so on, until you are down to just a single molecule of water. When you break that molecule apart, you will suddenly have two entirely different substances - oxygen and hydrogen, which each have their own set of properties. These are atoms.

When you break an atom apart, something new happens: the properties are lost. Every atom breaks down into the same thing; protons and neutrons and electrons, the primordial building blocks of matter. All protons are alike, as are all neutrons and so on - the thing that makes one atom different from another is simply how many of each type of subatomic particle it has in it.

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