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Mix and Match Elements exist alone, but oſt en you’ll fi nd them mixed together. Something made of two or more elements is called a compound. A compound may be very diff erent from the elements that make it up. T e elements combine to create something new. Take water, for example. It’s made of the


elements hydrogen (H) and oxygen (O). Both are gases. When two hydrogen atoms mix with an oxygen atom, they create a chemical reaction. T e gases turn into a liquid compound called water, or H2


O. T at sounds pretty tame. In a lab, though, S ome people collect rocks. Others collect


stamps, coins, or even bugs. I’m a collector, too. I look for something a little diff erent, though. I collect chemical elements. An element is a substance made up of only


one kind of atom. And what’s an atom? An atom is the smallest part of a substance. Every solid, liquid, and gas is made of atoms. In some ways, being an element collector is


easy. T at’s because everything in the universe is made up of elements. T ese substances are the building blocks of every living and nonliving thing. Some elements, like oxygen, are easy to


collect. I can bottle that up. Other elements are more challenging, like fi nding radioactive plutonium. I can also collect items that are made from elements—rocks, stamps, coins, and bugs are all made of elements. T ere are currently 118 elements. Of these,


only 94 naturally exist on Earth. Some are gases. Many are metals. You’ve probably heard of some elements,


like gold. Gold is a metal. You can dig it up out of the ground. You can also fi nd it in a jewelry store. It’s considered rare because there isn’t much of it on our planet. You might know helium, too. Helium is a gas. As it is lighter than air, it’s used to fi ll balloons and make them fl oat.


12 NATIONAL GEOGRAPHIC EXPLORER


Gates are fi lled with water and stay down flat on the seafloor until needed.


the reaction between hydrogen and oxygen is anything but! To be safe, I fi rst put on ear protection. T en I mix the hydrogen and oxygen inside soap bubbles. At first, nothing happens. T en I add a spark. Bang! Light flashes. T e reaction releases a lot of energy as it turns the two elements into a compound. Drops of water spray in every direction.


Air is pumped into the gates, pushing out the water. The gate then floats upward on its hinge.


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