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. 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 called water, or H2
O. So, how does
this happen? I mix the hydrogen and oxygen inside
S ome people collect rocks. Other people
collect stamps, coins, or even bugs. I collect 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? It’s the smallest part of a substance. Every solid, liquid, and gas is made of atoms. In one way, being an element collector is easy. Everything in the universe is made up of elements. T ese substances are the building blocks of every living and nonliving thing. 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 jewelry. 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
soap bubbles. T en I add a spark. T e gases explode. T is reaction turns the elements into a compound. Drops of water spray in every direction.
Air is pumped into the gates, pushing out the water. The gate then fl oats upward on its hinge.
Gates are fi lled with water and stay down fl at on the seafl oor until needed.
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