What is an Atom?
An atom is the smallest piece of an element that still has the properties of that element. It is made of three types of smaller particles: protons, neutrons, and electrons. The protons and neutrons are clustered together in the center, called the nucleus, while electrons move around the nucleus in regions called orbitals. Even though atoms are incredibly tiny, they are not the smallest things that exist—protons, neutrons, and electrons are made of even smaller particles called quarks and leptons.
How Atoms Make Everything
Everything you see and touch is made of atoms joined together. When atoms connect to each other, they form molecules. For example, two hydrogen atoms and one oxygen atom bond together to make a water molecule. Millions and billions of molecules stack together to create all the materials around you—your body, this device, air, food, rocks, and stars. Different combinations of atoms create different substances with different properties. Iron atoms make metal, carbon atoms make diamonds and graphite, and hydrogen and oxygen atoms make water.
The 118 Elements
Scientists have discovered 118 different types of atoms, called elements. Each element has atoms with a specific number of protons. Hydrogen has 1 proton, helium has 2, carbon has 6, oxygen has 8, and gold has 79. The periodic table is a chart that organizes all these elements by their properties. Most elements occur naturally in the world, though scientists have created some in laboratories.
Atoms in Your Body
Your body is made almost entirely of just six elements: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. Oxygen makes up about 65 percent of your body by weight, mostly from water. Carbon is the backbone of all living things. Calcium strengthens your bones and teeth. These atoms are constantly moving, reacting, and being replaced through eating, breathing, and other life processes.
How Scientists Know About Atoms
Atoms are too small to see with regular microscopes—they are about 0.0000001 millimeters across. Scientists use special tools like electron microscopes and can study atoms by observing how they behave and interact. In the early 1900s, scientists discovered that atoms have a nucleus, and over time they learned about the particles inside. Today, scientists can even move individual atoms and see them using special microscopes. 
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