The same atoms that were in a dinosaur millions of years ago may be inside you today. This is even true for atoms from a star billions of years before that.
Atoms are the building blocks of all matter. They are the tiny particles, or pieces, that make up everything in the universe. Atoms can join together with other atoms. Then they form molecules.
From port-a-potties to supernovas, matter makes up everything visible in the known universe. Because matter is never created or destroyed, it cycles through our world.
Matter Can't Be Created or Destroyed
Matter is anything that has mass and takes up space. It includes molecules, atoms, fundamental particles, and any substance that has these particles. Matter can change form through physical and chemical changes. Still, through any of these changes, matter is conserved. That is, the same amount of matter exists before and after the change. None is created or destroyed. This concept is called the Law of Conservation of Mass.
In a physical change, a substance's physical properties may change. However, its chemical makeup does not. Water, for example, is made up of two hydrogen atoms and one oxygen atom. Water is the only known substance on Earth that exists naturally in three states. These are solid, liquid, and gas. To change between these states, water must undergo physical changes. When water freezes, it becomes hard and less dense. Still, it is chemically the same. There are the same number of water molecules present before and after the change. Water's chemical properties remain constant.
Hydrogen and Oxygen Undergo Changes To Make Water
To form water, however, hydrogen and oxygen atoms must undergo chemical changes. For a chemical change to occur, bonds between atoms must break or form. This changes the chemical properties of the substances involved. Both hydrogen and oxygen exist naturally as bonded pairs. These are H2 and O2, respectively. In the right conditions and with enough energy, these bonds will break. The atoms will join to form H2O (water). Chemists write out this chemical reaction as:
2H2 + O2 -> 2H2O
This equation says that it takes two molecules of hydrogen and one molecule of oxygen to form two molecules of water. Notice there are the same number of hydrogen atoms and oxygen atoms on either side of the equation. In chemical changes, just as in physical changes, matter is conserved. There's a difference in this case though. The substances before and after the change have different physical and chemical properties.
At standard temperature and pressure, hydrogen and oxygen are gases. Meanwhile, water is a colorless, odorless liquid.
In Nature, Water Goes through a Lot of Physical Changes
Ecosystems have many chemical and physical changes happening all at once. Matter is conserved in each and every one—no exceptions. Consider a stream flowing through a canyon. How many chemical and physical changes are happening at any given moment?
First, let's consider the water. For many canyon streams, the water comes from higher elevations. It begins as snow. Of course, that's not where the water began. It's been cycled all over the world since Earth first had water.
The snow must undergo a physical change—melting—to join the stream. As the liquid water flows through the canyon, it may evaporate into water vapor. This is another physical change. Water gives a very clear example of how matter cycles through our world. It frequently changes form but never disappearing.
A Chemical Change Takes Place in Photosynthesis
Next, consider the plants and algae living in and along the stream. These organisms convert light energy from the sun into chemical energy. This is stored in sugars. The process is called photosynthesis. However, the light energy doesn't produce the atoms in those sugars. That would break the Law of Conservation of Mass. It simply provides energy for a chemical change to occur. The atoms come from carbon dioxide in the air and water in the soil. Light energy allows these bonds to break and reform to produce sugar and oxygen. This is shown in the chemical equation for photosynthesis:
6CO2 + 6H2O + light -> C6H12O6 (sugar) + 6O2
This equation says that six carbon dioxide molecules combine with six water molecules. Together, they form one sugar molecule and six molecules of oxygen. If you added up all the carbon, hydrogen, and oxygen atoms on either side of the equation, the sums would be equal. Matter is conserved in this chemical change.
Animals around the stream eat these plants. Their bodies use the stored chemical energy to power their cells and move. They use the nutrients in their food to grow and repair their bodies. The atoms for new cells must come from somewhere. Any food that enters an animal's body must either leave its body or become part of it.
Our Atoms Are Very Old
The bottom line is this. Matter cycles through the universe in many different forms. In any physical or chemical change, matter doesn't appear or disappear. Atoms created in the stars a very long time ago make up every living and nonliving thing on Earth—even you. It's impossible to know how far and through what forms your atoms traveled to make you. And it's impossible to know where they will end up next.