Atoms are important because they form the basic building blocks of all visible matter in the universe. There are 92 types of atoms that exist in nature, and other types of atoms can be made in the lab. The different types of atoms are called elements.
Hydrogen, gold and iron are examples of elements comprised of unique types of a single kind of atom. Atoms can also combine together in fixed ratios to form molecules. Water is a molecule composed of two hydrogen atoms and a single oxygen atom bonded together.
Atoms are comprised of even smaller particles called neutrons, protons and electrons. The neutrons and protons have positive charges and are very large and heavy compared to the electrons. These two particles exist in the nucleus of the atom, which is located at its center. The majority of the mass of the atom is located in the nucleus.
The electrons orbit the atomic nucleus much in the way that the Earth orbits the sun. The electrons can store energy in the motion of these electrons around the nucleus. This chemical energy enables the atom to partake in chemical reactions to form compounds. The electrons can gain energy from light, jumping further away from the atom. When they jump back, they can shed this extra energy by emitting light of their own.