The world we see is only a fraction of what really exists in the universe. What our eyes fail to pick up is the existence of very small things, the building blocks of everything in the universe -- atoms.

Atoms are the smallest part of an element that still retain the uniqueness of that element. They are made of three basic things -- protons, neutrons, and electrons. These three subatomic particles are made of still smaller things called quarks.

Both protons and neutrons are located in the nucleus, or center, of the atom. Protons carry a positive charge, while neutrons carry none at all. Electrons are not found in the nucleus; they orbit it, ready to bond with electrons from other atoms. Electrons carry a negative charge. The amount of protons in a stable atom, or a pre-reaction atom, is equal to the amount of electrons. Electrons are arranged in layers, called shells, on the outside of an atom. Think of several concentric circles, and each electron as a point on one of the circles.

Sometimes, an atom is called an element. An element is a specific kind of atom, arranged in a specific way. Hydrogen is an element. It has one proton, one electron, and no neutrons. Gold is also an element. It has 79 protons and electrons, and 118 neutrons. The difference in the amount of protons, electrons, and neutrons is what makes each element unique.

All known elements are listed on a very useful device known as the Periodic Table of the Elements. On this table, elements are listed according to atomic number (how many protons they have), their atomic weight (the weight of the matter at the nucleus of an atom), and what types of elements they are (alkali metals, transition metals, alkali earth metals, other metals, nonmetals, noble gases, lanthanides, and actinides). New elements are constantly being added to the table, mostly in the form of artificially-discovered isotopes that exist for a few seconds, if that.

To find out how many neutrons an atom has, simply subtract the atomic number from the atomic weight.

Speaking of isotopes, an isotope is a different form of an element. If an element has one proton and one electron, it is an atom of hydrogen - no other element has one proton and one electron. If a neutron is added, the element no longer becomes hydrogen, but deuterium. Since the element has one proton and one electron, though, effectively giving some of the qualities of hydrogen, it is an isotope of hydrogen.

When two or more different kinds of atoms join together, the result is a molecule. A molecule is the building block of the compound. Some common molecules are those of water (two atoms of hydrogen, one of oxygen), carbon dioxide (one carbon atom, two oxygen atoms), and salt (one atom of sodium, one atom of chlorine).

Two or more molecules put together is called a compound. Some everyday compounds are water (H₂0), calcium carbonate (CaCO₃), and sugar (C₆H₁₂O₆).

When atoms bond to form compounds, the whole atom is not involved. Instead, it is the electrons on the very outside shell that form bonds with electrons from the outer shells of other atoms. There are three different kinds of bonds -- ionic, where the two electrons bond with no sharing of electrical charge, non-polar covalent (the electrons share the charge), and polar covalent (the electrons share, but the larger one receives more of the charge for a longer amount of time).

An atom that has a positive or negative charge is said to be an "ion." Normally, atoms have no charge, as the amount of protons and electrons are equal and they cancel each other out. When an atom gives up or accepts more electrons, it becomes either positive or negative. The amount of positive or negative charge an atom has is called its valence.

A very important rule when dealing with chemistry is the rule of conservation of atoms. It stipulates that in a chemical reaction, atoms are neither created nor destroyed. All of the atoms remain in existence, they may merely be free atoms now, or in combination with other atoms. The main point is that chemical reactions do not create or destroy atoms. Nuclear reactions, however, are a different case.