Element

Elements are the different varieties of atoms that exist. They are the basic components of matter, which cannot be broken down into smaller parts (except by much higher energy nuclear reactions). The twenty or so heaviest are so unstable they only ever occur in intentional lab experiments, and even then not for very long. Elements are distinguished from each other by their numbers of protons. For instance, any atom with 26 protons is iron. Protons and neutrons congregate in the nucleus, and electrons zip around far away from the nucleus. Chemical reactions result from the addition or subtraction of electrons. Removing an electron creates a positive charge, and adding an electron creates a negative charge to an atom.

The elements have different chemical properties, and so do their compounds.

Overview
Each element has distinct chemical properties in its pure form, and some (like carbon, phosphorus, and tin) have very different pure forms. or allotropes. Diamonds and graphite as both carbon, although they look very different. The allotropes of phosphorus are known by their colors and act very differently. Tin has a metallic form and a non-metallic form. The various elements are related due to the rather repetitious and simplistic structures that recur as they get larger, probably due to our creator's laziness. These similarities allow the elements to be grouped in the Bible of Chemistry, known as the Periodic Table.

Well, even without a Creator, elements with completed outer shells of electrons (the "noble" elements helium, argon, neon, krypton, and xenon; radon is likely much the same except that its radioactivity makes its chemistry "messy") refuse to accept electrons and (except for krypton and xenon) hold on so tightly to their outer electrons that they remain distinct atoms. It is possible to make compounds out of krypton and xenon with great difficulty, and these compounds are usually quite unstable. The elements with one or two more electrons than a noble gas typically cast off their outermost electrons much as a stripper removes her clothes, but to form positive ions instead of showing some shocking anatomy; elements with two or one fewer electrons than a noble gas tend to grab enough electrons to create a negative ion. The difference between negative and positive ions is shocking, and positive and negative ions tend to form salt-like ionic compounds like salt (sodium chloride) or calcium oxide. Chemistry is much more complicated for elements more than two electrons away from having an inert-gas structure.