Surrounding the nucleus is the electron cloud, where you can find the negatively-charged electrons.?? The cloud is made up of various shells, depending on how many electrons are connected to the element.

The first shell in any element can hold two electrons, and after that every shell can hold at least eight electrons. (More complex atoms can hold up to 32!) When the outer shell of an element is completely filled, the element is stable and not as reactive as one that isn't totally filled. Reactive elements like to attach themselves to other elements or steal electrons from other elements all the time!

One unusual example is the element hydrogen, which has only one electron! That means that it's very reactive, because it "wants" to get another electron to balance itself out.

Elements that don't have the normal number of electrons in their outer shells are called ions.?? (Editor's Note: Hey, I know where you've heard of that before -- the Ion Engine Game!?? I wonder what that means....)

The elements also have their own symbol, a one, two, or three-lettered abbreviation for the certain element. While some abbreviations come from the first letter of the name (such as F for fluorine), others come from two significant letters in the element (Mg for Magnesium, Ne for Neon) or are based on its name in another language (like Hg for Mercury, meaning hydrargyrum, or 'liquid silver' in Latin).

There are 112 known elements, but not all of them were discovered at the same time. In fact, Ununbium, the 112th element, was discovered in 1996, just about eight years ago.

A man named Dmitri Mendeleev was actually a big part of the development of the periodic table. In 1869, Mendeleev had constructed his own version of the periodic table. Every day, he sorted the known elements in different ways. One day it was by atomic number and another day by the date they were discovered. His experiments in organizing have led to the way we sort the elements today -- by atomic mass.

There are three types of elements, mainly -- solids, liquids, and gases. The only two elements that are liquids at room temperature in the periodic table are bromine and mercury! In the periodic table, there are also different groupings which shows how reactive the various elements are. Look below:

• Alkali Metals: These are found in Group 1 of the Periodic Table. They are highly reactive metals that might explode if exposed to water.
• Alkaline Earth Metals: These are found in Group 2 of the Periodic Table. They are also reactive metals, but not as reactive as the Alkali metals. They are not found free in nature.
• Transition Metals: These are found in groups 3 through 12 of the Periodic Table. They are ductile and malleable (easily molded) and conduct both electricity and heat.
• "Other" Metals: These 7 elements are located in groups 13, 14, and 15, are all solid, have a high density, and are opaque.
• Metalloids: These 7 elements are found along the stair-step to the right of the Periodic Table. They have properties of both metals and non-metals.
• Non-Metals: Located in groups 14-16 of the Periodic Table. They are very brittle, and not too ductile of malleable. They cannot reflect light and have no metallic luster.
• Halogens: Found in group 7 of the Periodic Table. All of the halogens are known as 'salts', and exist in all 3 states of matter.
• Noble Gases: Found in Group 18 of the Periodic Table. These elements are all stable.
• Rare Earth Elements: These are the elements in the Lanthanide and Actinide series. There are 30 rare earth elements, about half of them man-made.