They are those little dots at the end of sentences. Seriously though, a period is a horizontal row in the periodic table of elements. All the known elements fit into the first seven periods.

8/10/98 - Atomic Numbers

Atomic numbers are the number of protons in an atomic nucleus of an element.

8/11/98 - Condensation

Condensation is the process of a gas being converted back into a liquid. It usually happens when the gas cools to below it's boiling point. A good example of this is water droplets forming above a stove when you are cooking noodles.

8/12/98 - Gases

They types that you put in your car are definitely not what we're thinking of here. A gas is matter in a form that has a very low density and it will expand by itself if placed in a larger containter until it is evenly distributed throughout the container.

8/13/98 - Groups

Groups are vetical rows on the periodic table of elements. Elements that belong in the same group together will usually have very similar chemical propterties.

8/14/98 - Heterogeneous Mixtures

A heterogeneous mixture is a sample of matter that has more than one substance contained in it, and more than one phase at the same time.

8/15/98 - Metals

Metals are elements that usually have a nice shine to them, they can be beaten into sheets or other tools, spun out into wire, and they are excellent conductors of electricity and heat.