This is where I come when the bathroom is airing out, beautiful isn't it? Another beautiful thing is knowledge, let me drop some on you. Isotopes are two or more of the same element that have different amounts of neutrons in their nuclei. They are only different in atomic mass but not in chemical properties, so its not a different element.
Sorry for the change in scenery, it was getting dark and I had to head home. To continue off our last topic, Isotopes can be stable or unstable. What makes an unstable nucleus is the size of the nucleus, if the nucleus becomes too large it will be unstable and will try to get rid of its abundance of protons/ neutrons.
Okay, time to get into some math. Everybody favorite! The average atomic mass of an element is the sum of the isotopes masses, each one multiplied by its natural abundance on Earth. The equation is m1, f1+m2, f2,... so on and so-forth. Anyhow, it getting late so i'm gonna hit the hay. You can see yourself out.