Re: I dont understand what an isotope is, can you explain it to me?

I am sure you understand the basic structure of an atom. An atom has three basic particles: electrons, neutrons, and protons. The center of an atom is composed of the protons and neutrons while the electrons circle around. Now, each and every element is composed of these three particles but in different amounts. For each element, there is a basic number of these particles to make up its structure. For example, oxygen has 8 neutrons, 8 protons, and 8 electrons. An isotope is basically this same atom (protons and electrons) but with a few extra or a few less neutrons in its make up. Some "isotopes" of oxygen have as few as 4 neutrons and up to 16 extra neutrons in their make up, but all still have 8 protons and 8 electrons. All these different "isotopes" are considered oxygen because their protons and electrons remain at the same number. It's just the number of neutrons in the atom that changes.

Since these isotopes contain the same number of electrons, they will all behave the same chemically to a great degree. In a chemical reaction, oxygen-14 (or ¹⁴O), oxygen-16, and oxygen-22 all behave the same. However, in some instances, an isostope with too many neutrons will be too big physically to pass through cell membranes, creating some interesting situations.

This difference in neutron numbers helps determine whether an element will be radioactive or not. When all three particles are in the right proportions, the element will be stable and not decay. They will not always be in the same amounts, but close in numbers. It is when the number of protons or neutrons change above or below that special number that the atom is considered radioactive and will decay to another element or atom that is stable.

As you seem to be aware, there are many naturally occurring radiaoctive elemetns in nature. Potassium (K) is one of them. Potassium's natural states are K-39 (19 protons, 20 neutrons), K-40 (19 protons, 21 neutrons), and K-41 (19 protons, 22 neutrons). Potassium represents one of the cases where the number of protons and neutrons do not have to be the same number to make a stable isotope. However, K-40 is radioactive with an extremely long half-life of 128,000,000,000 years. For some, this is considered a "stable" form of the element.

Postassium has approximately 20 isotopes. All of them will have 19 protons and electrons. The number of neutrons each isotope of potassium will have ranges from 16 to 35. This will mean that the atomic number (the total number of neutrons and protons) will range from 35 (19 + 16 = 35)to 54 (19 + 35 = 54). This will then let you know that the isotopes of potatssium range from K-35 to K-54. The typical half-life for postassium isotopes (besides the stable ones) range from 22 hours to just a few milliseconds.

I hope I have explained this well enough to you. If you have any additional questions, please feel free to let me know.

References:
Cember, Herman. Introduction to Health Physics, 3rd Edition. McGraw-Hill:USA. 1996.

Walker, F. William, Josef Parrington, and Frank Feiner. Nuclides and Isotopes: Chart of the Nuclides, 14th Edition. GE Nuclear Energy/GE Electric Company:San Jose, California. 1989.