﻿ GED Science Practice Test: Radioactivity, Isotopes And Radiocarbon Dating | Open Window Learning
Periodic Table, Atom Structure, and Chemical Bonding

We have focused a great deal on electrons, particularly valence electrons, in this unit, because they are so important to understanding many of the physical and chemical properties of elements and compounds.  We now turn our attention to neutrons, and how variances in the number of neutrons in an atom explains radioactivity, which is an important property of some materials.

Isotopes: The number of neutrons in the nucleus of an atom of a specific element can vary. For example, carbon, which normally has 6 protons and 6 neutrons, also has versions that have 7 neutrons or 8 neutrons in the nucleus. These kinds of atoms, that have atypical numbers of neutrons in the nucleus, are known as isotopes.  Because different isotopes of an element have different numbers of neutrons, different isotopes of an element also have different atomic masses.  The following table shows the three common isotopes of carbon:

There is a great deal of other information in the diagram above.  Isotopes are named with a combination of their element symbol and their mass (e.g., carbon-12, carbon-13, etc.).  Some isotopes are more common than other isotopes; carbon-12 makes up ~99% of the carbon on earth, while carbon-13 makes up ~1% of the carbon on earth.  If you look up the atomic mass of carbon in a periodic table, you will notice that it is not a whole number, despite the fact that the atomic mass represents the number of protons and neutrons (which, themselves, can only be whole numbers).  The mass of carbon in the periodic table is 12.0107.  The reason for this is that the atomic mass in the periodic table is an average of the atomic masses of the isotopes of carbon.  The atomic mass of carbon (12.0107) is close to 12, because most of the carbon on earth (99%) is carbon-12.  However, the small amount of carbon-13 and carbon-14 raise the average atomic mass just a bit.

Isotopes of an element differ in their stability.  Carbon-12, for example, is stable, while carbon-14 is not.  This instability leads an atom to give off particles or energy in order to become more stable.  The emission of these particles or energy is called radioactivity.  The particles or energy, themselves are called radiation.  The following diagram shows the different types of radiation and their ability to penetrate certain substances.

Taken from: http://www.daviddarling.info/encyclopedia/G/gamma_rays.html

As you can see in the diagram, some type of radiation can penetrate through human body tissues.  Even alpha particles, which can be stopped by a sheet of paper, can interact with human skin.  Radiation can lead to changes within the DNA of our cells, causing mutations, and possibly cancer.  However, radiation is not all bad.  Radiation is how x-rays and some cancer therapies work.