General ---------Biology ------------------Online------------------   ----wilcox@sunline.net Ch.  2 :  The Chemical Context of Life

Predicting how various elements will "behave" when they interact to form molecules requires an understanding of an elements atomic structure. 

An element's atomic number, atomic weight, and if it is an isotope, are all important.  But, the most important feature in predicting how atoms will interact to form molecules is the atom's "electron configuration."  Consider the electron configuration of hydrogen versus helium.  (In the figures below, the nucleus is shown as a blob; the protons and neutrons are not shown.  If you want to refresh your memory on how these nuclei "look," click on the nuclei below.)

Recall:  the first electron shell  holds up to two (2) electrons.  The second shell holds up to 8 electrons.  Atoms are "driven" to fill their outer electron shell.   Based on these observations, answer the following question:  Based on their electron configuration, predict which atom will be more "reactive" (interact to form molecules) hydrogen or helium?  _____________ (answer)

Summary: You should understand 

• electron configuration is most important in determining reactivity
• atoms with full outer shells are not reactive and are called "inert"
• atoms with space in their outer shell, will react to form molecules