A Lithium Atom has 2 electron shells

        This 3rd electron is forced to spin around the nucleus in a second electron shell because of the repulsive forces between the 3 negatively charged electrons.  Just as 2 positively charged protons repel each other if they are brought too close together, so will negatively charged electrons repel each other.  No neutral particles exist however to help hold electrons together in their shells (neutrons only exist in the nucleus).  When an electron will not fit into an existing level because of repulsive forces between negative charges, the atom will work around this problem by adding the electron to a new shell, more distant from the nucleus than the existing shell (or shells).

        In other words, electron shells have a limited capacity for electrons.  As you might expect, the farther an electron shell is from the nucleus, the larger it is.  You can calculate the total capacity of an electron shell using the formula 2n², where n equals the number of the electron shell.  For example, for the 1st electron shell n = 1 and 2 x 1² = 2, telling us that the capacity of the 1st shell is 2 electrons as we have already seen.  For the 2nd shell (n = 2) and 2 x 2² = 8.  For an atom to fill its 2nd electron shell, it would need 10 electrons: 2 to fill the 1st shell and 8 to fill the 2nd.  The 3rd shell has a total capacity of 2 x 3² = 18 electrons.  But things get a bit tricky here.  Electron shells actually have sublevels.  The first sublevel (the s sublevel) holds 2 electrons.  The second, p, sublevel holds 6.  The third, d, sublevel holds 10.  After levels 3s and 3p are filled, electron shell #3 acts as if it has reached capacity with only 8 total electrons.  In other words, in an atom with 20 electrons (which is the element calcium, Ca) the first 2 electrons are located in the 1st shell, the next 8 in shell #2, the following 8 in shell #3 and the remaining 2 electrons are located in shell #4.

        As you can see, at this point atomic theory begins to get complicated.  Just as we saw in the last lesson that the electron is not a simple particle but a more complex wave, here we find that filling electron shells is not as simple as stacking books on a shelf.  This is because at the atomic level things are just plain wierd.  Particles travel through time and space like something out of a bad Star-Trek re-run.  It all boils down to something called quantum theory.  We'll try to keep things as simple as possible, but if you would like, you can learn more about atomic structure and quantum theory using some of the links listed at the bottom of this page.
 
 

By convention there is color,  by convention sweetness,  by convention bitterness,  but in reality there are atoms and space.  -Democritus (400 BC)

        Why does this matter?  What significance do electron shells have on the fact that "you'd rather be fishing"?  As it turns out, the reason hydrogen atoms stick to oxygen atoms to form the water in the fish pond depends on electron structure.  Actually, just about everything depends on electron structure.  The chemical properties of an atom will depend on the number of electrons in the atom's outermost (or valence) electron shell.  Thus lithium behaves more similarly to hydrogen than it does to helium because both lithium and hydrogen have 1 electron in their outermost shell.  The atoms can be arranged according to their electron structure and chemical properties.  This is where we encounter the Periodic Table of Elements.

        The Periodic Table of Elements (or the Periodic Table for short) was first proposed by a Russian chemist named Dmitri Mendeleev in 1871.  A modern version of the Table appears below.  Atoms are ordered by their atomic number in the Periodic Table.  The Table is set up so as to indicate the number of electron shells in each atom and the number of valence electrons (electrons in the outermost shell) in the atom.  As you descend rows in the Table, the number of electron shells in the atom increases.  For example, hydrogen (H) in the 1st row has 1 shell, lithium (Li) in the 2nd row has 2 shells, sodium (Na) 3 shells, etc.  As you read the Table from left to right in any one row, the number of valence electrons increases.  For example, hydrogen has 1 electron (in the first shell).  Helium (He), the 2nd element in the first row, has 2 electrons (thus filling its valence shell).  Let's look at lithium (Li) again.  From the atomic number we know that Li has 3 electrons.  From its position on the Periodic Table (and from our discussion above) we know that Li has 1 valence electron: 2 electrons fill Li's 1st shell and 1 orbits in the second shell.  From its position on the Table we know that berylium (Be) has 2 valence electrons in its 2nd shell.  Can you predict the structure of the next element, Boron (B)?

        Each element in the Table below is linked to information on Chris Heilman's Pictorial Periodic Table.  To return to this page hit the 'Back' button on your web browser.

        Using the Table row and group numbers to predict the number of valence electrons in an atom works reasonably well for the metals, metalloids and nonmetals (see the color legend above). However, with the transition metals our simplified explanation of valence electrons begins to break down and quantum theory is needed to explain behavior.  We will limit our discussion to the metals, metalloids and nonmetals.  In our simplified approach to atomic structure, you can predict the number of valence electrons in an atom by ignoring the transition metals and counting from left to right in group A of the Periodic Table.  Thus, elements in group IA have 1 valence electron, elements in group IIA have 2, IIIA have 3 valence electrons, etc.  If you are having trouble visualizing this, an abbreviated Periodic Table (with the transition metals removed) can be viewed by clicking below.

For those interested, some links to other resources have been included below.

• A Practice Test on Atomic Structure
• More in-depth information on the structure of the atom:
• The Atomic Model: an interesting summary of developments in atomic theory
• The Physics 2000 Quantum Zone: very cool, but not for the faint of heart
• The Particle Adventure: quarks, muons, universal forces and more
• Electron Orbital Shapes: that says it all
• Portrait of an Atom: an interesting artistic rendition of atomic structure

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Copyright © 1998-1999, All Rights Reserved, Anthony Carpi
The Pictorial Periodic Table © Chris Heilman