Lewis Symbols
As far we have looked at atoms that form ions and then are held together by electrostatic attraction between the positive and negative charge. These are collectively refered to as ionic compounds.   However some molecules do not play the give and take electron game, and simply share the electrons in what will be called a covalent bond. Quite often the electrons are not shared equally and result in polar compounds. In a few rare examples the electrons are shared evenly and we find that these are non-polar covalent compounds.

A useful tool in understanding covalent bonds are Lewis symbols, named after G.N. Lewis (1875-1946).  To draw a Lewis symbol for an element write the element's symbol surrounded by a number of dots. The number of dots represent the number of electrons in the valence shell in the outer shell of the element.  For example, Li and H both have 1 electron in their outer shells, 2s and 1s respectfully. They would be written as Li .   and   H .
 
Each element in Group IA have the same Lewis symbol, H . , Li . , K . ,  Rb . ,  Cs .
 
The Lewis symbols for the eight A-group elements of period 2** are:

Group  IA      IIA       IIIA     IVA     VA      VIA    VIIA    VIIIA
                                    .          .          .          ..         ..          ..
             Li .   . Be .    . B .     . C .    . N :     . O :    . F :     : Ne :
                                                .          .           .          ..           ..


The elements below these in the periodic table are given identical symbols. Notice that when an element has more than four electrons, the additional electrons are shown paired with others.  Also notice for the Group A elements that the number of valence electrons is equal to the group number.

** For beryllium, boron, and carbon, the number of unpaired elcetrons in the Lewis symbol does not agree with the number predicted from the atom's electron configuration. Boron, for example, has two electrons paired in the 2s orbital and a third electron in one of the 2p orbitals. So there is actually only one unpaired electron in a boron atom. The Lewis symbols are drawn as shown however because when beryllium, boron, and carbon form bonds, they behave as if they have two, three or four unpaired electrons, respectively.

Although we use Lewis symbols mostly to follow the fate of valence electrons in covalent bonds, they can also be used to describe what happens during the formation of ions. For example, when a sodium atom reacts with a chlorine atom,  the electron transfer can be depicted as
                                                     ..                                           ..
                                      Na .  +  . Cl :  ------->   Na+   +   [ : Cl : ]-
                                                     ..                                         ..
 
The valence shell of the Na ion is emptied and so no dots remain. The outer shell of the chlorine, which has seven electrons, gains the sodium's electron to give a total of eight.  The brackets are drawn around the choride ion to show that the electrons represented by the dots are its exclusive property.