2.6: Valence Electrons and Electron Dot Formula
Valence Electrons
In any given atom or element there are two types of electrons. There are the core electrons and the valence electrons. Electrons in the outer-most shell, called the valence shell, tend to react (be gained, lost, or shared) during a chemical reaction. You might imagine that, if two atoms bumped into each other, it would be the outer electrons that would interact first.
The number of valence electrons for each main group element can be determined by the column, or group, it occupies on the periodic table.
For example, the elements in the first column (labeled 1A), all have one valence electron.
The second column (2A) has two valence electrons.
We skip the block of ten elements in the middle (transition metals).
The elements in columns 3A, 4A, 5A, 6A, and 7A, and 8A* have three, four, five, six, seven, and eight* valence electrons, respectively.
*Note that helium (He) only has two valence electrons.
Electron Dot Structures of Main Group Elements
Chemists use simple diagrams to show an atom’s valence electrons. These diagrams show only valence electrons which are represented as dots. The dots are arranged to the right and left and above and below the symbol, with no more than two dots on a side . For example, the representation for sodium atom is as follows:
and the representation for chlorine atom is as follows:
These diagrams are called Lewis electron dot diagram s, after Gilbert N. Lewis, the American chemist who introduced them. To write an element’s Lewis dot symbol, place the dots representing its valence electrons, one at a time, around the element’s chemical symbol.
For the e-dot structures, it does not matter what sides the dots are placed on in Lewis diagrams as long as each side has a maximum of two dots. The following figure shows the electron dot structures of the elements, lithium through neon, which is the entire second period of the periodic table. For the main group elements, the number of valence electrons is the same as the group number listed at the top of the periodic table.
This applies to the main group elements. Groups 1B-8B (transition elements) do not follow the predicted patterns well.