2.50. Valence Electrons http://www.ck12.org
Electron Dot Diagrams
Because valence electrons are so important, atoms are often represented by simple diagrams that show only their
valence electrons. These are called electron dot diagrams, and three are shown below. In this type of diagram, an
element’s chemical symbol is surrounded by dots that represent the valence electrons. Typically, the dots are drawn
as if there is a square surrounding the element symbol with up to two dots per side. An element never has more than
eight valence electrons, so there can’t be more than eight dots per atom.
Q:Carbon (C) has four valence electrons. What does an electron dot diagram for this element look like?
A:An electron dot diagram for carbon looks like this:
Valence Electrons and the Periodic Table
The number of valence electrons in an atom is reflected by its position in the periodic table of the elements (see the
periodic table in theFigure2.97). Across each row, or period, of the periodic table, the number of valence electrons
in groups 1–2 and 13–18 increases by one from one element to the next. Within each column, or group, of the table,
all the elements have the same number of valence electrons. This explains why all the elements in the same group
have very similar chemical properties.
For elements in groups 1–2 and 13–18, the number of valence electrons is easy to tell directly from the periodic
table. This is illustrated in the simplified periodic table in theFigure2.98. It shows just the numbers of valence
electrons in each of these groups. For elements in groups 3–12, determining the number of valence electrons is more
complicated. You can learn more about the valence electrons of elements in these groups at this URL: http://www.c
olorado.edu/physics/2000/periodic_table/transition_elements.html.
Q:Based on both periodic tables above (Figures2.97 and 2.98), what are examples of elements that have just one
valence electron? What are examples of elements that have eight valence electrons? How many valence electrons
does oxygen (O) have?
A:Any element in group 1 has just one valence electron. Examples include hydrogen (H), lithium (Li), and sodium
(Na). Any element in group 18 has eight valence electrons (except for helium, which has a total of just two electrons).
Examples include neon (Ne), argon (Ar), and krypton (Kr). Oxygen, like all the other elements in group 16, has six
valence electrons.
Valence Electrons and Reactivity
The table salt pictured in theFigure2.99 contains two elements that are so reactive they are rarely found alone
in nature. Instead, they undergo chemical reactions with other elements and form compounds. Table salt is the