What is valence?
Valence is the number of chemical bonds an atom can form. Since hydrogen has a valence of one, you can
measure the valence of an element by the number of atoms of hydrogen that one atom of the element can combine with, or
replace. Look at these formulas: HCl, H2O, NH3, and CH4. What are the valences
of chlorine, oxygen, nitrogen, and carbon? Answer
The concept of valence is useful because it allows us to guess at the formulas for compounds two elements will form.
For example, a compound between oxygen and fluorine would be expected to have the formula F2O, because
fluorine has a valence of one, and oxygen has a valence of two.
However, there is a problem. Many elements have several possible valences. Sulfur, for example, usually has a valence of 2 (as in H2S).
But in some compounds, it has a valence of four (as in the highly reactive compound SF4) or even six
(as in SF6, which is one of the most inert chemical compounds known). So the formulas of compounds
guessed using the most common valences of elements are not always correct.
What are valence electrons?
Valence electrons are electrons that can participate in chemical bonds with other atoms. For main group elements, the number
of valence electrons is easy to determine: it equals the element's group number (a Roman numeral you'll find at the top of the column the
element is in, on the periodic table).
What are the number of valence electrons for chlorine, oxygen, nitrogen, and carbon? Answer
For main group elements, the number of valence electrons follows naturally from the atom's electronic configuration. In all of these elements, the s and p electrons are valence electrons.
In the transition metals, it is possible to have some of the d electrons participate in bonding in addition
to the outer s electrons. It isn't easy to tell by how many d electrons act this way just by looking at the transition metal
atom's electronic configuration. That makes it risky to try to draw Lewis structures for transition metal compounds.