What is the difference between valence, and number of valence electrons?
Valence electrons are the outermost electrons of an atom.
Here is a video which discusses how to draw Lewis structures for atoms showing their number of valence electrons.
Video from: Noel Pauller
Valence refers to the ability of an element to combine with other atoms. There are two competing definitions.
- Valence is the maximum number of hydrogen or chlorine atoms that can combine with an atom of the element.
According to this (official) definition, the valence of carbon is four, because carbon can form CH₄ and CCl₄.
- Valence is the number of hydrogen atoms or twice the number of oxygen atoms that can combine with an atom of the element.
Many chemists use the second definition. By this definition, carbon has more than one valence. The valence of carbon can be either two or four, because carbon forms CO and CO₂.
According to the first definition, nitrogen has a valence of 4, because it forms NH₄⁺ ions.
But nitrogen also forms NO and NH₃. According to the second definition, it also has valences of 2 and 3.