Is there a difference between valence and valence electrons?

Question

Is there a difference between valence and valence electrons?

Answer 1

Valence electrons are the number of electrons in the valence shell. Valence if the number of electrons ADDED or REMOVED to/from the shell to produce the net valence charge (pseudo or "real [ionic]).

Answer 2

Well, yes, usually.  Depends what you mean by the word "valence".

What does the word “valence” mean?

The answer depends enormously on the context in which the word "valence" is used, who's using it, and what they're trying to learn or convey by doing so. Historically, its meaning has changed as our understanding of atomic and molecular structure has advanced, and the older meanings (which predate any real understanding of chemical bonding) can still creep in. This wouldn't be so much of a problem, except that old definitions and new definitions are often talking about contradictory ideas and models of bonding.  The old definitions really ought to be discouraged and abandoned.

In the modern sense, the one that most chemists usually use, “valence” refers to the highest-energy electrons in an atom or molecule. For an atom, that usually (but not always) means those with the highest principal quantum number n.

e.g. As is (1s)2 (2s)2 (2p)6 (3s)2 (3p)6 (3d)10 (4s)2 (4p)3

The five n = 4 electrons are the valence electrons: (4s)2 (4p)3

All the others are the core electrons.  There are exceptions, but that's the jist.  It's also a concept that extends to molecules.  Knowing how many valence electrons a compound has, and what orbitals they're sitting in, determines the structure, bonding and many of the properties of a compound.  Which is why the word is right there in valence bond theory, one of the most successful theories of chemical bonding ever devised.

However, historically, "valence" and "valent" have meant different things.

Sometimes “valence” can mean the number of bonds formed by an element, its “combining power”.  In this sense it corresponds to the number of electrons an atom is using to form covalent bonds.   e.g. N has a typical valence of 3, because it tends to form three bonds, O has a valence of 2, because it tends to form two bonds.  Sometimes you can relate this to the number of valence electrons by the previous definition (B has 3ve, usually forms 3 bonds), but not always (N has 5ve, usually forms 3 bonds), because an element doesn’t always commit all its valence electrons to covalent bonding.  Sometimes it's related to the formal charge on the closed-shell ion you expect to form: [C}4- [N]3- [O]2- [F]- usually form 4 3 2 1 bonds respectively, but that doesn't work as well for metals: although Mg has two valence electrons and makes MgCl2, that compound doesn't contain two covalent Mg-Cl bonds per Mg centre (it's an ionic complex).  It also doesn't help that most elements have a variable “valence” in this sense: SF2 SF4 SF6 are all stable, you can make [NH2]– or NH3 or [NH4]+, NO or NO2 or [NO3]–.  This use of the word stopped being useful ninety years ago; it should be strongly discouraged, and replaced with either “coordination number” or just “number of bonds”.

Worse yet, the words "valence" and “valent” can also refer to oxidation state.  It still crops up in this sense, especially in TM chemistry, but again such a use should be discouraged.  e.g. FeCl3 contains [Fe]3+, “trivalent” iron; Ni(CO)4 contains [Ni]0, “zero valent” nickel.  This definition can relate back and show congruence with the previous two definitions for some main group elements: Al has three valence electrons, tends to form three covalent bonds (when it forms a covalent compound), and formally adopts an oxidation state of +3 e.g. AlBr3. But for transition metals, the three definitions have nothing to do with one another.  Note the confusion: Ni in Ni(CO)4 has 10 valence electrons, four bonds, and a zero-valent oxidation state.  Fe(II) has six valence electrons, an oxidation state of two, and I can think of Fe(II) compounds right now with 2, 3, 4, 5, or 6 covalent bonds.

So, yes, there's a difference.  "Valence electrons" is a useful concept with a precise definition rooted in modern models of atomic structure and molecular bonding.  "Valence" is an arcane and no-longer-useful idea, used to mean different things in different contexts (often mutually contradictory), and should be replaced with phrases like "oxidation state" and "coordination number".