NaCl versus NaCl2
Sodium and Chlorine produce NaCl. Why not NaCl2?
Is it related to the energy required?
The simplest explanation is that sodium tends to form a cation that has a
1+ oxidation state, and chlorine tends to form an anion that has a 1-
oxidation state and therefore, a neutral molecule forms in a 1:1
combination of these ions. . . . but that is more of a simplification of a
more complex idea.
The more in-depth explanation is that ionization energy, the energy
required to excite an outer-shell electron to an infinite distance is a
positive number (energy is needed). Therefore, when Na = Na+ + e- this
needs energy and results in a more energetic, unstable state. An even
higher energy is required to go from Na+ = Na2+ + e-. Likewise electron
affinity is a also a positive number. That is for Cl + e- = Cl-, energy is
required. --Think of this as atoms being stable and to form ions energy is
required because ions are more unstable then the parent atoms. So why
would such ions form in the first place? The reason is that the Na+ and
the Cl- combine to form NaCl and this releases an amount of energy (is
negative) and this release of energy is higher than the other to positive
Thus, imagine the following equations:
(1) Na = Na+ + e- (positive, requires energy)
(2) Cl + e- = Cl- (positive, requires energy)
(3) Na+ + Cl- = NaCl (negative, releases energy)
If we add these three equations together, we get:
(4) Na + Cl = NaCl
Which because the third equation is so highly negative, releases a lot of
energy, the resulting 4th equation is actually negative, and therefore
produces a more stable system.
Now, imagine the situation that arises with the making of NaCl2.
This would mean that:
Another equation (call it 1b) would have to develop such that
Na+ = Na2+ + e- (highly positive, requires a lot of energy)
And equation 2 would have to be used twice (to account for the Cl2)
Thus the new equation (adding up eq 1, 1a, 2 twice, and using a modified
equation 3: (3b) Na2+ + 2Cl- = NaCl2)
Would result in a positive energy value simply because equation 1a and
equation 2 (used twice) would give such a high positive that NaCl2 is
actually requiring energy to form.
Thus, it is not favorable and so we form the more favored NaCl instead.
Greg (Roberto Gregorius)
The electron configuration of the element sodium, Na, is the neon core
[Ne] + 1 electron in the 3s orbital. The element chlorine, Cl, is the neon
core [Ne] + 3s2, 3p5 one electron shy of the argon core, [Ar] 3s2, 3p6.
The combination of Na + Cl ------> Na (+1) + Cl (-1) results in a Ne core
for Na(+1) and a Ar core for Cl(-1). These filled shell configurations are
very stable energetically. There is no "driving force" for NaCl to bond
with another atom of chlorine. The addition of another atom of chlorine
would not be favored.
Click here to return to the Chemistry Archives
Update: June 2012