Valence and Reactivity
Date: October 2008
Which element will be more reactive - one having ONE electron in its
p shell or another having SEVEN electrons in its p shell?
There are more factors at work than just the number of electrons in the outer
shell - how many filled shells already - how large the atom, what charge is
created? All have an influence.
Your two given examples though, would be operating in different directions.
With only one electron in the outer shell, the atom would be aiming to lose that
electron, and would aim toward a valence of +1, and would do so with some vigour
- we have a strong oxidiser.
The element with seven electrons will try to gain one more to complete the shell
- it will be a strong reducing agent.
Should these two meet, they would be expected to react vigourously - but since
they are operating in opposite directions I think it would be hard to say which
is more reactive.
Good question, but I think your words need a little clarification. Since the
p-orbital only has the ability to hold 6 electrons, I am assuming that your
question is really what is more reactive: an element having 1 valence electron
or 7 valence electrons. Valence electrons are electrons in the outermost
shells, so two electrons would fill the s-orbital first, then followed by up to
6 electrons in the p-orbitals.
That being said, I am not sure if there are numbers to actually answer your
question directly, but I will give you some general comments. One quick note
is that there are a good number of MOLECULES that can defy this rule, but
generally ELEMENTS follow it pretty strongly except for those that are very
small like hydrogen and helium.
First, it is important to realize that we are talking about two specific column
on the periodic table. The alkali elements (column I or 1) and the halogens
(column VIIa or 17). Second, it is important to realize that both columns are
driven by the same force: The Octet Rule. This rule states that atoms will
tend to accept or loose electrons if doing so will result in a full outer shell.
Each of the alkali elements have one s-orbital electron in their outermost shell
and they all react to get rid of it to leave a full valence shell. Halogens are
on the other end of the spectrum. They all have 7 valence electrons and react
to accept an electron to make a full shell. Again, the driving principle is the
same in either case: to leave an atom with a full valence shell. One neat fact
about the alkali metals: The one s-orbital electron is only loosely attracted
to the nucleus, so the electron will tend to travel from atom to atom. This is
what gives metals the property of electrical conductance.
Now, on to your question about reactivity! The problem with this question is
that the alkali metals react in a completely different way and with different
compounds than the halogens. In fact, they will react well with each other
because the metals have one electron to donate and the halogens have one electron
to accept. The resulting salts will be very stable. For example, table
salt/sodium chloride or other salts like cesium fluoride and lithium iodide. It
is useful to understand the reactivity trends for each column. The metals get
more reactive, as the radius of the atom increases. Excluding hydrogen, lithium
is the smallest atom in the column and if you were to have a chunk of this metal
and put it in water, it would fizz and bubble a bit, but nothing all that
impressive. As you move up to sodium, the reaction will be much more significant
and with potassium, the reaction will be very significant. Again, going up to
cesium would create a violent explosion if it were to be exposed even to just
the moisture in the air. Many of these metals are stored in oil to prevent this
The halogen trend is just the opposite. Instead of increasing reactivity as you
move down the column, the most reactive is fluorine and the least reactive is
iodine. (Antimony is generally not a common element, nor is francium in the
metals column.) Again, the reason is the atomic radius. As the radius
increases, the distance of the valance shell from the nucleus increases, and
more importantly, so does the number of total protons and electrons. As an
example, if you have 8 protons and 7 electrons very close to each other, the
1/8 (one eighth) difference can be expressed as a percentage--12.5% difference
between total protons and total electrons. But if you have 40 proton and 39
electrons, then you only have a 1/40th difference, which is a 2.5% difference.
The difference is smaller, so the driving force to add an electron will not be
as great. The distance also has a very relevant result too. As the distance
between a proton and an electron increases, the attraction decreases
exponentially (to the square of the distance to be exact).
So while I did not directly answer your question, these trends in each column
are important to know when evaluating what will react more.
Depends on which period the two elements are in. If they are in same period
(row) of Periodic Table, reactivity is about same. Bigger atoms more easily
lose outer (valence) electrons. Smaller atoms have greater attraction for
eighth electron than larger atoms.
Reactivity is a tricky question. (1) What would be the measure of reactivity -
how much energy is released, how fast the reaction goes, how many different
types of reaction the substance can do -all of these can be measured and in
their own way speak to the "reactivity" of a substance. (2) Since the reactions
of Group IA and Group VIIA are different because of the number of electrons in
their outer shell (not p as you put it) - that is, they do not react with the
same things, then how exactly do you compare reactivity if you choose speed (or
rate of reaction) as your measuring standard?
More importantly, in a reaction that, say, forms NaCl - in this case Na has 1
electron in the outermost orbital and Cl has seven - when they react, how do
you say that Na, say, was more reactive than Cl when they are reacting with each
other? I understand that you want to know whether losing the one electron is a
more favored reaction than the gaining of the one electron - but since one does
not lose electrons without it being gained by another -this makes the question
really difficult to answer.
The only way this can be answered somewhat is to ask how much energy does it
take to remove an electron from an atom having one electron in its outer shell,
and compare it to adding an electron to one that has seven electrons in its outer
shell - and the data indicates that Group IA elements tend to *require*
approximately 500kJ/mol to remove that one electron. On the other hand, Group
VIIA atoms *release* energy, about 325kJ/mol to gain an extra electron. Remember
that the signs are different since one is a gain in energy and the other is a
release of energy. So compare carefully - also understanding that reactivity
(as in rate or speed) has little to do with initial and final energies.
Greg (Roberto Gregorius)
The number of electrons in a valence shell is not a good measure of an element's
reactivity. First, "Reactivity" depends upon many other factors than the number
of electrons in the valence shell. For example: Reactivity with respect to what
other reactant. Second, and more importantly, only 6 electrons can be occupied
in the "p" orbitals of any element, so SEVEN electrons in a "p" orbital is not
possible by the Pauli exclusion principle.
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Update: June 2012