Hi again,
When you go down from lithium to sodium to potassium, you are increasing the atoms reactive behavior (potassium violently reacts whereas lithium reacts less so).
What is the cause of this reactive behavior? According to aamc, it is the ionization potential, but why can't it be electronegativity or polarizability? The less electronegative potassium holds its electrons less tightly, and thus promotes more reactivity. Also, it is more polarizable, and therefore more prone to reaction...what am I missing here?
I don't have a perfect answer, but I'll give it the best explanation I can.
Broadly, the differences between electronegativity, ionization potential, etc. is outlined below:
Electronegativity - a measure of an atom's tendency to gain and retain an electron from a neighboring atom within a bond.
Electronegativity can be measured by the dipole moment. This means that the element in question is already bound and therefore reactivity is not in the question.
Polarizability (I don't have a formal definition, but it's something related to) - the ability for a particle/molecule to be polarized in an electric field
To my understanding, somewhat related to the dielectric constant of a given compound, or its ability to "line up" in an electric field.
Electron Affinity - the tendency of an element to gain an electron
Electron Affinity can ultimately be thought of almost as the "opposite" of ionization energy. Because ionization energy is the energy required to remove an electron, whereas electron affinity is the tendency to gain an electron. This can either be considered a subset of ionization or probably more accurately the other half of the same coin of ionization.
Ionization Energy - the energy required to remove an electron from the valence shell.
This energy is directly related to the electron shielding of an element, which is dependent on both the effective charge and atomic radius of the element. The greater the effective charge (eff charge increases from left to right), the lower the electron shielding, and therefore more energy is required to remove an electron. Also, as you go down a group the radius is greater and the shielding is greater. For the sake of comparing Group 1, effective charge is irrelavent, but electron shielding [by atomic radius] is still important.
Ionization energy can also be directly correlated to the oxidation potential. In other words, the easier it is to ionize an atom (lower ionization energy), the easier it is to oxidize the atom (greatest oxidation potential) by 1 electron, and therefore the element reacts more readily.
As I mentioned before, oxidation and reduction potentials are really two halves to the same coin; which is why the elements with the greatest oxidation potential (lowest ionization energy) or greatest reduction potential (highest ionization energy excluding noble gasses) are the most reactive elements (bottom left and top right of periodic table).
This explanation is very convoluted when considering elements not at the extreme, and there are other considerations such as half-orbital stability which also have trends in ionization energy. Actually now that I think of it, stability is another good way to answer this. Ultimately less stable = more reactive in every situation I can think of. [And ionization energy has a trend that mirrors stability]
MUCH LESS CERTAIN ABOUT EVERYTHING BELOW
Specifically with respect to the discussion of Group I elements, the topic of exothermic vs endothermic seems appropriate, but is in fact misleading. [I'm actually a little uncertain here, clarification would be helpful =)]
Enthalpy of Heat - (with highly reactive, and in this case, highly exothermic reactions, this may be misleading) - while elements such as lithium and sodium will have more negative enthalpy's of heat = more exothermic = more heat energy given off in the reaction. This might suggest that lithium has a hotter reaction which intuitively suggests more reactive, but may be misleading. I think the true basis of
reactivity depends on the ease with which to reduce/oxidize as explained above or perhaps even the Gibb's free energy associated with the reaction, but I lack the data to show a trend or any proof of this. Certainly oxidation/reduction potential is directly proportional to reactivity.
