From the ionization energies we might expect lithium to be the weakest of the alkali metals as a reducing agent in water. However, the standard reduction potentials indicate that it is the strongest. This reversal results mainly from the very large energy of hydration of the small Li+ ion. Please note that the reaction takes place in water and not in a gas phase. What this means is that because of its relatively high charge density, the Li+ ion very effectively attracts water molecules. A large quantity of energy is released in the process, favoring the formation of the Li+ ion and making lithium a strong reducing agent in aqueous solution.

A word about ionization energy: The definition according to my chemistry book is: The first ionization energy is the energy required to remove the most loosely held electron from one mole of gaseous atoms to produce 1 mole of gaseous ions each with a charge of 1⁺.

X(g) ----------->X(g)⁺ + e^-

The state symbols - (g) - are essential. When you are talking about ionization energies, everything must be present in the gas state.

Now , let us understand the ionization energy for the alkali group:Why is the sodium value less than that of lithium? >There are 11 protons in a sodium atom but only 3 in a lithium atom, so the nuclear charge is much greater. You might have expected a much larger ionization energy in sodium, but offsetting the nuclear charge is a greater distance from the nucleus and more screening.

Li 1s²2s¹ 1st I.E. = 519 kJ mol-1

Na 1s²2s² 2p⁶ 3s¹
1st I.E. = 494 kJ mol-1

Lithium's outer electron is in the second level, and only has the 1s² electrons to screen it. The 2s¹ electron feels the pull of 3 protons screened by 2 electrons - a net pull from the center of 1⁺. The sodium's outer electron is in the third level, and is screened from the 11 protons in the nucleus by a total of 10 inner electrons. The 3s¹ electron also feels a net pull of 1⁺ from the center of the atom. In other words, the effect of the extra protons is compensated for by the effect of the extra screening electrons. The only factor left is the extra distance between the outer electron and the nucleus in sodium's case. That lowers the ionization energy.

OK, one more thing that is interesting in the alkali group. We know that lithium is the strongest reducing agent, so why is it that it reacts more slowly with water than sodium or potassium? Maybe you can think about this one and let me know if you have an explanation. (One hint: what factor controls the rate of a reaction?.

I believe that this apparent contradiction you asked about can be best explained by reexamining the definitions of these properties. The ionization energies for removing an electron from an atom are measured for gases - for lithium, this could be expressed as: Li (g)--> Li⁺(g) + e-However, the reduction potential refers to an electron loss while in an aqueous solution, or: Li --> Li⁺(aq) + e-The ionization energies are basically a measure of the forces between the(negatively charged) electron and a the (positively charged) nucleus (with other effects such as shielding, etc.). The reduction potential, on the other hand, also takes into account the interaction with water molecules.(Li has a very strong interaction with water molecules due its relatively large charge density from its small size). So, as it turns out, Li can readily lose an electron in water and be a very reactive metal.

The issue is that ionization energy is measured against a gas phase lithium molecule or atom. Here the energy is adequately modeled by a hydrogen atom with screening (as are all the alkali metals). Lithium is indeed the hardest to ionize as an individual atom since the screening effect is the weakest. On the other hand, Reduction Potentials are measured typically in water or hydrogen electro-chemical cells. In such a cell, the ionization potential half-reaction is measured from metallic lithium to the dissolved ion. While the two measurements are related, there is no reason that the potential energy for the water reaction should be the same as that for the gaseous Li 2 phase. Note that although the ionization potential decreases monotonically for the alkali metals as they get heavier, the reduction potential is not monotonic -- I suspect there is a thermodynamic process active here with water acting as a medium and as the oxidation and reduction agent. There is the possibility that water has a physically favorable reaction with lithium metal, due to its lattice spacing, versus the other alkali metals.