Re: Why is Lithium first on the Activity Series of Metals?

It is certainly true that lithium is, as the periodic table would suggest, the least reactive of the alkali metals. You only have to observe the reactions of the alkali metals with water to see this (preferably from a video -- extreme safety issues. Should only be tried by a qualified chemist).

The Activity Series you are referring to actually relates specifically to aqueous (i.e. water) solutions. There are some special features of the lithium ion that sets it apart from other alkali metal ions, and makes it a much more low-energy species in aqueous solution.

The lithium ion is much smaller than any of the other alkali metal ions. You might know that when we talk about Mg(2+) or Al(3+) in aqueous solution, we are really talking about Mg(H2O)6 (2+) and Al(H2O)6 (3+), because these small multiply charged positive ions strongly attract the negative oxygen parts of the water molecules. Lithium, even though it has only a single positive charge, is small enough that it can also attract water molecules in this way. Sodium and the other alkali metals only bind to water very much more weakly. They are large, and have only a single charge.

If you look at ionization potentials of the alkali metals, which relate to the gas phase ions, you will find the order
Cs<Rb<K<Na<Li,
as you would expect from the periodic table.
But if you look at reduction potentials, known as the Electrochemical Series or your Activity Series, you find the order
Li<Cs<Rb<K<Na
because these values relate to the aqueous ions.

This effect is discussed in detail in many University level Inorganic Chemistry texts (e.g. Greenwood & Earnshaw, Chemistry of the Elements, p. 87). It is not usually covered in high school chemistry, beyond the barest mention.

Incidentally, the electrochemical series is determined by electrode potentials. The idea of actually setting up a Cs(metal)|Cs+(aqueous) electrode in a half-cell with a standard hydrogen electrode to measure the emf is a nonsense: the Cs metal would explode when it contacted the water! But there is a fairly straightforward way of working out what the electrode potential "would be" from the standard free energies of the various materials involved, which can be obtained experimentally.