Re: activity series, Why is Li most reactive

Hmmm... the first sentence in your question seems to be more of a statement
than an actual question. If you are asking whether or not Lithium is the
most reactive of the Alkali Metals, then the answer is "no, it's not". In
fact, depending upon the reaction, it is the least reactive.

I should explain that "reactivity" is a very subjective term. For example,
Lithium will react with Nitrogen to give Lithium Nitride and none of the
other elements in the first column will do this. In this regard, Lithium
is definitely the most "reactive". But I don't think this is what you are
after.

The question of "reactivity" for the Gp.1 elements really comes down to a 
question of ionization:
"Will the element ionize to give a cation and how easily will it do so?"

The reason that elements lose electrons is because electrons aren't held in
place very tightly. The attraction between the negatively charged electron
and the positively charged nucleus can be described in simple electrostatic
terms, with consideration for the nuclear charge and the electron shell.
When enough energy is applied to the atom, the electron is "bounced" out of
its shell and escapes - ionization has occurred. This has the advantage of 
yielding a "closed shell" configuration for the ionized atom, which has a
much higher ionization potential and is thus "stable".

How easy is removing an electron from a Gp. 1 element? 
Well, it depends upon the "effective nuclear charge" - the amount of 
nuclear charge that the electron is experiencing - and the shell that the 
electron is in. The larger the effective nuclear charge or the lower the 
shell, the harder it is to remove the electron. In numerical terms, it 
takes 513.3 kJ/mol to remove an electron from Lithium whereas it only takes 
only 375.7 kJ/mol to remove an electron from Cesium. This makes Cesium a 
much more "reactive" element - in fact, one of the most reactive in the
periodic table.