[taregg (OP)]

most of the websites they say francium and the most say lithium..
but i think Li  is more rective than Fr for electrochimical reaction
and Fr is more reactive than Li for reactivity reaction...is it right.

[Kryptid]

It depends on what one means by reactivity.

The properties of francium are not well understood due to its very short half-life, so I'll use cesium in my next example. Cesium is often cited as reacting more vigorously with water than lithium, and this is (at least to the human eyes) true. The reason for this is the very low melting point of cesium. Since liquids can react more quickly than solids, the ability for cesium to melt much more quickly than lithium allows it to react at a faster rate with water. Francium would probably be similar to cesium in this respect. However, if one were to evaluate the energy released in chemical reactions, lithium would often come out on top due to its smaller ionic radius. This in effect gives it a more concentrated charge and allows it to form more stable compounds.

[taregg (OP)]

instead ...francium and cesium becuse its radioactivity elements..let us say lithium and potassium ....like potassiom more reactive in water than lithium ...but for lithium what type of strongest  reaction can react with...

[chiralSPO]

Supercryptid is correct. Here are some numbers to elaborate:

The energy required to remove an electron from Li (520 kJ/mol) is greater than that for K (420 kJ/mol) or Cs (380 kJ/mol; Fr should be even lower). So in that sense the heavier elements are more reactive (lower barrier to reaction = faster reaction = more reactive).

However if you consider the heat of reaction, say with water:
Li + H2O --> LiOH + 0.5H2 + 201.8 kJ
K + H2O --> KOH + 0.5H2 + 139.2 kJ
Cs + H2O --> CsOH + 0.5 H2 + 130.4 kJ

The reaction with Li releases more heat than with K or Cs, which as Supercryptid pointed out, has to do with the ionic radii of the metals.

[taregg (OP)]

what a bout hydrocarbon fuel...does Li give more burning heat than Na...

[chiralSPO]

Alkali metals do not react with hydrocarbon fuels at any reasonable temperature. Both are susceptible to oxidation, but neither are capable of oxidizing much. You need to react a reducing agent and an oxidizing agent to get this sort of reaction to work.

Alkali metals will react vigorously with elements like oxygen, sulfur, fluorine, chlorine and bromine and compounds like NO2, XeF6, etc. etc.

[AndyJohnDOU]

so assuming that reactivity increases in group 1 the further down the group
so Lithium < Sodium < Potassium etc...
Presumably the same for group 2 elements
Beryllium < Magnesium < Calcium etc...
and Group 2 < Group 1, ie Magnesium < Sodium
So Beryllium is the least reactive out of Groups 1 & 2
and Francium is the most reactive out of Groups 1 & 2
are there any elements in Group 2 that are more reactive than Group 1 elements
ie is there a cross-over in reactivity between groups 1 & 2
thanks

[chiralSPO]

Ionization energy trend
easiest to remove ↔ hardest to remove
Cs<Rb<K<Ba<Na<Li<Sr<Ca<Mg<Be

so in this sense, Ba is more reactive than Li and Na, but if we look at ionic radii, Ba is way bigger than either of these, so I would say that there is not any alkaline earth metal more reactive than any alkali metal (unless you count H as an alkali metal, in which case, it is the least reactive of all of them; though H is sometimes more like a halogen than a metal, so this comparison is not very good)